have already said that quartz-sand can wear a hole through corundum.
This leads me to express my acknowledgments to General Tilghman, who
is the inventor of the sand-Blast. [Footnote: The absorbent power,
if I may use the phrase, exerted by the industrial arts in the United
States, is forcibly illustrated by the rapid transfer of men like
Mr. Tilghman from the life of the soldier to that of the civilian.
General McClellan, now a civil engineer, whom I had the honour of
frequently meeting in New York, is a most eminent example of the same
kind. At the end of the war, indeed, a million and a half of men were
thus drawn, in an astonishingly short time, from military to civil
life.] To his spontaneous kindness I am indebted for some beautiful
illustrations of his process. In one thick plate of glass a figure
has been worked out to a depth of three eighths of an inch. A second
plate, seven eighths of an inch thick, is entirely perforated. In a
circular plate of marble, nearly half an inch thick, open work of most
intricate and elaborate description has been executed. It would
probably take many days to perform this work by any ordinary process;
with the sand-blast it was accomplished in an hour. So much for the
strength of the blast; its delicacy is illustrated by this beautiful
example of line engraving, etched on glass by means of the Blast.
This power of erosion, so strikingly displayed when sand is urged by
air, renders us better able to conceive its action when urged by
water. The erosive power of a river is vastly augmented by the solid
matter carried along with it. Sand or pebbles, caught in a river
vortex, can wear away the hardest rock potholes' and deep cylindrical
shafts being thus produced. An extraordinary instance of this kind of
erosion is to be seen in the Val Tournanche, above the village of this
name. The gorge at Handeck has been thus cut out. Such waterfalls
were once frequent in the valleys of Switzerland; for hardly any
valley is without one or more transverse barriers of resisting
material, over which the river flowing through the valley once fell as
a cataract. Near Pontresina, in the Engadin, there is such a case; a
hard gneiss being there worn away to form a gorge, through which the
river from the Morteratsch glacier rushes. The barrier of the Kirchet
above Meyringen is also a case in point. Behind it was a lake,
derived from the glacier of the Aar, and over the barrier the lake
poured its excess of water. Here the rock, being limestone, was in
part dissolved; but added to this we had the action of the sand and
gravel carried along by the water, which, on striking the rock,
chipped it away like the particles of the sand-Blast. Thus, by
solution and mechanical erosion, the great chasm of the
Finsteraarschlucht was formed. It is demonstrable that the water
which flows at the bottoms of such deep fissures once flowed at the
level of their present edges, and tumbled down the lower faces of the
barriers. Almost every valley in Switzerland furnishes examples of
this kind; the untenable hypothesis of earthquakes, once so readily
resorted to in accounting for these gorges, being now for the most
part abandoned. To produce the Canons of Western America, no other
cause is needed than the integration of effects individually
infinitesimal.
And now we come to Niagara. Soon after Europeans had taken possession
of the country, the conviction appears to have arisen that the deep
channel of the river Niagara below the falls had been excavated by the
cataract. In Mr. Bakewell's 'Introduction to Geology,' the prevalence
of this belief has been referred to. It is expressed thus by
Professor Joseph Henry in the 'Transactions of the Albany Institute:'
[Footnote: Quoted by Bakewell.] 'In viewing the position of the
falls, and the features of the country round, it is impossible not to
be impressed with the idea that this great natural raceway has been
formed by the continued action of the irresistible Niagara, and that
the falls, beginning at Lewiston, have, in the course of ages, worn
back the rocky strata to their present site.' The same view is
advocated by Sir Charles Lyell, by Mr. Hall, by M. Agassiz, by
Professor Ramsay, indeed by most of those who have inspected the
place.
A connected image of the origin and progress of the cataract is easily
obtained. Walking northward from the village of Niagara Falls by the
side of the river, we have to our left the deep and comparatively
narrow gorge, through which the Niagara flows. The bounding cliffs of
this gorge are from 300 to 350 feet high. We reach the whirlpool,
trend to the north-east, and after a little time gradually resume our
northward course. Finally, at about seven miles from the present
falls, we come to the edge of a declivity, which informs us that we
have been hitherto walking on table-land. At some hundreds of feet
below us is a comparatively level plain, which stretches to Lake
Ontario. The declivity marks the end of the precipitous gorge of the
Niagara. Here the river escapes from its steep mural boundaries, and
in a widened bed pursues its way to the lake which finally receives
its waters.
The fact that in historic times, even within the memory of man, the
fall has sensibly receded, prompts the question, How far has this
recession gone? At what point did the ledge which thus continually
creeps backwards begin its retrograde course? To minds disciplined in
such researches the answer has been, and will be--At the precipitous
declivity which crossed the Niagara from Lewiston on the American to
Queenston on the Canadian side. Over this transverse barrier the
united affluents of all the upper lakes once poured their waters, and
here the work of erosion began. The dam, moreover, was demonstrably
of sufficient height to cause the river above it to submerge Goat
Island; and this would perfectly account for the finding by Sir
Charles Lyell, Mr. Hall, and others, in the sand and gravel of the
island, the same fluviatile shells as are now found in the Niagara
River higher up. It would also account for those deposits along the
sides of the river, the discovery of which enabled Lyell, Hall, and
Ramsay to reduce to demonstration the popular belief that the Niagara
once flowed through a shallow valley.
The physics of the problem of excavation, which I made clear to my
mind before quitting Niagara, are revealed by a close inspection of
the present Horseshoe Fall. We see evidently that the greatest weight
of water bends over the very apex of the Horseshoe. In a passage in
his excellent chapter on Niagara Falls, Mr. Hall alludes to this fact.
Here we have the most copious and the most violent whirling of the
shattered liquid; here the most powerful eddies recoil against the
shale. From this portion of the fall, indeed, the spray sometimes
rises without solution of continuity to the region of clouds, becoming
gradually more attenuated, and passing finally through the condition
of true cloud into invisible vapour, which is sometimes reprecipitated
higher up. All the phenomena point distinctly to the centre of the
river as the place of greatest mechanical energy, and from the centre
the vigour of the fall gradually dies away towards the sides. The
Horseshoe form, with the concavity facing downwards, is an obvious and
necessary consequence of this action. Right along the middle of the
river the apex of the curve pushes its way backwards, cutting along
the centre a deep and comparatively narrow groove, and draining the
sides as it passes them. [Footnote: In the discourse the excavation of
the centre and drainage of the sides action was illustrated by a model
devised by my assistant, Mr. John Cottrell.] Hence the remarkable
discrepancy between the widths of the Niagara above and below the
Horseshoe. All along its course, from Lewiston Heights to its present
position, the form of the fall was probably that of a horseshoe; for
this is merely the expression of the greater depth, and consequently
greater excavating power, of the centre of the river. The gorge,
moreover, varies in width, as the depth of the centre of the ancient
river varied, being narrowest where that depth was greatest.
The vast comparative erosive energy of the Horseshoe Fall comes
strikingly into view when it and the American Fall are compared
together. The American branch of the river is cut at a right angle by
the gorge of the Niagara. Here the Horseshoe Fall was the real
excavator. It cut the rock, and formed the precipice, over which the
American Fall tumbles. But since its formation, the erosive action of
the American Fall has been almost nil, while the Horseshoe has cut its
way for 600 yards across the end of Goat Island, and is now doubling
back to excavate its channel parallel to the length of the island.
This point, which impressed me forcibly, has not, I have just learned,
escaped the acute observation of Professor Ramsay. [Footnote: His
words are: 'Where the body of water is small in the American Fall, the
edge has only receded a few yards (where most eroded) during the time
that the Canadian Fall has receded from the north corner of Goat
Island to the innermost curve of the Horseshoe Fall.'--Quarterly
Journal of Geological Society, May 1859.] The river bends; the
Horseshoe immediately accommodates itself to the bending, and will
follow implicitly the direction of the deepest water in the upper
stream. The flexures of the gorge are determined by those of the
river channel above it. Were the Niagara centre above the fall
sinuous, the gorge would obediently follow its sinuosities. Once
suggested, no doubt geographers will be able to point out many
examples of this action. The Zambesi is thought to present a great
difficulty to the erosion theory, because of the sinuosity of the
chasm below the Victoria Falls. But, assuming the basalt to be of
tolerably uniform texture, had the river been examined before the
formation of this sinuous channel, the present zigzag course of the
gorge below the fall could, I am persuaded, have been predicted, while
the sounding of the present river would enable us to predict the
course to be pursued by the erosion in the future.
But not only has the Niagara River cut the gorge; it has carried away
the chips of its own workshop. The shale, being probably crumbled, is
easily carried away. But at the base of the fall we find the huge
boulders already described, and by some means or other these are
removed down the river. The ice which fills the gorge in winter, and
which grapples with the boulders, has been regarded as the
transporting agent. Probably it is so to some extent. But erosion
acts without ceasing on the abutting points of the boulders, thus
withdrawing their support and urging them gradually down the river.
Solution also does its portion of the work. That solid matter is
carried down is proved by the difference of depth between the Niagara
River and Lake Ontario, where the river enters it. The depth falls
from 72 feet to 20 feet, in consequence of the deposition of solid
matter caused by the diminished motion of the river. [Footnote: Near
the mouth of the gorge at Queenston, the depth, according to the
Admiralty Chart, is 180 feet; well within the gorge it is 132 feet.]
The annexed highly instructive map has been reduced from one published
in Mr. Hall's 'Geology of New York.' It is based on surveys executed
in 1842, by Messrs. Gibson and Evershed. The ragged edge of the
American Fall north of Goat Island marks the amount of erosion which
it has been able to accomplish, while the Horseshoe Fall was cutting
its way southward across the end of Goat Island to its present
position. The American Fall is 168 feet high, a precipice cut down,
not by itself, but by the Horseshoe Fall. The latter in 1842 was 159
feet high, and, as shown by the map, is already turning eastward, to
excavate its gorge along the centre of the upper river. P is the apex
of the Horseshoe, and T marks the site of the Terrapin Tower, with the
promontory adjacent, round which I was conducted by Conroy. Probably
since 1842 the Horseshoe has worked back beyond the position here
assigned to it.
In conclusion, we may say a word regarding the proximate future of
Niagara. At the rate of excavation assigned to it by Sir Charles
Lyell, namely, a foot a year, five thousand years or so will carry the
Horseshoe Fall far higher than Goat Island. As the gorge recedes it
will drain, as it has hitherto done, the banks right and left of it,
thus leaving a nearly level terrace between Goat Island and the edge
of the gorge. Higher up it will totally drain the American branch of
the river; the channel of which in due time will become cultivable
land. The American Fall will then be transformed into a dry
precipice, forming a simple continuation of the cliffy boundary of the
Niagara gorge. At the place occupied by the fall at this moment we
shall have the gorge enclosing a right angle, a second whirlpool being
the consequence. To those who visit Niagara a few millenniums hence I
leave the verification of this prediction. All that can be said is,
that if the causes now in action continue to act, it will prove itself
literally true.
*****
Fig. 6.
POSTSCRIPT.
A year or so after I had quitted the United States, a man sixty years
of age, while engaged in painting one of the bridges which connect
Goat Island with the Three Sisters, slipped through the rails of the
bridge into the rapids, and was carried impetuously towards the
Horseshoe Fall. He was urged against a rock which rose above the
water, and with the grasp of desperation he clung to it. The
population of the village of Niagara Falls was soon upon the island,
and ropes were brought, but there was none to use them. In the midst
of the excitement, a tall powerful young fellow was observed making
his way silently through the crowd. He reached a rope; selected from
the bystanders a number of men, and placed one end of the rope in
their hands. The other end he fastened round himself, and choosing a
point considerably above that to which the man clung, he plunged into
the rapids. He was carried violently downwards, but he caught the
rock, secured the old painter and saved him. Newspapers from all
parts of the Union poured in upon me, describing this gallant act of
my guide Conroy.
********************
VIII. THE PARALLEL ROADS OF GLEN ROY.
[Footnote: A discourse delivered at the Royal Institution of Great
Britain on June 9, 1876.]
THE first published allusion to the Parallel Roads of Glen Roy occurs
in the appendix to the third volume of Pennant's 'Tour in Scotland,' a
work published in 1776. 'In the face of these hills,' says this
writer, 'both sides of the glen, there are three roads at small
distances from each other and directly opposite on each side. These
roads have been measured in the complete parts of them, and found to
be 26 paces of a man 5 feet 10 inches high. The two highest are
pretty near each other, about 50 yards, and the lowest double that
distance from the nearest to it. They are carried along the sides of
the glen with the utmost regularity, nearly as exact as drawn with a
line of rule and compass.'
The correct heights of the three roads of Glen Roy are respectively
1150, 1070, and 860 feet above the sea. Hence a vertical distance of
80 feet separates the two highest, while the lowest road is 210 feet
below the middle one.
These 'roads' are usually shelves or terraces formed in the yielding
drift which here covers the slopes of the mountains. They are all
sensibly horizontal and therefore parallel. Pennant accepted as
reasonable the explanation of them given by the country people in his
time. They thought that the roads 'were designed for the chase, and
that the terraces were made after the spots were cleared in lines from
wood, in order to tempt the animals into the open paths after they
were rouzed, in order that they might come within reach of the bowmen
who might conceal themselves in the woods above and below.'
In these attempts of 'the country people' we have an illustration of
that impulse to which all scientific knowledge is due--the desire to
know the causes of things; and it is a matter of surprise that in the
case of the parallel roads, with their weird appearance challenging
enquiry, this impulse did not make itself more rapidly and
energetically felt. Their remoteness may perhaps account for the fact
that until the year 1817 no systematic description of them, and no
scientific attempt at an explanation of them, appeared. In that year
Dr. MacCulloch, who was then President of the Geological Society,
presented to that Society a memoir, in which the roads were discussed,
and pronounced to be the margins of lakes once embosomed in Glen Roy.
Why there should be three roads, or why the lakes should stand at
these particular levels, was left unexplained.
To Dr. MacCulloch succeeded a man, possibly not so learned as a
geologist, but obviously fitted by nature to grapple with her facts
and to put them in their proper setting. I refer to Sir Thomas
Dick-Lauder, who presented to the Royal Society of Edinburgh, on the
2nd of March, 1818, his paper on the Parallel Roads of Glen. Roy. In
looking over the literature of this subject, which is now copious, it
is interesting to observe the differentiation of minds, and to single
out those who went by a kind of instinct to the core of the question,
from those who erred in it, or who learnedly occupied themselves with
its analogies, adjuncts, and details. There is no man, in my opinion,
connected with the history of the subject, who has shown, in relation
to it, this spirit of penetration, this force of scientific insight,
more conspicuously than Sir Thomas Dick-Lauder. Two distinct mental
processes are involved in the treatment of such a question. Firstly,
the faithful and sufficient observation of the data; and secondly,
that higher mental process in which the constructive imagination comes
into play, connecting the separate facts of observation with their
common cause, and weaving them into an organic whole. In neither of
these requirements did Sir Thomas Dick-Lauder fail.
Adjacent to Glen Roy is a valley called Glen Gluoy, along the sides of
which ran a single shelf, or terrace, formed obviously in the same
manner as the parallel roads of Glen Roy. The two shelves on the
opposing sides of the glen were at precisely the same level, and
Dick-Lauder wished to see whether, and how, they became united at the
head of the glen. He followed the shelves into the recesses of the
mountains. The bottom of the valley, as it rose, came ever nearer to
them, until finally, at the head of Glen Gluoy, he reached a col, or
watershed, of precisely the same elevation as the road which swept
round the glen.
The correct height of this col is 1170 feet above the sea; that is to
say, 20 feet above the highest road in Glen Roy.
From this col a lateral branch-valley--Glen Turrit--led down to Glen
Roy. Our explorer descended from the col to the highest road of the
latter glen, and pursued it exactly as he had pursued the road in Glen
Gluoy. For a time it belted the mountain sides at a considerable
height above the bottom of the valley; but this rose as he proceeded,
coming ever nearer to the highest shelf, until finally he reached a
col, or watershed, looking into Glen Spey, and of precisely the same
elevation as the highest road of Glen Roy.
He then dropped down to the lowest of these roads, and followed it
towards the mouth of the glen. Its elevation above the bottom of the
valley gradually increased; not because the shelf rose, but because it
remained level while the valley sloped downwards. He found this
lowest road doubling round the hills at the mouth of Glen Roy, and
running along the sides of the mountains which flank Glen Spean. He
followed it eastwards.
PARALLEL ROADS OF GLEN ROY.
After a Sketch by Sir Thomas Dick-Lauder.
The bottom of the Spean Valley, like the others, gradually rose, and
therefore gradually approached the road on the adjacent mountain-side.
He came to Loch Laggan, the surface of which rose almost to the level
of the road, and beyond the head of this lake he found, as in the
other two cases, a col, or watershed, at Makul, of exactly the same
level as the single road in Glen Spean, which, it will be remembered,
is a continuation of the lowest road in Glen Roy.
Here we have a series of facts of obvious significance as regards the
solution of this problem. The effort of the mind to form a coherent
image from such facts may be compared with the effort of the eyes to
cause the pictures of a stereoscope to coalesce. For a time we
exercise a certain strain, the object remaining vague and indistinct.
Suddenly its various parts seem to run together, the object starting
forth in clear and definite relief. Such, I take it, was the effect
of his ponderings upon the mind of Sir Thomas Dick-Lauder. His
solution was this: Taking all their features into account, he was
convinced that water only could have produced the terraces. But how
had the water been collected? He saw clearly that, supposing the
mouth of Glen Gluoy to be stopped by a barrier sufficiently high, if
the waters from the mountains flanking the glen were allowed to
collect, they would form behind the barrier a lake, the surface of
which would gradually rise until it reached the level of the col at
the head of the glen. The rising would then cease; the superfluous
water of Glen Gluoy discharging itself over the col into Glen Roy. As
long as the barrier stopping the mouth of Glen Gluoy continued high
enough, we should have in that glen a lake at the precise level of its
shelf, which lake, acting upon the loose drift of the flanking
mountains, would form the shelf revealed by observation.
So much for Glen Gluoy. But suppose the mouth of Glen Roy also
stopped by a similar barrier. Behind it also the water from the
adjacent mountains would collect. The surface of the lake thus formed
would gradually rise, until it had reached the level of the col which
divides Glen Roy from Glen Spey. Here the rising of the lake would
cease; its superabundant water being poured over the col into the
valley of the Spey. This state of things would continue as long as a
sufficiently high barrier remained at the mouth of Glen Roy. The lake
thus dammed in, with its surface at the level of the highest parallel
road, would act, as in Glen Gluoy, upon the friable drift
overspreading the mountains, and would form the highest road or
terrace of Glen Roy.
And now let us suppose the barrier to be so far removed from the mouth
of Glen Roy as to establish a connection between it and the upper part
of Glen Spean, while the lower part of the latter glen still continued
to be blocked up. Upper Glen Spean and Glen Roy would then be
occupied by a continuous lake, the level of which would obviously be
determined by the col at the head of Loch Laggan. The water in Glen
Roy would sink from the level it had previously maintained, to the
level of its new place of escape. This new lake-surface would
correspond exactly with the lowest parallel road, and it would form
that road by its action upon the drift of the adjacent mountains.
In presence of the observed facts, this solution commends itself
strongly to the scientific mind. The question next occurs, What was
the character of the assumed barrier which stopped the glens? There
are at the present moment vast masses of detritus in certain portions
of Glen Spean, and of such detritus Sir Thomas Dick-Lauder imagined
his barriers to have been formed. By some unknown convulsion, this
detritus had been heaped up. But, once given, and once granted that
it was subsequently removed in the manner indicated, the single road
of Glen Gluoy and the highest and lowest roads of Glen Roy would be
explained in a satisfactory manner.