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SINKING AND BORING WELLS.
[Illustration: BORING SHEAR FRAME.]
WATER SUPPLY.
THE PRESENT PRACTICE OF SINKING AND BORING WELLS;
With Geological Considerations and Examples of Wells Executed.
by
ERNEST SPON,
Member of the Society of Engineers; of the Franklin Institute;
of the Iron and Steel Institute; and of the Geologists' Association.
[Illustration: Ornate Monogram.]
London:
E. & F. N. Spon, 48, Charing Cross.
New York: 446, Broome Street.
1875.
CONTENTS.
CHAP. PAGE
PREFACE. v.
I. GEOLOGICAL CONSIDERATIONS. 1
II. THE NEW RED SANDSTONE. 35
III. WELL SINKING. 40
IV. WELL BORING. 60
V. AMERICAN TUBE WELL. 81
VI. WELL BORING AT GREAT DEPTHS. 85
VII. EXAMPLES OF WELLS EXECUTED, AND OF DISTRICTS
SUPPLIED BY WELLS. 155
VIII. TABLES AND MISCELLANEOUS INFORMATION. 202
INDEX. 211
E. & F. N. SPON'S NEW BOOKS. Advertisements
PREFACE.
In modern times the tendency of the inhabitants of a country to dwell
together in large communities, and the consequent need for accumulating
in a particular locality a sufficient supply of water for household,
social, and industrial purposes, have rendered necessary the
construction of such engineering works as impounding reservoirs and
wells, by means of which the abundant measure of sparsely populated
districts may be utilized, and water obtained not only free from those
impurities which it collects in densely populated districts, but also in
greater quantity than the natural sources of the district are capable of
supplying.
Of the works mentioned, wells have fairly a primary claim upon the
notice of the sanitary engineer, for, without undervaluing other sources
of supply, the water from them certainly possesses the advantage over
that from rivers and surface drainage, of being without organic
admixture and unimpregnated with those deadly spores which find their
way into surface waters and are so fatal in seasons of epidemic
visitation. A great deal of the irregularity in the action of wells, and
the consequent distrust with which they are regarded by many, is
attributable either to improper situation or to the haphazard manner in
which the search for underground water is frequently conducted. As
regards the first cause, it cannot be too strongly stated that extreme
caution is necessary in the choice of situations for wells, and that a
sound geological knowledge of the country in which the attempt is to be
made should precede any sinking or boring for this purpose, otherwise
much useless expense may be incurred without a chance of success.
Indeed, the power of indicating those points where wells may, in all
probability, be successfully established, is one of the chief practical
applications of geology to the useful purposes of life.
Two cases in point are before me as I write; in the one 15,000_l._ has
been spent in sinking a shaft and driving headings which yield but
little water, found abundantly at the same depth in a mine adjoining;
and in the other a town would be, but for its surface wells, entirely
without water, the waterworks having been idle for weeks, and the
sinkers are feebly endeavouring to obtain water by deep sinkings, in a
position where its occurrence in any quantity is physically impossible.
Ample supplies could be obtained in both these cases by shifting the
situation a few hundred yards.
The subject-matter of the following pages is divided into chapters which
treat of geological considerations, the new red sandstone, well sinking,
well boring, the American tube well, well boring at great depths, and
examples of wells executed and of localities supplied respectively, with
tables and miscellaneous information. Each system with its adjuncts has
been kept complete in itself, instead of separating the various tools
and appliances into classes, the plan adopted in the most approved
French and German technical works. This, however, when too rigidly
adhered to, as is the case with German works in particular, renders it
troublesome for even a practised engineer to grasp a strange system in
its entirety, while the pupil is wearied and retarded in his reading by
an over-elaborate classification.
It may, perhaps, be remarked that undue prominence has been given to the
tertiary and cretaceous formations, but it is urged in extenuation that
they happen to underlie two of the most important cities in Europe, and
that they have, in consequence, received a more thorough investigation
than has been accorded to other districts. The records of wells in many
formations are singularly scanty and unreliable, but it is hoped that
the time is not far distant when the water-bearing characteristics of
strata, such as the new red sandstone and permian, will receive proper
attention, and that correct official records of well-work will be found
in every locality, as this alone can rescue an important branch of
hydraulic engineering from the charge of empiricism.
In the course of the work the writings of G. R. Burnell, C.E., Baldwin
Latham, C.E., M. Dru, Emerson Bainbridge, C.E., G. C. Greenwell, and
other well known authorities, have been freely referred to, particular
recourse having been had to the works of Professor Prestwich, F.G.S.
I am indebted to Geo. G. André, C.E., F.G.S., Messrs. S. Baker and Son,
and Messrs. T. Docwra and Son, for many suggestions and much valuable
information; to Messrs. Docwra special thanks are due for some of the
important sections illustrating chapter vii.
Any claim to attention the book may deserve is based upon its being an
attempt to embody, in a collected form, facts and information derived
from practice, or from various sources not accessible to the majority of
those engaged in the superintendence, or otherwise interested in the
construction of wells.
ERNEST SPON.
16, CRAVEN STREET, CHARING CROSS,
_June, 1875_.
SINKING AND BORING WELLS.
CHAPTER I.
GEOLOGICAL CONSIDERATIONS.
Nearly every civil engineer is familiar with the fact that certain
porous soils, such as sand or gravel, absorb water with rapidity, and
that the ground composed of them soon dries up after showers. If a well
be sunk in such soils, we often penetrate to considerable depths before
we meet with water; but this is usually found on our approaching some
lower part of the porous formation where it rests on an impervious bed;
for here the water, unable to make its way downwards in a direct line,
accumulates as in a reservoir, and is ready to ooze out into any opening
which may be made, in the same manner as we see the salt water filtrate
into and fill any hollow which we dig in the sands of the shore at low
tide. A spring, then, is the lowest point or lip of an underground
reservoir of water in the stratification. A well, therefore, sunk in
such strata will most probably furnish, besides the volume of the
spring, an additional supply of water.
The transmission of water through a porous medium being so rapid, we may
easily understand why springs are thrown out on the side of a hill,
where the upper set of strata consist of chalk, sand, and other
permeable substances, whilst those lying beneath are composed of clay or
other retentive soils. The only difficulty, indeed, is to explain why
the water does not ooze out everywhere along the line of junction of the
two formations, so as to form one continuous land-soak, instead of a
few springs only, and these oftentimes far distant from each other. The
principal cause of such a concentration of the waters at a few points
is, first, the existence of inequalities in the upper surface of the
impermeable stratum, which lead the water, as valleys do on the external
surface of a country, into certain low levels and channels; and
secondly, the frequency of rents and fissures, which act as natural
drains. That the generality of springs owe their supply to the
atmosphere is evident from this, that they vary in the different seasons
of the year, becoming languid or entirely ceasing to flow after long
droughts, and being again replenished after a continuance of rain. Many
of them are probably indebted for the constancy and uniformity of their
volume to the great extent of the subterranean reservoirs with which
they communicate, and the time required for these to empty themselves by
percolation. Such a gradual and regulated discharge is exhibited, though
in a less perfect degree, in all great lakes, for these are not sensibly
affected in their levels by a sudden shower, but are only slightly
raised, and their channels of efflux, instead of being swollen suddenly
like the bed of a torrent, carry off the surplus water gradually.
An Artesian well, so called from the province of Artois, in France, is a
shaft sunk or bored through impermeable strata, until a water-bearing
stratum is tapped, when the water is forced upwards by the hydrostatic
pressure due to the superior level at which the rain-water was received.
Among the causes of the failure of Artesian wells, we may mention those
numerous rents and faults which abound in some rocks, and the deep
ravines and valleys by which many countries are traversed; for when
these natural lines of drainage exist, there remains only a small
quantity of water to escape by artificial issues. We are also liable to
be baffled by the great thickness either of porous or impervious strata,
or by the dip of the beds, which may carry off the waters from adjoining
high lands to some trough in an opposite direction,--as when the borings
are made at the foot of an escarpment where the strata incline inwards,
or in a direction opposite to the face of the cliffs.
[Illustration: Effect of Strata on Artesian Well.
Fig. 1.]
[Illustration: Effect of a Fault.
Fig. 2.]
As instances of the way in which the character of the strata may
influence the water-bearing capacity of any given locality, we give the
following examples, taken from Baldwin Latham's papers on 'The Supply of
Water to Towns.' Fig. 1 illustrates the causes which sometimes conduce
to a limited supply of water in Artesian wells. Rain descending on the
outcrop E F of the porous stratum A, which lies between the impervious
stratum B B, will make its appearance in the form of a spring at S; but
such spring will not yield any great quantity of water, as the area E F,
which receives the rainfall, is limited in its extent. A well sunk at W,
in a stratum of the above description, would not be likely to furnish a
large supply of water, if any. The effect of a fault is shown in Fig. 2.
A spring will in all probability make its appearance at the point S, and
give large quantities of water, as the whole body of water flowing
through the porous strata A is intercepted by being thrown against the
impermeable stratum B. Permeable rock intersected by a dyke and
overlying an impermeable stratum is seen in Fig. 3. The water flowing
through A, if intersected by a dyke D, will appear at S in the form of a
spring, and if the area of A is of large extent, then the spring S will
be very copious. As to the depth necessary to bore certain wells, in a
case similar to Fig. 4, owing to the fault, a well sunk at A would
require to be sunk deeper than the well B, although both wells derive
their supply from the same description of strata. If there is any
inclination in the water-bearing strata, or if there is a current of
water only in one direction, then one of the wells would prove a failure
owing to the proximity of the fault, while the other would furnish an
abundant supply of water.
[Illustration: Permeable Rock Intersected by a Dyke.
Fig. 3.]
It should be borne in mind that there are two primary geological
conditions upon which the quantity of water that may be supplied to the
water-bearing strata depends; they are, the extent of superficial area
presented by these deposits, by which the quantity of rain-water
received on their surface in any given time is determined; and the
character and thickness of the strata, as by this the proportion of
water that can be absorbed, and the quantity which the whole volume of
the permeable strata can transmit, is regulated. The operation of these
general principles will constantly vary in accordance with local
phenomena, all of which must, in each separate case, be taken into
consideration.
[Illustration: Well Depth either side of a Fault.
Fig. 4.]
The mere distance of hills or mountains need not discourage us from
making trials; for the waters which fall on these higher lands readily
penetrate to great depths through highly-inclined or vertical strata, or
through the fissures of shattered rocks; and after flowing for a great
distance, must often reascend and be brought up again by other fissures,
so as to approach the surface in the lower country. Here they may be
concealed beneath a covering of undisturbed horizontal beds, which it
may be necessary to pierce in order to reach them. The course of water
flowing underground is not strictly analogous to that of rivers on the
surface, there being, in the one case, a constant descent from a higher
to a lower level from the source of the stream to the sea; whereas, in
the other, the water may at one time sink far below the level of the
ocean, and afterwards rise again high above it.
For the purposes under consideration, we may range the various strata of
which the outer crust of the earth is composed under four heads, namely:
1, drift; 2, alluvion; 3, the tertiary and secondary beds, composed of
loose, arenaceous and permeable strata, impervious, argillaceous and
marly strata, and thick strata of compact rock, more or less broken up
by fissures, as the Norwich red and coralline crag, the Molasse
sandstones, the Bagshot sands, the London clay, and the Woolwich beds,
in the tertiary division; and the chalk, chalk marl, gault, the
greensands, the Wealden clay, and the Hastings sand; the oolites, the
has, the Rhætic beds, and Keuper, and the new red sandstone, in the
secondary division; and 4, the primary beds, as the magnesian limestone,
the lower red sand, and the coal measures, which consist mainly of
alternating beds of sandstones and shales with coal.
The first of these divisions, the drift, consisting mainly of sand and
gravel, having been formed by the action of flowing water, is very
irregular in thickness, and exists frequently in detached masses. This
irregularity is due to the inequalities of the surface at the period
when the drift was brought down. Hollows then existing would often be
filled up, while either none was deposited on level surfaces, or, if
deposited, was subsequently removed by denudation. Hence we cannot infer
when boring through deposits of this character that the same, or nearly
the same, thickness will be found at even a few yards' distance. In
valleys this deposit may exist to a great depth, the slopes of hills are
frequently covered with drift, which has either been arrested by the
elevated surface or brought down from the upper portions of that surface
by the action of rain. In the former case the deposits will probably
consist of gravel, and in the latter, of the same elements as the hill
itself.
The permeability of such beds will, of course, depend wholly upon the
nature of the deposit. Some rocks produce deposits through which water
percolates readily, while others allow a passage only through such
fissures as may exist. Sand and gravel constitute an extremely absorbent
medium, while an argillaceous deposit may be wholly impervious. In
mountainous districts springs may often be found in the drift; their
existence in such formations will, however, depend upon the position and
character of the rock strata; thus, if the drift cover an elevated and
extensive slope of a nature similar to that of the rocks by which it is
formed, springs due to infiltration through this covering will certainly
exist near the foot of the slope. Upon the opposite slope, the small
spaces which exist between the different beds of rock receive these
infiltrations directly, and serve to completely drain the deposit which,
in the former case, is, on the contrary, saturated with water. If,
however, the foliations or the joints of the rocks afford no issue to
the water, whether such a circumstance be due to the character of their
formation, or to the stopping up of the issues by the drift itself,
these results will not be produced.
It will be obvious how, in this way, by passing under a mass of drift
the water descending from the top of hill slopes reappears at their
foot in the form of springs. If now we suppose these issues stopped, or
covered by an impervious stratum of great thickness, and this stratum
pierced by a boring, the water will ascend through this new outlet to a
level above that of its original issue, in virtue of the head of water
measured from the points at which the infiltration takes place to the
point in which it is struck by the boring.
Alluvion, like drift, consists of fragments of various strata carried
away and deposited by flowing water; it differs from the latter only
in being more extensive and regular, and, generally, in being composed
of elements brought from a great distance, and having no analogy with
the strata with which it is in contact. Usually it consists of sand,
gravel, rolled pebbles, marls or clays. The older deposits often occupy
very elevated districts, which they overlie throughout a large extent
of surface. At the period when the large rivers were formed, the
valleys were filled up with alluvial deposits, which at the present day
are covered by vegetable soil, and a rich growth of plants, through
which the water percolates more slowly than formerly. The permeability
of these deposits allows the water to flow away subterraneously to
a great distance from the points at which it enters. Springs are
common in the alluvion, and more frequently than in the case of drift,
they can be found by boring. As the surface, which is covered by the
deposit, is extensive, the water circulates from a distance through
permeable strata often overlaid by others that are impervious. If at a
considerable distance from the points of infiltration, and at a lower
level, a boring be put down, the water will ascend in the bore-hole
in virtue of its tendency to place itself in equilibrium. Where the
country is open and uninhabited, the water from shallow wells sunk
in alluvion is generally found to be good enough and in sufficient
quantity for domestic purposes.
The strata of the tertiary and secondary beds, especially the latter,
are far more extensive than the preceding, and yield much larger
quantities of water. The chalk is the great water-bearing stratum for
the larger portion of the south of England. The water in it can be
obtained either by means of ordinary shafts, or by Artesian wells bored
sometimes to great depths, from which the water will frequently rise
to the surface. It should be observed that water does not circulate
through the chalk by general permeation of the mass, but through
fissures. A rule given by some for the level at which water may be
found in this stratum is, "Take the level of the highest source of
supply, and that of the lowest to be found. The mean level will be the
depth at which water will be found at any intermediate point, after
allowing an inclination of at least 10 feet a mile." This rule will
also apply to the greensand. This formation contains large quantities
of water, which is more evenly distributed than in the chalk. The gault
clay is interposed between the upper and the lower greensand, the
latter of which also furnishes good supplies. In boring into the upper
greensand, caution should be observed so as not to pierce the gault
clay, because water which permeates through that system becomes either
ferruginous, or contaminated by salts and other impurities.
The next strata in which water is found are the upper and inferior
oolites, between which are the Kimmeridge and Oxford clays, which are
separated by the coral rag. There are instances in which the Oxford clay
is met with immediately below the Kimmeridge, rendering any attempt at
boring useless, because the water in the Oxford clay is generally so
impure as to be unfit for use. And with regard to finding water in the
oolitic limestone, it is impossible to determine with any amount of
precision the depth at which it may be reached, owing to the numerous
faults which occur in the formation. It will therefore be necessary to
employ the greatest care before proceeding with any borings. Lower down
in the order are the upper has, the marlstone, the lower has, and the
new red sandstone. In the marlstone, between the upper and lower beds of
the has, there may be found a large supply of water, but the level of
this is as a rule too low to rise to the surface through a boring. It
will be necessary to sink shafts in the ordinary way to reach it. In the
new red sandstone, also, to find the water, borings must be made to a
considerable depth, but when this formation exists a copious supply may
be confidently anticipated, and when found the water is of excellent
quality.
Every permeable stratum may yield water, and its ability to do this, and
the quantity it can yield, depend upon its position and extent. When
underlaid by an impervious stratum, it constitutes a reservoir of water
from which a supply may be drawn by means of a sinking or a bore-hole.
If the permeable stratum be also overlaid by an impervious stratum, the
water will be under pressure and will ascend the bore-hole to a height
that will depend on the height of the points of infiltration above the
bottom of the bore-hole. The quantity to be obtained in such a case as
we have already pointed out, will depend upon the extent of surface
possessed by the outcrop of the permeable stratum. In searching for
water under such conditions a careful examination of the geological
features of the district must be made. Frequently an extended view of
the surface of the district, such as may be obtained from an eminence,
and a consideration of the particular configuration of that surface,
will be sufficient to enable the practical eye to discover the various
routes which are followed by the subterranean water, and to predicate
with some degree of certainty that at a given point water will be found
in abundance, or that no water at all exists at that point. To do this,
it is sufficient to note the dip and the surfaces of the strata which
are exposed to the rains. When these strata are nearly horizontal, water
can penetrate them only through their fissures or pores; when, on the
contrary, they lie at right-angles, they absorb the larger portion of
the water that falls upon their outcrop. When such strata are
intercepted by valleys, numerous springs will exist. But if, instead of
being intercepted, the strata rise around a common point, they form a
kind of irregular basin, in the centre of which the water will
accumulate. In this case the surface springs will be less numerous than
when the strata are broken. But it is possible to obtain water under
pressure in the lower portions of the basin, if the point at which the
trial is made is situate below the outcrop.
The primary rocks afford generally but little water. Having been
subjected to violent convulsions, they are thrown into every possible
position and broken by numerous fissures; and as no permeable stratum is
interposed, as in the more recent formations, no reservoir of water
exists. In the unstratified rocks, the water circulates in all
directions through the fissures that traverse them, and thus occupies no
fixed level. It is also impossible to discover by a surface examination
where the fissures may be struck by a boring. For purposes of water
supply, therefore, these rocks are of little importance. It must be
remarked here, however, that large quantities of water are frequently
met with in the magnesian limestone and the lower red sand, which form
the upper portion of the primary series.
Joseph Prestwich, jun., in his 'Geological Inquiry respecting the
Water-bearing Strata round London,' gives the following valuable epitome
of the geological conditions affecting the value of water-bearing
deposits; and although the illustrations are confined to the Tertiary
deposits, the same mode of inquiry will apply with but little
modification to any other formation.
The main points are--
The extent of the superficial area occupied by the water-bearing
deposit.
The lithological character and thickness of the water-bearing deposit,
and the extent of its underground range.
The position of the outcrop of the deposit, whether in valleys or hills,
and whether its outcrop is denuded, or covered with any description of
drift.
The general elevation of the country occupied by this outcrop above the
levels of the district in which it is proposed to sink wells.
The quantity of rain which falls in the district under consideration,
and whether, in addition, it receives any portion of the drainage from
adjoining tracts, when the strata are impermeable.
The disturbances which may affect the water-bearing strata, and break
their continuous character, as by this the subterranean flow of water
would be impeded or prevented.
EXTENT OF SUPERFICIAL AREA.
To proceed to the application of the questions in the particular
instance of the lower tertiary strata. With regard to the first
question, it is evident that a series of permeable strata encased
between two impermeable formations can receive a supply of water at
those points only where they crop out and are exposed on the surface of
the land. The primary conditions affecting the result depend upon the
fall of rain in the district where the outcrop takes place; the quantity
of rain-water which any permeable strata can gather being in the same
ratio as their respective areas. If the mean annual fall in any district
amounts to 24 inches, then each square mile will receive a daily average
of 950,947 gallons of rain-water. It is therefore a matter of essential
importance to ascertain, with as much accuracy as possible, the extent
of exposed surface of any water-bearing deposit, so as to determine the
maximum quantity of rain-water it is capable of receiving.
The surface formed by the outcropping of any deposit in a country of
hill and valley is necessarily extremely limited, and it would be
difficult to measure in the ordinary way. Prestwich therefore used
another method, which seems to give results sufficiently accurate for
the purpose. It is a plan borrowed from geographers, that of cutting out
from a map on paper of uniform thickness and on a large scale, say one
inch to the mile, and weighing the superficial area of each deposit.
Knowing the weight of a square of 100 miles cut out of the same paper,
it is easy to estimate roughly the area in square miles of any other
surface, whatever may be its figure.
MINERAL CHARACTER OF THE FORMATION.
The second question relates to the mineral character of the formation,
and the effect it will have upon the quantity of water which it may hold
or transmit.
If the strata consist of sand, water will pass through them with
facility, and they will also hold a considerable quantity between the
interstices of their component grains; whereas a bed of pure clay will
not allow of the passage of water. These are the two extremes of the
case; the intermixture of these materials in the same bed will of
course, according to their relative proportions, modify the transmission
of water. Prestwich found by experiment that a silicious sand of
ordinary character will hold on an average rather more than one-third of
its bulk of water, or from two to two and a half gallons in one cubic
foot. In strata so composed the water may be termed free, as it passes
easily in all directions, and under the pressure of a column of water is
comparatively but little impeded by capillary attraction. These are the
conditions of a true permeable stratum. Where the strata are more
compact and solid, as in sandstone, limestone, and oolite, although all
such rocks imbibe more or less water, yet the water so absorbed does not
pass freely through the mass, but is held in the pores of the rock by
capillary attraction, and parted with very slowly; so that in such
deposits water can be freely transmitted only in the planes of bedding
and in fissures. If the water-bearing deposit is of uniform lithological
character over a large area, then the proposition is reduced to its
simplest form; but when, as in the deposit between the London clay and
the chalk, the strata consist of variable mineral ingredients, it
becomes essential to estimate the extent of these variations; for very
different conclusions might be drawn from an inspection of the Lower
Tertiary strata at different localities.
[Illustration: _a_ London clay, _b_ Sands and clay, _c_ Chalk.
Fig. 5.]
In the fine section exposed in the cliffs between Herne Bay and the
Reculvers, in England, a considerable mass of fossiliferous sands is
seen to rise from beneath the London clay. Fig. 5 represents a view of a
portion of this cliff a mile and a half east of Herne Bay and continued
downwards, by estimation below the surface of the ground to the chalk.
In this section there is evidently a very large proportion of sand, and
consequently a large capacity for water. Again, at Upnor, near
Rochester, the sands marked 3 are as much as 60 to 80 feet thick, and
continue so to Gravesend, Purfleet, and Erith. In the first of these
places they may be seen capping Windmill Hill; in the second, forming
the hill, now removed, on which the lighthouse is built; and in the
third, in the large ballast pits on the banks of the river Thames. The
average thickness of these sands in this district may be about 50 to 60
feet. In their range from east to west, the beds 2 become more clayey
and less permeable, and 1, very thin. As we approach London the
thickness of 3 also diminishes. In the ballast pits at the west end of
Woolwich, this sand-bed is not more than 35 feet thick, and as it passes
under London becomes still thinner.
[Illustration: Fig. 6.
General Section of Strata below London.]
Fig. 6 is a general or average section of the strata on which London
stands. The increase in the proportion of the argillaceous strata, and
the decrease of the beds of sand, in the Lower Tertiary strata is here
very apparent, and from this point westward to Hungerford, clays
decidedly predominate; while at the same time the series presents such
rapid variations, even on the same level and at short distances, that no
two sections are alike. On the southern boundary of the Tertiary
district, from Croydon to Leatherhead, the sands 3 maintain a thickness
of 20 to 40 feet, whilst the associated beds of clay are of inferior
importance. We will take another section, Fig. 7, representing the
usual features of the deposit in the northern part of the Tertiary
district. It is from a cutting at a brickfield west of the small village
of Hedgerley, 6 miles northward of Windsor.
[Illustration: Unusual Deposit Features 6 miles North of Windsor.
Fig. 7.]
Here we see a large development of the mottled clays, and but little
sand. A somewhat similar section is exhibited at Oak End, near Chalfont
St. Giles. But to show how rapidly this series changes its character,
the section of a pit only a third of a mile westward of the one at
Hedgerley is given in Fig. 8.
[Illustration: Section of a Pit.
Fig. 8.]
In this latter section the mottled clays have nearly disappeared, and
are replaced by beds of sand with thin seams of mottled clays. At
Twyford, near Reading, and at Old Basing, near Basingstoke, the mottled
clays again occupy, as at Hedgerley, nearly the whole space between the
London clays and the chalk. Near Reading a good section of these beds
was exhibited in the Sonning cutting of the Great Western Railway; they
consisted chiefly of mottled clays. At the Katsgrove pits, Reading, the
beds are more sandy. Referring back to Fig. 6, it may be noticed that
there is generally a small quantity of water found in the bed marked 1,
in parts of the neighbourhood of London. Owing, however, to the constant
presence of green and ferruginous sands, traces of vegetable matters and
remains of fossil shells, the water is usually indifferent and
chalybeate. The well-diggers term this a slow spring. They well express
the difference by saying that the water creeps up from this stratum,
whereas that it bursts up from the lower sands 3, which is the great
water-bearing stratum. In the irregular sand-beds interstratified with
the mottled clays between these two strata water is also found, but not
in any large quantity.
[Illustration: Section at Pebble Hill.
Fig. 9.]
Fig. 9 is a section at the western extremity of the Tertiary district at
Pebble Hill, near Hungerford. Here again the mottled clays are in
considerable force, sands forming the smaller part of the series.
The following lists exhibit the aggregate thickness of all the beds of
sand occurring between the London clay and the chalk at various
localities in the Tertiary district. It will appear from them that the
mean results of the whole is very different from any of those obtained
in separate divisions of the country. The mean thickness of the deposit
throughout the whole Tertiary area may be taken at 62 feet, of which 36
feet consist of sands and 26 feet of clays; but as only a portion of
this district contributes to the water supply of London, it will
facilitate our inquiry if we divide it into two parts, the one westward
of and including London, and the other eastward of it, introducing also
some further subdivisions into each.
MEASUREMENT OF SECTIONS EASTWARD OF LONDON.
+---------------------+-----+-----+
|Southern Boundary. |Sand.|Clay.|
+---------------------+-----+-----+
| | ft. | ft. |
|Lewisham | 65 | 26 |
|Woolwich | 66 | 18 |
|Upnor | 80? | 8 |
|Herne Bay | 70? | 50 |
| | | |
| | | |
| | | |
| | | |
| +-----+-----+
| Average | 70 | 25 |
+---------------------+-----+-----+
+---------------------+-----+-----+
|Northern Boundary. |Sand.|Clay.|
+---------------------+-----+-----+
| | ft.| ft. |
|Hertford | 26 | 3 |
|Beaumont Green, | | |
| near Hoddesdon | 16 | 10 |
|Broxbourne | 28 | 2 |
|Gestingthorpe, | | |
| near Sudbury | 50?| ? |
|Whitton, | | |
| near Ipswich | 60?| 5 |
| +-----+-----+
| Average | 36 | 5 |
+---------------------+-----+-----+
The mean of the three columns in two western sections gives a thickness
to this formation of 57 feet, of which only 19 feet are sand and
permeable to water, and the remaining 38 feet consist of impermeable
clays, affording no supply of water.
The area, both at the surface and underground, over which they extend
is about 1086 square miles.
MEASUREMENT OF SECTIONS WESTWARD OF LONDON.
+--------------------------------------------+
| On or near the Southern Boundary |
| of the Tertiary District. |
+--------------------------------+-----+-----+
| |Sand.|Clay.|
| +-----+-----+
| | ft.| ft.|
|Streatham | 30 | 25 |
|Mitcham | 47 | 34 |
|Croydon | 35?| 20?|
|Epsom | 31 | 23 |
|Fetcham | 35 | 20 |
|Guildford | 10?| 40 |
|Chinham, near Basingstoke | 20?| 30 |
|Itchingswell, near Kingsclere | 22 | 34 |
|Highclere | 24 | 27 |
|Pebble Hill, near Hungerford | 9 | 39 |
| | | |
| | | |
| | | |
| +-----+-----+
| Average | 26 | 29 |
+--------------------------------+-----+-----+
+--------------------------------------------+
| On a Central Line in the |
| Tertiary District. |
+--------------------------------+-----+-----+
| |Sand.|Clay.|
| +-----+-----+
| Sand.Clay.| ft.| ft.|
| ft. ft. | | |
|London: | | |
| Millbank 49 40} | | |
| Trafalgar Square 49 30} | | |
| Tottenham Court Road 35 30} | | |
| Pentonville 34 44} | 46 | 39 |
| Barclay's Brewery 55 42} | | |
| Lombard Street 53 35} | | |
| The Mint 49 38} | | |
| Whitechapel 45 50} | | |
|Garrett, near Wandsworth | 20 | 52 |
|Isleworth | 17 | 70 |
|Twickenham | 7 | 50 |
|Chobham | 3 | 45 |
| +-----+-----+
| Average | 18 | 51 |
+--------------------------------+-----+-----+
+--------------------------------------------+
| On or near the Northern Boundary |
| of the Tertiary District |
+--------------------------------+-----+-----+
| |Sand.|Clay.|
| +-----+-----+
| | ft.| ft.|
|Hatfield | 23 | 2 |
|Watford | 25 | 10 |
|Pinner | 12 | 32 |
|Oak End, Chalfont St. Giles | 3 | 40 |
|Hedgerley, near Slough | 5 | 45 |
|Starveall, " " | 13 | 20 |
|Twyford | 5 | 60 |
|Sonning, near Reading | 12 | 54 |
|Reading | 16 | 33 |
|Newbury | 20 | 36 |
|Pebble Hill | 9 | 39 |
| | | |
| | | |
| +-----+-----+
| Average | 13 | 34 |
+--------------------------------+-----+-----+
The average total thickness of the eastern district deduced from the
nine sections we have taken gives 68 feet, of which 53 feet are sands
and 15 feet clays. The larger area, 1849 square miles, over which the
eastern portion of the Tertiary series extends, and the greater volume
of the water-bearing beds, constitute important differences in favour of
this district; and if there had been no geological disturbances to
interfere with the continuous character of the strata, we might have
looked to this quarter for a large supply of water to the Artesian wells
of London.
[Illustration: A Geological Section.
Fig. 10.]
From these tables it will be readily perceived that the strata of which
the water-bearing deposits are composed are very variable in their
relative thickness. They consist, in fact, of alternating beds of clay
and sand, in proportions constantly changing. In one place, as at
Hedgerley, the aggregate beds of sand may be 5 feet thick, and the clays
45 feet; whilst at another, as at Leatherhead, the sands may be 35, and
the clays 20 feet thick, and some such variation is observable in every
locality. But although we may thus in some measure judge of the capacity
of these beds for water, this method fails to show whether the
communication from one part of the area to another is free, or impeded
by causes connected with mineral character. Now as we know that these
beds not only vary in their thickness, but that they also frequently
thin out, and sometimes pass one into another, it may happen that a very
large development of clay at any one place may altogether stop the
transit of the water in that locality. Thus in Fig. 10 the beds of sand
at y allow of the free passage of water, but at x, where clays occupy
the whole thickness, it cannot pass; the obstruction which this cause
may offer to the underground flow of water can only be determined by
experience. It must not, however, be supposed that such a variation in
the strata is permanent or general along any given line. It is always
local, some of the beds of clay commonly thinning out after a certain
horizontal range, so that, although the water may be impeded or
retarded in a direct course, it most probably can, in part or
altogether, pass round by some point where the strata have not undergone
the same alteration.
POSITION AND GENERAL CONDITIONS OF THE OUTCROP.
This involves some considerations to which an exact value cannot at
present be given, yet which require notice, as they to a great extent
determine the proportion of water which can pass from the surface into
the mass of the water-bearing strata. In the first place, when the
outcrop of these strata occurs in a valley, as represented in Fig. 11,
it is evident that _b_ may not only retain all the water which might
fall on its surface, but also would receive a proportion of that
draining off from the strata of _a_ and _c_. This form of the surface
generally prevails wherever the water-bearing strata are softer and less
coherent than the strata above and below them.
[Illustration: Water-bearing Strata in a Valley.
Fig. 11.]
It may be observed in the Lower Tertiary series at Sutton, Carshalton,
and Croydon, where a small and shallow valley, excavated in these sands
and mottled clays, ranges parallel with the chalk hills.
It is apparent again between Epsom and Leatherhead, and also in some
places between Guildford and Farnham, as well as between Odiham and
Kingsclere. The Southampton Railway crosses this small valley on an
embankment at Old Basing.
This may be considered as the prevailing, but not exclusive, form of
structure from Croydon to near Hungerford. The advantage, however, to be
gained from it in point of water supply is much limited by the rather
high angle at which the strata are inclined, as well as by their small
development, which greatly restrict the breadth of the surface occupied
by the outcrop. It rarely exceeds a quarter of a mile, and is generally
very much less, often not more than 100 to 200 feet. The next
modification of outcrop, represented in Fig. 12, is one not uncommon on
the south side of the Tertiary district. The strata _b_ here crop out on
the slope of the chalk hills, and the rain falling upon them, unless
rapidly absorbed, tends to drain at once from their surface into the
adjacent valleys. V, L, shows the line of valley level.
[Illustration: Strata on Slope of Chalk Hills.
Fig. 12.]
This arrangement is not unfrequent between Kingsclere and Inkpen, and
also between Guildford and Leatherhead. Eastward of London it is
exhibited on a larger scale at the base of the chalk hills, in places
between Chatham and Faversham, a line along which the sands of the Lower
Tertiary strata, _b_, are more fully developed than elsewhere. As,
however, the surface of _b_ is there usually more coincident with the
valley level, V, L, of the district, it is in a better position for
retaining more of the rainfall.
[Illustration: Chalk Stratum at Base of a Hill.
Fig. 13.]
A third position of outcrop, much more unfavourable for the
water-bearing strata, prevails generally along the greater part of the
northern boundary of the Tertiary strata. Instead of forming a valley,
or outcropping at the base of the chalk hills, almost the whole length
of this outcrop lies on the slope of the hills, as in Fig. 13, where the
chalk _c_ forms the base of the hill and the lower ground at its foot,
whilst the London clay, _a_, caps the summit, thus restricting the
outcrop of _b_ to a very narrow zone and a sloping surface. This form
of structure is exhibited in the hills round Sonning, Reading,
Hedgerley, Rickmansworth, and Watford; thence by Shenley Hill, Hatfield,
Hertford, Sudbury; and also at Hadleigh this position of outcrop is
continued. If, as on the southern side of the Tertiary district, the
outcrop were continued in a nearly unbroken line, then these
unfavourable conditions would prevail uninterruptedly; but the hills are
in broken groups, and intersected at short distances by transverse
valleys, as that of the Kennet at Reading, of the Loddon at Twyford, of
the Colne at Uxbridge, and so on. Between Watford and Hatfield there is
a constant succession of small valleys running back for short distances
from the Lower district of the chalk, through the hills of the Tertiary
district. The Valley of the Lea at Roydon and Hoddesdon is a similar and
stronger case in point. The effect of these transverse valleys is to
open out a larger surface of the strata _b_ than would otherwise be
exposed, for if the horizontal line, V, L, Fig. 13, were carried back
beyond the point _x_, to meet the prolongation of _b_, then these Lower
Tertiary strata would not only be intersected by the line of valley
level, but would form a much smaller angle with the plane V, L, and
therefore spread over a larger area than where they crop out on the side
of the hills.
The foregoing are the three most general forms of outcrop, but
occasionally the outcrop takes place wholly or partly on the summit of a
hill, as, near the Reculvers in the neighbourhood of Canterbury, of
Sittingbourne, and at the Addington Hills, near Croydon, in which cases
the area of the Lower Tertiary is expanded. When the dip is very slight,
and the beds nearly horizontal, the Lower Tertiary sands occasionally
spread over a still larger extent of surface, as between Stoke Pogis,
Burnham Common, and Beaconsfield, and in the case of the flat-topped
hill, forming Blackheath and Bexley Heath, as in Fig. 14. Favourable as
such districts might at first appear to be from the extent of their
exposed surface, nevertheless they rarely contribute to the water supply
of the wells sunk into the Lower Tertiary sands under London, the
continuity of the strata being broken by intersecting valleys; thus the
district last mentioned is bounded on the north by the valley of the
Thames, on the west by that of Ravensbourne, and on the east by the
valley of the Cray; consequently the rain-water, which has been absorbed
by the very permeable strata on the intermediate higher ground, passes
out on the sides of the hills, into the surface channels in the valleys,
or into the chalk. Almost all the wells at Bexley Heath, for their
supply of water, have, in fact, to be sunk into the chalk through the
overlying 100 to 133 feet of sand and pebble beds, _b_.
[Illustration: Section at Blackheath and Bexley Heath.
Fig. 14.]
Thus far we have considered this question, as if, in each instance, the
outcropping edges of the water-bearing strata, _b_, were laid bare, and
presented no impediment to the absorption of the rain-water falling
immediately upon their surface, or passing on to it from some more
impermeable deposits. But there is another consideration which
influences materially the extent of the water supply.
If the strata _b_ were always bare, we should have to consider their
outcrop as an absorbent surface, of power varying according to the
lithological character and dip of the strata only. But the outcropping
edges of the strata do not commonly present bare and denuded surfaces.
Thus a large extent of the country round London is more or less covered
by beds of drift, which protect the outcropping beds of _b_, and turn
off a portion of the water falling upon them.
The drift differs considerably in its power of interference with the
passage of the rain-water into the strata beneath. The ochreous sandy
flint gravel, forming so generally the subsoil of London, admits of the
passage of water. All the shallow surface springs, from 10 to 20 feet
deep, are produced by water which has fallen on, and passed through,
this gravel, _g_, Fig. 15, down to the top of the London clay, _a_, on
the irregular surface of which it is held up.
[Illustration: Section through London Subsoil.
Fig. 15.]
When the London clay is wanting, this gravel lies immediately upon the
Lower Tertiary strata, as in the valley between Windsor and Maidenhead,
and in that of the Kennet between Newbury and Thatcham, transmitting to
the underlying strata part of the surface water. Where beds of brick
earth occur in the drift, as between West Drayton and Uxbridge, the
passage of the surface water into the underlying strata is intercepted.
Sometimes the drift is composed of gravel mixed very irregularly with
broken up London clay, and although commonly not more than 3 to 8 feet
thick, it is generally impermeable.
Over a considerable portion of Suffolk and part of Essex, a drift,
composed of coarse and usually light-coloured sand with fine gravel,
occurs. Water percolates through it with extreme facility, but it is
generally covered by a thick mass of stiff tenacious bluish grey clay,
perfectly impervious. This clay drift, or boulder clay, caps, to a depth
of from 10 to 50 feet or more, almost all the hills in the northern
division of Essex, and a large portion of Suffolk and Norfolk. It so
conceals the underlying strata that it is difficult to trace the course
of the outcrop of the Lower Tertiary sands between Ware and Ipswich; and
often, as in Fig. 16, notwithstanding the breadth, apart from this cause
of the outcrop of the Tertiary sands, _b_, and of the drift of sand and
gravel, 2, they are both so covered by the boulder clay, 1, that the
small surface exposed can be of comparatively little value.
[Illustration: Outcrop covered by Boulder Clay.
Fig. 16.]
There are also, in some valleys, river deposits of silt, mud, and
gravel. These are, however, of little importance to the subject before
us. Under ordinary conditions they are generally sufficiently impervious
to prevent the water from passing through the beds beneath.
HEIGHT OF WATER-BEARING STRATA ABOVE SURFACE OF COUNTRY.
The height of the districts, wherein the water-bearing strata crop out,
above that of the surface of the country in which the wells are placed,
should be made the subject of careful consideration, as upon this point
depends the level to which the water in Artesian wells may ascend.
Again, taking the London district as an example, Prestwich remarks that,
as the country rises on both sides of the Thames to the edge of the
chalk escarpments, and as the outcrop of the Lower Tertiary strata is
intermediate between these escarpments and the Thames, it follows that
the outcrop of these lower beds must, in all cases, be on a higher level
than the Thames itself, where it flows through the centre of the
Tertiary district. Its altitude is, of course, very variable, as shown
in the following list of its approximate height above Trinity high
water-mark at London. These heights are taken where the Tertiaries are
at their lowest level in the several localities mentioned.
--------------------------------+-------------------------
South of London. | North of London.
--------------------------------+-------------------------
Croydon about 130 feet. | Thetford about 200 feet.
Leatherhead " 90 " | Watford " 170 "
Guildford " 96 " | Slough " 60 "
Old Basing " 250 " | Reading " 120 "
Near Hungerford " 360 " | Newbury " 236 "
--------------------------------+-------------------------
Eastward of London these strata crop out at a gradually decreasing
level. In consequence, therefore, of the outcrop of the water-bearing
strata being thus much above the surface of the central Tertiary
district bordering the Thames, the water in these strata beneath London
tended originally to rise above that surface.
As, however, these beds crop out on a level with the Thames immediately
east of the city between Deptford, Blackwall, and Bow, the water, having
this natural issue so near, could never have risen in London much above
the level of the river.
RAINFALL IN THE DISTRICT WHERE THE WATER-BEARING STRATA CROP OUT.
When inquiring into the probable relative value of any water-bearing
strata, it is necessary to compare the rainfall in their respective
districts.
Rain is of all meteorological phenomena the most capricious, both as
regards its frequency and the amount which falls in a given time. In
some places it rarely or never falls, whilst in others it rains almost
every day; and there does not yet exist any theory from which a probable
estimate of the rainfall in a given district can be deduced
independently of direct observation. But although dealing with one of
the most capricious of the elements, we nevertheless find a workable
average in the quantity of rain to be expected in any particular place,
if careful and continued observations are made with the rain-gauge. G.
J. Symons, the meteorologist, to whose continued investigations we are
indebted for our most reliable data upon the subject of rainfall, gives
the following practical instructions for using a rain-gauge;--
"The mouth of the gauge must be set quite level, and so fixed that it
will remain so; it should never be less than 6 inches above the ground,
nor more than 1 foot except when a greater elevation is absolutely
necessary to obtain a proper exposure.
"It must be set on a level piece of ground, at a distance from shrubs,
trees, walls, and buildings, at the very least as many feet from their
base as they are in height.
"If a thoroughly clear site cannot be obtained, shelter is most
endurable from N.W., N., and E., less so from S., S.E., and W., and not
at all from S.W. or N.E.
"Special prohibition must issue as to keeping all tall-growing flowers
away from the gauges.
"In order to prevent rust, it will be desirable to give the japanned
gauges a coat of paint every two or three years.
"The gauge should, if possible, be emptied daily at 9 A.M., and the
amount entered against the previous day.
"When making an observation, care should be taken to hold the glass
upright.
"It can hardly be necessary to give here a treatise on decimal
arithmetic; suffice it therefore to say that rain-gauge glasses usually
hold half an inch of rain (0·50) and that each 1/100 (0·01) is marked;
if the fall is less than half an inch, the number of hundredths is read
off at once, if it is over half an inch, the glass must be filled up to
the half inch (0·50), and the remainder (say 0·22) measured afterwards,
the total (0·50 + 0·22) = 0·72 being entered. If less than 1/10 (0·10)
has fallen, the cipher must always be prefixed; thus if the measure is
full up to the seventh line, it must be entered as 0·07, that is, no
inches, no tenths, and seven hundredths. For the sake of clearness it
has been found necessary to lay down an invariable rule that there shall
always be two figures to the right of the decimal point. If there be
only one figure, as in the case of one-tenth of an inch, usually written
0·1, a cipher must be added, making it 0·10. Neglect of this rule causes
much inconvenience.
"In snow three methods may be adopted--it is well to try them all. 1.
Melt what is caught in the funnel, and measure that as rain. 2. Select a
place where the snow has not drifted, invert the funnel, and turning it
round, lift and melt what is enclosed. 3. Measure with a rule the
average depth of snow, and take one-twelfth as the equivalent of water.
Some observers use in snowy weather a cylinder of the same diameter as
the rain-gauge, and of considerable depth. If the wind is at all rough,
all the snow is blown out of a flat-funnelled rain-gauge."
A drainage area is almost always a district of country enclosed by a
ridge or watershed line, continuous except at the place where the waters
of the basin find an outlet. It may be, and generally is, divided by
branch ridge-lines into a number of smaller basins, each drained by its
own stream into the main stream. In order to measure the area of a
catchment basin a plan of the country is required, which either shows
the ridge-lines or gives data for finding their positions by means of
detached levels, or of contour lines.
When a catchment basin is very extensive it is advisable to measure the
smaller basins of which it consists, as the depths of rainfall in them
may be different; and sometimes, also, for the same reason, to divide
those basins into portions at different distances from the mountain
chains, where rain-clouds are chiefly formed.
The exceptional cases, in which the boundary of a drainage area is not a
ridge-line on the surface of the country, are those in which the
rain-water sinks into a porous stratum until its descent is stopped by
an impervious stratum, and in which, consequently, one boundary at least
of the drainage area depends on the figure of the impervious stratum,
being, in fact, a ridge-line on the upper surface of that stratum,
instead of on the ground, and very often marking the upper edge of the
outcrop of that stratum. If the porous stratum is partly covered by a
second impervious stratum, the nearest ridge-line on the latter stratum
to the point where the porous stratum crops out will be another boundary
of the drainage area. In order to determine a drainage area under these
circumstances it is necessary to have a geological map and sections of
the district.
The depth of rainfall in a given time varies to a great extent at
different seasons, in different years, and in different places. The
extreme limits of annual depth of rainfall in different parts of the
world may be held to be respectively nothing and 150 inches. The average
annual depth of rainfall in different parts of Britain ranges from 22
inches to 140 inches, and the least annual depth recorded in Britain is
about 15 inches.
The rainfall in different parts of a given country is, in general,
greatest in those districts which lie towards the quarter from which the
prevailing winds blow; in Great Britain, for instance, the western
districts have the most rain. Upon a given mountain ridge, however, the
reverse is the case, the greatest rainfall taking place on that side
which lies to leeward, as regards the prevailing winds. To the same
cause may be ascribed the fact that the rainfall is greater in
mountainous than in flat districts, and greater at points near high
mountain summits than at points farther from them; and the difference
due to elevation is often greater by far than that due to 100 miles
geographical distance.
The most important data respecting the depth of rainfall in a given
district, for practical purposes, are, the least annual rainfall; mean
annual rainfall; greatest annual rainfall; distribution of the rainfall
at different seasons, and especially, the longest continuous drought;
greatest flood rainfall, or continuous fall of rain in a short period.
The available rainfall of a district is that part of the total rainfall
which remains to be stored in reservoirs, or carried away by streams,
after deducting the loss through evaporation, through permanent
absorption by plants and by the ground, and other causes.
The proportion borne by the available to the total rainfall varies very
much, being affected by the rapidity of the rainfall and the compactness
or porosity of the soil, the steepness or flatness of the ground, the
nature and quantity of the vegetation upon it, the temperature and
moisture of the air, which will affect the rate of evaporation, the
existence of artificial drains, and other circumstances. The following
are examples:
Available Rainfall.
Ground. ÷
Total Rainfall.
Steep surfaces of granite, gneiss, and slate, nearly 1
Moorland and hilly pasture from ·8 to ·6
Flat cultivated country from ·5 to ·4
Chalk 0
Deep-seated springs and wells give from ·3 to ·4 of the total rainfall.
Stephenson found that for the chalk district round Watford the
evaporation was about 34 per cent., the quantity carried off by streams
23·2 per cent., leaving 42·8 per cent., which sank below the surface to
form springs. In formations less absorbent than the chalk it can be
calculated roughly, that streams carry off one-third, that another
third evaporates, and that the remaining third of the total rainfall
sinks into the earth.
Such data as the above may be used in approximately estimating the
probable available rainfall of a district; but a much more accurate and
satisfactory method is to measure the actual discharge of the streams,
and the quantity lost by evaporation, at the same time that the
rain-gauge observations are made, and so to find the actual proportion
of available to total rainfall.
The following Table gives the mean annual rainfall in various parts of
the world;--
TABLE OF RAINFALL. Collected by G. J. Symons.
-----------------------------+--------------+----------+-------
| Period | | Mean
Country and Station. | of | Latitude.| Annual
| Observations.| | Fall.
-----------------------------+--------------+----------+-------
EUROPE. | years | ° ´ | ins.
AUSTRIA--Cracow | 5 | 50 4N | 33·1
Prague | 47 | 50 5 | 15·1
Vienna | 10 | 48 12 | 19·6
BELGIUM--Brussels | 20 | 50 51 | 28·6
Ghent | 13 | 51 4 | 30·6
Louvain | 12 | 50 33 | 28·6
DENMARK--Copenhagen | 12 | 55 41 | 22·3
FRANCE--Bayonne | 10 | 43 29 | 56·2
Bordeaux | 32 | 44 50 | 32·4
Brest | 30 | 48 23 | 38·8
Dijon | 20 | 47 14 | 31·1
FRANCE--Lyons | .. | 45 46 | 37·0
Marseilles | 60 | 43 17 | 19·0
Montpelier | 51 | 43 36 | 30·3
Nice | 20 | 43 43 | 55·2
Paris | 44 | 48 50 | 22·9
Pau | 12 | 43 19 | 37·1
Rouen | 10 | 49 27 | 33·7
Toulon | .. | 43 4 | 19·7
Toulouse | 52 | 43 36 | 24·9
GREAT BRITAIN-- | | |
England, London | 40 | 51 31 | 24·0
" Manchester | 40 | 53 29 | 36·0
" Exeter | 40 | 50 44 | 33·0
" Lincoln | 40 | 53 15 | 20·0
Wales, Cardiff | 40 | 51 28 | 43·0
" Llandudno | 40 | 53 19 | 30·0
Scotland, Edinburgh | 40 | 55 57 | 24·0
" Glasgow | 40 | 55 52 | 39·0
" Aberdeen | 40 | 57 8 | 31·0
Ireland, Cork | 40 | 51 54 | 40·0
" Dublin | 40 | 53 23 | 30·0
" Galway | 40 | 53 15 | 50·0
HOLLAND--Rotterdam | .. | 51 55 | 22·0
ICELAND--Reikiavik | 5 | 64 8 | 28·0
IONIAN ISLES--Corfu | 22 | 39 37 | 42·4
ITALY--Florence | 8 | 43 46 | 35·9
Milan | 68 | 45 29 | 38·0
Naples | 8 | 40 52 | 39·3
Rome | 40 | 41 53 | 30·9
Turin | 4 | 45 5 | 38·6
Venice | 19 | 45 25 | 34·1
MALTA | .. | 35 54 | 15·0
NORWAY--Bergen | 10 | 60 24 | 84·8
Christiania | .. | 59 54 | 26·7
PORTUGAL--Coimbra | | |
(in Vale of Mondego) | 2 | 40 13 | 224·0?
Lisbon | 20 | 38 42 | 23·0
PRUSSIA--Berlin | 6 | 52 30 | 23·6
Cologne | 10 | 50 55 | 24·0
Hanover | 3 | 52 24 | 22·4
Potsdam | 10 | 52 24 | 20·3
RUSSIA--St. Petersburg | 14 | 59 56 | 16·2
Archangel | 1 | 64 32 | 14·5
Astrakhan | 4 | 46 24 | 6·1
Finland, Uleaborg | .. | 65 0 | 13·5
SICILY--Palermo | 24 | 38 8 | 22·8
SPAIN--Madrid | .. | 40 24 | 9·0
Oviedo | 1 | 43 22 | 111·1
SWEDEN--Stockholm | 8 | 59 20 | 19·7
SWITZERLAND--Geneva | 72 | 46 12 | 31·8
Great St. Bernard | 43 | 45 50 | 58·5
Lausanne | 8 | 46 30 | 38·5
| | |
ASIA. | | |
CHINA--Canton | 14 | 23 6 | 69·3
Macao | .. | 22 24 | 68·3
Pekin | 7 | 39 54 | 26·9
INDIA-- | | |
Ceylon, Colombo | .. | 6 56 | 91·7
" Kandy | .. | 7 18 | 84·0
" Adam's Peak | .. | 6 50 | 100·0
Bombay | 33 | 18 56 | 84·7
Calcutta | 20 | 22 35 | 66·9
Cherrapongee | .. | 25 16 | 610·3?
Darjeeling | .. | 27 3 | 127·3
Madras | 22 | 13 4 | 44·6
Mahabuleshwur | 15 | 17 56 | 254·0
Malabar, Tellicherry | .. | 11 44 | 116·0
Palamcotta | 5 | 8 30 | 21·1
Patna | .. | 25 40 | 36·7
Poonah | 4 | 18 30 | 23·4
MALAY--Pulo Penang | .. | 5 25 | 100·5
Singapore | .. | 1 17 | 190·0
PERSIA--Lencoran | 3 | 38 44 | 42·8
Ooroomiah | 1 | 37 28 | 21·5
RUSSIA--Barnaoul | 15 | 53 20 | 11·8
Nertchinsk | 12 | 51 18 | 17·5
Okhotsk | 2 | 59 13 | 35·2
Tiflis | 6 | 41 42 | 19·3
Tobolsk | 2 | 58 12 | 23·0
TURKEY--Palestine, Jerusalem | { 14 | 31 47 | 65·0?
| { 3 | 31 47 | 16·3
Smyrna | .. | 38 26 | 27·6
| | |
AFRICA. | | |
ABYSSINIA--Gondar | .. | 12 36 | 37·3
ALGERIA--Algiers | 10 | 36 47 | 37·0
Constantina | .. | 36 24 | 30·8
Mostaganem | 1 | 35 50 | 22·0
Oran | 2 | 35 50 | 22·1
ASCENSION | 2 | 8 8S | 11·5
CAPE COLONY--Cape Town | 20 | 33 52 | 24·3
GUINEA--Christiansborg | .. | 5 30N | 19·2
MADEIRA | 4 | 33 30 | 30·9
MAURITIUS--Port Louis | .. | 20 3S | 35·2
NATAL--Maritzburgh | .. | 29 36 | 27·6
ST. HELENA | 3 | 15 55N | 18·8
SIERRA LEONE | .. | 8 30 | 86·0
TENERIFFE | 2 | 28 28 | 22·3
| | |
NORTH AMERICA. | | |
BRITISH COLUMBIA-- | | |
New Westminster | 3 | 49 12 | 54·1
CANADA--Montreal, | | |
St. Martin's | 2 | 45 31 | 47·3
Toronto | 16 | 43 39 | 31·4
HONDURAS--Belize | 1 | 17 29 | 153·0
MEXICO--Vera Cruz | .. | 19 12 | 66·1
RUSSIAN AMERICA--Sitka | 7 | 57 3 | 89·9
UNITED STATES--Arkansas, | | |
Fort Smith | 15 | 35 23 | 42·1
California, San Francisco | 9 | 37 48 | 23·4
Nebraska, Fort Kearny | 6 | 40 38 | 28·8
New Mexico, Socorro | 2 | 34 10 | 7·9
New York, West Point | 12 | 41 23 | 46·5
Ohio, Cincinnati | 20 | 39 6 | 46·9
Pennsylvania, Philadelphia | 19 | 39 57 | 43·6
South Carolina, Charlestown| 15 | 32 46 | 48·3
Texas, Matamoras | 6 | 25 54 | 35·2
WEST INDIES--Antigua | .. | 17 3 | 39·5
Barbadoes | 10 | 13 12 | 75·0
" St. Philip | 20 | 13 13 | 56·1
Cuba, Havannah | 2 | 23 9 | 50·2
Matanzas | 1 | 23 2 | 55·3
Grenada | .. | 12 8 | 126·0
Guadaloupe, Basseterre | .. | 16 5 | 126·9
" Matonba | .. | 16 5 | 285·8
Jamaica, Caraib | .. | 18 3 | 97·0
" Kingstown | .. | 17 58 | 83·0
St. Domingo, Cape Haitien | .. | 19 43 | 127·9
" Tivoli | .. | 19 0 | 106·7
Trinidad | .. | 10 40 | 62·9
Virgin Isles, St. Thomas' | .. | 18 17 | 60·6
" Tortola | .. | 18 27 | 65·1
| | |
SOUTH AMERICA. | | |
BRAZIL--Rio Janeiro | .. | 22 54S | 58·7
S. Luis de Maranhao | .. | 3 0 | 276·0
GUYANA--Cayenne | 6 | 4 56 | 138·3
Demerara, George Town | 5 | 6 50 | 87·9
Paramaribo | .. | 6 0 | 229·2
NEW GRANADA--La Baja | 6 | 7 22 | 54·1
Marmato | 15 | 5 29 | 90·0
Santa Fé de Bogota | 6 | 4 36 | 43·8
VENEZUELA--Cumana | .. | 10 27 | 7·5
Curaçoa | .. | 12 15N | 26·6
| | |
AUSTRALIA. | | |
NEW SOUTH WALES--Bathurst | 3 | 33 24S | 22·7
Deniliquin | 2 | 35 32 | 13·8
Newcastle | 3 | 32 57 | 55·3
Port Macquarie | 12 | 31 29 | 70·8
Sydney | 6 | 33 52 | 46·2
NEW ZEALAND--Auckland | 2 | 36 50 | 31·2
Christchurch | 3 | 43 45 | 31·7
Nelson | 2 | 41 18 | 38·4
Taranaki | 2 | 39 3 | 52·7
Wellington | 2 | 41 17 | 37·8
SOUTH AUSTRALIA--Adelaide | 6 | 34 55 | 19·2
TASMANIA--Hobart Town | 12 | 42 54 | 20·3
VICTORIA--Melbourne | 6 | 37 49 | 30·9
Port Phillip | 11 | 38 30 | 29·2
WEST AUSTRALIA--Albany | .. | 35 0 | 32·1
York | 1 | 31 55 | 25·4
| | |
POLYNESIA. | | |
SOCIETY ISLANDS-- | | |
Tahiti, Papiete | 5 | 17 32 | 45·7
-----------------------------+--------------+----------+--------
DISTURBANCES OF THE STRATA.
The last question to be considered relates to the disturbances which may
have affected the strata; for whatever may be the absorbent power of the
strata, the yield of water will be more or less diminished whenever the
channels of communication have suffered break or fracture.
If the strata remained continuous and unbroken, we should merely have to
ascertain the dimensions and lithological character of the strata in
order to determine their water value. But if the strata is broken, the
interference with the subterranean transmission of water will be
proportionate to the extent of the disturbance.
Although the Tertiary formations around London have probably suffered
less from the action of disturbing forces than the strata of any other
district of the same extent in England, yet they nevertheless now
exhibit considerable alterations from their original position.
The principal change has been that which, by elevation of the sides or
depression of the centre of the district, gave the Tertiary deposits
their present trough-shaped form, assuming it not to be the result of
original deposition. If no further change had taken place we might have
expected to find an uninterrupted communication in the Lower Tertiary
strata from their northern outcrop at Hertford to their southern outcrop
at Croydon, as well as from Newbury on the west to the sea on the east;
and the entire length of 260 miles of outcrop would have contributed to
the general supply of water at the centre.
But this is far from being the case; several disturbing causes have
deranged the regularity of original structure. The principal one has
caused a low axis of elevation, or rather a line of flexure running east
and west, following nearly the course of the Thames from the Nore to
Deptford, and apparently continued thence beyond Windsor. It brings up
the chalk at Cliff, Purfleet, Woolwich, and Loampit Hill to varied but
moderate elevations above the river level. Between Lewisham and Deptford
the chalk disappears below the Tertiary series, and does not come to the
surface till we reach the neighbourhood of Windsor and Maidenhead.
There is also, probably, another line of disturbance running between
some points north and south and intersecting the first line at Deptford.
It passes apparently near Beckenham and Lewisham, and then, crossing the
Thames near Deptford, continues up a part, if not along the whole length
of the valley of the Lea towards Hoddesdon. This disturbance appears in
some places to have resulted in a fracture or a fault in the strata,
placing the beds on the east of it on a higher level than those on the
west; and at other places merely to have produced a curvature in the
strata. Prestwich states that he was unable to give its exact course,
but its effect, at all events upon the water supply of London, is
important, as, in conjunction with the first or Thames valley
disturbance, it cuts off the supplies from the whole of Kent, and
interferes most materially with the supply from Essex; for in its course
up the valley of the Lea it either brings up the Lower Tertiary strata
to the surface, as at Stratford and Bow, or else, as farther up the
valley, it raises them to within 40 or 60 feet of the surface.
The Tertiary district thus appears, on a general view, to be divided
naturally into four portions by lines running nearly north and south,
the former line passing immediately south, and the latter east of
London, which stands at the south-east corner of the north-western
division, and consequently it must not be viewed as the centre of one
large and unbroken area, so far as the Tertiary strata are concerned.