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THE
JOURNAL OF GEOLOGY
_MAY-JUNE, 1893._
CONTENTS
On The Typical Laurentian Area of Canada 325
Melilite-Nepheline-Basalt and Nepheline-Basanite from Southern
Texas 340
Some Dynamic Phenomena Shown by the Baraboo Quartzite Ranges of
Central Wisconsin 347
The Chemical Relation of Iron and Manganese In Sedimentary Rocks 356
Some Rivers of Connecticut 371
Geological History of the Laurentian Basin 394
Editorials 408
Reviews 410
Analytical Abstracts of Current Literature 419
Acknowledgments 423
ON THE TYPICAL LAURENTIAN AREA OF CANADA.
The name Laurentian was given by Logan in 1854 to the great series
of rocks forming the Laurentides or Laurentian Mountains, a district
of mountainous country rising to the north of the River and Gulf of
St. Lawrence, and extending in an unbroken stretch along the shore of
the latter from Quebec to Labrador, a distance of nine hundred miles.
This district, with its continuation to the west as far as Lake Huron,
being situated in the Province of Quebec and the adjacent portion of
the Province of Ontario, and forming part of the main Protaxis of the
continent, is the “Original Laurentian Area” of Logan. The Laurentian
rocks are now known to extend far beyond the limits of this area to
the west and north, constituting, as they do, by far the greater part
of the Protaxis, and underlying (with subordinate patches of Huronian)
an area of somewhat over two million square miles.[1] The area above
referred to is, however, the one which was first studied and described;
it is the “Typical Laurentian area,” and to it the observations in the
present paper will be as far as possible confined.
A general exploration of the area in question, and a more detailed
study of a small part of it--the Grenville District--situated in the
counties of Argenteuil and Terrebonne in the Province of Quebec, was
carried out by Logan and his assistants in the early years of the
Canadian Geological Survey. An excellent résumé of the results of these
studies is given in the “Geology of Canada,” published in 1863, which
contains not only a good description of the general petrographical
character and arrangement of the rocks which make up the area, but
is accompanied by an atlas containing two maps illustrating this
description, one showing the general distribution of the Laurentian in
the eastern part of the Dominion, and the other its stratigraphical
relations in the smaller area above referred to.
As a result of these studies, Logan announced his belief that the
Laurentian System consisted of two great unconformable series of
sedimentary rocks, to which he gave the names Upper and Lower
Laurentian. The latter he considered to be divisible into a lower
and an upper portion, which sub-divisions he regarded as probably
conformable to one another. In the course of time these several series
came to be known as the Anorthosite or Norian Series, the Grenville
Series and the Fundamental or Ottawa Gneiss. Logan’s views may then be
represented as follows:
Anorthosite or Norian Series, Upper Laurentian.
Grenville Series, Upper portion } Lower
Fundamental or Ottawa Gneiss, Lower portion } Laurentian.
Subsequently, in the southeastern corner of the Province of Ontario,
in the district lying to the north of the eastern end of Lake Ontario,
another series of rocks was discovered--the so-called Hastings Series.
Logan supposed this to come in above the Grenville Series, while
Vennor, who subsequently examined the district, believed it to be
equivalent to the lower part of the Grenville Series already mentioned.
When these investigations were carried out, the microscope had not
as yet been seriously employed in petrographical work. The precise
composition of many of the rocks making up the several series was not
recognized, the effects produced by great dynamic action were not duly
considered, and the foliation possessed in a high degree by some and
to a certain extent by almost all these rocks was considered, in all
cases, to be a more or less obliterated survival of original bedding.
The detailed mapping in the field, accompanied by microscopical work in
the laboratory, by which alone conclusive results can be obtained in
working out the structure of complicated areas of crystalline schists,
was not carried out, in fact in many districts the construction of
detailed maps was at that time practically impossible. It is not
surprising therefore that, although excellent in the main, some of the
results arrived at have since proved to be erroneous.
It is proposed, in the present paper, to place before the readers
of this JOURNAL in as brief a manner as possible, a general account
of the several series of rocks occurring in this area, and to point
out what, in the opinion of the present writer, seems to have been
satisfactorily established concerning the stratigraphical position and
mutual relations of these ancient rocks and what still remains to be
determined by further study, and in conclusion to give a short sketch
of the evolution of this portion of the continent.
_The Fundamental Gneiss._--Exposed over very wide stretches of country
in Canada, and making up in all probability by far the larger part of
the Archean Protaxis, is the “Fundamental Gneiss,” sometimes called,
from its great development about the upper waters of the Ottawa River,
the “Ottawa Gneiss.” It is composed essentially of orthoclase gneiss,
usually reddish or greyish in color. Of this there are a number of
varieties, differing from one another in size of grain, relative
proportion of constituent minerals and in the distinctness of the
foliation or banding. It is sometimes rich in quartz, while at other
times this mineral is present in but very small amount. It is usually
poor in mica and bisilicates. Dark bands of amphibolite are not
uncommon, while basic hornblende or pyroxene gneisses occur in some
places. Other schistose rocks are rarely found. Over great areas it is
often nearly uniform in character and possesses a foliation which can
only be recognized when exposures of considerable size are examined.
On this account it is often referred to as a granitoid gneiss, a
designation, however, which by no means accurately describes it as a
whole. At a locality cited by Sir William Logan, as one where it is
typically developed, namely, Trembling Mountain in the above mentioned
Grenville Area, it consists of a fine grained reddish orthoclase
gneiss, with distinct but not very decided foliation, containing here
and there bands of orthoclase gneiss of somewhat different character,
as well as bands or layers of a dark amphibolite.
How much of this Fundamental Gneiss really consists of eruptive
material is not known. The indistinct foliation, in many cases at
any rate, is not a survival of original bedding, but is clearly due
to movements in a plastic mass. It is often possible to recognize
the existence of an indistinctly foliated gneiss intruded into
more distinctly foliated gneiss. The gneiss, in some cases, shows
excellently well-marked cataclastic structure, while in other cases
this is not distinct. The evidence accumulated goes to show that the
Fundamental Gneiss consists of a complicated series of rocks of unknown
origin, but comprising a considerable amount of material of intrusive
character.
_The Grenville Series._--In certain parts of the Laurentian area, and
notably in the Grenville district before mentioned, the Laurentian has
a decidedly different petrographical development. Orthoclase gneiss is
still the predominating rock, but it presents a much greater variety in
mineralogical composition, and is much more frequently well foliated,
often occurring in well defined bands or layers like the strata of
later formations.
Amphibolites are abundant, also hornblende schists, heavy beds of
quartzite and numerous thick bands of crystalline limestone or marble,
all these rocks being interbanded or interstratified with one another.
In the vicinity of the limestones the variety in petrographical
character is especially noticeable; garnets often occur abundantly in
the gneiss, the quartzite and the hornblende schist, as well as in
the limestone itself, beds of pure garnet rock being found in places.
Pyroxene, wollastonite and other minerals are also abundant, while the
presence of graphite disseminated through the limestones and their
associated rocks, often in such abundance as to give rise to deposits
of economic value, is of especial significance. This mineral which
is not found in the Fundamental Gneiss, occurs usually in little
disseminated scales but occasionally in veins. The limestones are
thoroughly crystalline, generally somewhat coarse in grain and often
nearly pure. They usually, however, contain grains of serpentine,
pyroxene, mica, graphite or other minerals, of which over fifty species
have been noted. They are often interstratified in thin bands with
the gneiss, in places are very impure, and may be traced for great
distances along the strike, being apparently as continuous as any other
element of the series. This development of the Laurentian is known as
the Grenville Series, and has been considered by all observers to be
above and to rest upon the Fundamental Gneiss. In it are found all
the mineral deposits of economic value--apatite, iron ore, asbestos,
etc., which occur in the Laurentian. The rocks of this series, though
generally highly inclined, over some large areas lie nearly horizontal
or are inclined at very low angles, but even in such cases they show
evidence of having been subjected to great pressure, resulting in some
cases in the horizontal disruption of certain of the beds.
The areas occupied by the Grenville series although of very
considerable extent, being known to aggregate many thousand square
miles, are probably small as compared with those underlain by the
Fundamental Gneiss. The relative distribution of the two series has not
been ascertained except in a general way in the more easily-accessible
parts of the great Archean Protaxis. The Grenville series is known to
occupy a large part of its southern margin between the city of Quebec
and the Georgian Bay, while the discovery of crystalline limestone
in the gneiss elsewhere at several widely separated points, as for
instance on the Hamilton River in Labrador, in the southern part of
Baffin Land and on the Melville Peninsula, makes it probable that other
considerable areas will, with the progress of geological exploration,
be found in the far north. Over the greater part of the Protaxis,
however, the more monotonous development of the Fundamental Gneiss
seems to prevail.
The question of the origin and mutual relations of the Fundamental
Gneiss and the Grenville series is one about which, though much has
been written but little is known. Three views may be taken on the
matter--
(1) The Fundamental Gneiss may be supposed to contain what remains of
a primitive crust, penetrated by great masses of igneous rock erupted
through it--the whole having been subjected to repeated dynamic
action.[2] The Grenville Series may be an upward continuation or
development of the Fundamental Gneiss under altered conditions, marking
in the history of the world the transition from those conditions
under which a primitive crust formed to those in which sedimentation
under the present normal conditions took place. It would seem that if
the earth originally had a crust on which the first sediments were
deposited when the temperature became sufficiently low to permit water
to condense, that the said water, at a very high temperature and under
what are to us now inconceivable conditions but little removed from
fusion, might give rise to sediments not altogether similar to those
formed by the ordinary processes of erosion at the present time. Also
that, under the unique conditions which must have prevailed at that
early time, in the formation of a crust solidification, precipitation
and sedimentation might go on to a certain extent concomitantly,
and thus no well-defined break could be detected, or would in fact
exist, between a primitive crust formed by solidification from a fused
magma and the earliest aqueous sediments or deposits. The Fundamental
Gneiss and the Grenville Series might thus, as Logan supposed, form
one practically continuous series and represent parts of the original
crust, with the first crystalline or clastic sediments deposited on it,
the whole penetrated by eruptive rocks and folded up and altered by
repeated dynamic action at subsequent periods.
The general petrographical similarity of the two series, taken in
connection with the more varied nature of the Grenville Series, its
frequent stratified character, and the presence in it of limestones and
graphite indicating an approach to modern conditions and the advent of
life, together with the difficulty of clearly separating the two series
from one another and defining their respective limits, lends support to
this view.
(2) A second view is that the Grenville Series is distinct from the
Fundamental Gneiss reposing on it unconformably and of much more recent
age; that it consists of a highly altered series of clastic origin--the
Fundamental Gneiss having possibly some such origin as that mentioned
under the last heading, or representing a much older series of still
more highly altered sediments. This is supported by the fact that
some observers have thought they could in places trace out a line of
contact between the two. But in these cases it always becomes a matter
of serious doubt whether what has been considered to represent the
Fundamental Gneiss is not really a mass of intrusive rock, in which,
by pressure or motion, a somewhat gneissic structure has been induced.
If the Fundamental Gneiss, moreover, was ever an ordinary sediment,
it must have undergone a metamorphosis so profound that no trace of
clastic origin remains, unless the generally indistinct foliation or
banding of some portions of it be considered as such. It must also be
noted in this connection that, although the rocks of the Grenville
series are more frequently possessed of a decided foliation and are
often banded, bands of different composition alternating with one
another as in ordinary sedimentary deposits, and although in this
series crystalline limestones and quartzites occur, we have as yet no
absolutely conclusive proof that even they are of sedimentary origin.
The series is thoroughly crystalline, most of its members at least show
the effect of great dynamic action, and so far as the present writer is
aware, no undoubted conglomerate or finer grained rock showing distinct
clastic structure has ever been found. In view of this fact,--although
the series is, in all probability, made up in part at least and perhaps
wholly of sedimentary material,--the proposal to separate it from the
rest of the Laurentian and class it as Algonkian or Huronian seems at
least premature.
(3) A third view which has been advanced is that the Fundamental Gneiss
is nothing more than a great mass of eruptive granite or granitic
rock which has eaten upward, and in places penetrated the Grenville
series, or perhaps absorbed it, while the Grenville series itself
represents a series of highly altered sediments of Laurentian, Huronian
or subsequent age. The enormous extent and world-wide distribution
of the Fundamental Gneiss forming as it does wherever the base of
the geological column is exposed to view, the foundation or floor on
which all subsequent rocks are seen to rest, is opposed to this view
of its origin, as is also its persistent gneissic or banded character,
although, as above mentioned, much eruptive material is undoubtedly to
be found in it.
Which of these views is correct can be ascertained only as very careful
and detailed mapping, accompanied by accurate petrographical study,
is proceeded with. In the present state of our knowledge additional
argument and discussion will not help us toward the goal, while hasty
work and generalization serves but to retard the progress of our
knowledge.
_The Anorthosite Series._--Associated with both the series of rocks
just described there are, as has been mentioned, great eruptive masses
of granite, some of which have been folded in with the gneisses, while
others evidently erupted at a much later date, show no trace of dynamic
action.
In addition to these, basic eruptive rocks belonging to the gabbro
family occur in certain districts, sometimes in the form of
comparatively insignificant masses, but elsewhere underlying great
tracts of country. One on the upper waters of the Saguenay has an area
of no less than 5,800 square miles. These usually consist of a variety
of gabbro in which the magnesia-iron constituents are present in very
small amount, being in many cases entirely wanting, so that the rock
consists practically of pure plagioclase feldspar. These rocks were
called _anorthosites_ by Hunt, in the early reports of the Canadian
Geological Survey, on account of the great preponderance in them of
“Anorthose,” a general name given many years ago by Delesse to the
triclinic feldspars, as distinguished from “Orthose,” or orthoclase
feldspar, and thus equivalent to the term plagioclase now in general
use, but having no connection with anorthite, a variety of plagioclase
which is seldom present. After a careful study of these rocks, both
in the field and the laboratory, it is believed that this name should
be retained for this well-marked member of the gabbro family, which,
though not a common rock elsewhere, has an enormous distribution in the
Laurentian of Canada.
If an olivine gabbro be regarded as the central member, so to speak,
of the gabbro family, the replacement of the monoclinic by rhombic
pyroxene will give rise to an olivine norite. A gradual diminution in
the amount of plagioclase will give rise to a peridotite or gabbro
pyroxenite, a diminution in the amount of pyroxene to a troktolite or
plagioclase-olivine rock, while a diminution in the amount of olivine
and pyroxene will give rise to an anorthosite, which variety forms
the greater part of the intrusive masses in question. The gradual
passage of one variety into another can be distinctly traced in many
localities in the anorthosite masses. These anorthosites are in some
places massive, but very frequently show a distinct foliation, often
very perfect. In some places they occur interbanded with the gneiss and
crystalline limestone, while elsewhere they cut directly across the
strike of these rocks. The interbanded anorthosite, together with the
gneiss and limestone associated with it, was supposed by Logan to form
a distinct sedimentary series, to which the name “Upper Laurentian,”
or “Norian,” was given, because the discovery that elsewhere the
anorthosite runs across the strike of the gneiss was supposed to
indicate that this series covered up and unconformably overlay
the Grenville series, the igneous and intrusive character of the
anorthosite not being recognized on account of its frequently foliated
structure. It is now known that these anorthosites do not constitute an
independent formation, but are igneous rocks which occur, cutting both
the Grenville series and the Fundamental gneiss. They have, however,
in many cases been intruded before the cessation of the great dynamic
movements to which the Laurentian was subjected in pre-Cambrian times,
and thus frequently taking a line of least resistance and having been
intruded between the bands or strata of the Grenville series, have had
a foliation induced in them parallel to that of the gneiss, while in
other cases where they are more or less massive, they cut across the
strike of the latter.
In many cases the anorthosites which exhibit a perfect foliation may be
traced step by step into the massive variety, the gradual development
of a foliated structure in the rock being accompanied by a progressive
granulation of the constituents, most beautifully seen under the
microscope. The change, however, differs from any hitherto described
in that it is purely mechanical. There are no lines of shearing with
accompanying chemical changes, but a breaking up of the constituents
throughout the whole mass, though in some places this has progressed
much further than in others, unaccompanied by any alteration of augite
or hypersthene to hornblende, or of plagioclase to saussurite, these
minerals, though prone to such alteration under pressure remaining
quite unaltered, suffering merely a granulation with the arrangement
of the granulated material in parallel strings. This process can be
observed in all its stages, and there is reason to believe that it
has been brought about by pressure acting on the rocks when they were
deeply buried and very hot.[3] The anorthosite areas, of which there
are about a dozen of great extent with many of smaller size, are
distributed along the south and southeastern edge of the main Archean
Protaxis from Labrador to Lake Champlain, occupying in this way a
position similar to that of volcanoes along the edge of our present
continents. Curiously enough precisely similar occurrences of this
anorthosite have been found in connection with similar gneissic rocks,
supposed to be of Archean age, in Russia, Norway and Egypt. These
anorthosite rocks being intrusive, may be left out of consideration in
endeavoring to work out the succession of the Archean in this great
area.
The whole Laurentian system, including the anorthosites, is in many
places cut by numerous dykes of large size, which can often be traced
for great distances. These are of several kinds, the principal series
consisting of a beautiful fresh diabase often holding quartz in
considerable amount in micro-pegmatitic intergrowths with plagioclase.
Other sets of dykes and eruptive masses consisting of augite and mica
syenites, quartz-porphyries and other rocks are also known to occur but
have not as yet been carefully studied.
_The Hastings Series._--The stratigraphical relations of the Hastings
series have not as yet been satisfactorily determined. The rocks
constituting the series differ widely in petrographical character from
those of the Fundamental Gneiss and the Grenville series, both of
which are supposed to occur in its immediate neighborhood. The series
consists largely of calc-schists, mica-schists, dolomites, slates and
conglomerates, thus containing much material of undoubtedly clastic
origin. It has moreover a very local development, being confined, so
far as at present known, to one small corner of the area, as has been
mentioned. It was by Logan supposed to come in above the Grenville
series, while Vennor who subsequently examined the district, believed
it to be equivalent to the lower part of this series. That we have in
the Hastings series a comparatively unaltered part of the Grenville
series, made up largely of rocks whose origin is easily recognized,
would be a most important fact if established, and would, of course,
afford a key to the whole question of the origin of the latter. This
is a conclusion, however, which cannot be accepted until supported by
very clear and decisive evidence, especially as the stratigraphy of the
Hastings district is very complicated, the several series represented
in it being much folded and penetrated by great masses of eruptive
rocks. The whole district has also been subject to great dynamic
action, some of the pebbles in the conglomerates of the Hastings series
being distorted in a most remarkable manner. This series may prove
to be merely an outlying area of Huronian rocks folded in with the
Laurentian, and until the district has been studied in detail its
stratigraphical position must remain a matter of conjecture.
Leaving the Hastings series out of consideration therefore, we have
in this Original and Typical Laurentian area two developments of the
Laurentian, generally considered as constituting two series, namely the
Grenville or Upper series,
Fundamental, Ottawa, or Lower Gneiss.
_The Evolution of the Area._--In endeavoring to outline the main
events in the evolution of this area it will be necessary to extend
the limits of our observation somewhat and seek for evidence bearing
on the question in other parts of the Protaxis, where we meet with
developments of Huronian and various earlier Paleozoic strata not found
in the typical area itself.
From the highly contorted condition of the Laurentian rocks of this
area as well as from the abundant evidences of dynamic action which
they present both in the field and under the microscope, it is evident
that they have been subjected to great orographic forces, which in
very early times threw them up into mountain ranges, probably of
great height. Some of the associated eruptive rocks were intruded
before these movements began, or while they were in progress and have
accordingly been influenced by them, while others, having been intruded
later, have not been affected.
How high these mountains rose cannot of course be determined. Bell
states that some of the mountains on the Labrador coast now rise to a
height of from 5,000 to 6,000 feet, while Lieber has estimated that on
the coast of Northern Labrador they rise to a height of from 6,000 to
10,000 feet. Along the southern part of the Protaxis, where the country
is much lower, notwithstanding the enormous subaerial denudation and
glaciation which the area has repeatedly undergone, there are many
points still rising from 2,500 to 3,500 feet above sea level, while
Logan estimated that the average elevation is from 1,500 to 1,600 feet.
In the Adirondacks, which are but an outlying portion of this area,
there are elevations of over 5,400 feet. The high elevations attained
by these rocks in portions of the Protaxis in the north may, of course,
be due to differential elevation, but immediately along the southern
edge of the area there can have been but little differential change of
level as compared with the flat-lying Potsdam strata which border it
and lie but little above the present sea level. Further evidence of
the original height or continued uprising of the area is afforded by
the fact that all the material of which the North American continent
was built up (with the possible exception of some of the limestones)
was derived originally from the Archean Protaxis of the continent, a
considerable proportion of this at least coming from the main Protaxis
of which this typical Laurentian area forms a part. We must conclude
therefore that in early Cambrian or pre-Cambrian times, in portions of
the Protaxis at least, the Laurentian mountains rose several hundred
and possibly in places several thousand feet above the sea level.
The intrusion of the granites and anorthosites as well as the folding
of the whole system of rocks took place before Upper Cambrian times.
The whole series was moreover without doubt at that time in the
“metamorphic” condition in which we now find it, for along the margin
of the area the Potsdam sandstone rests in flat undisturbed beds on the
deeply eroded remnants of these old mountains, its basal beds often
consisting of a conglomerate with pebbles of the underlying gneissic
rocks. These Cambrian strata cover up the gneisses, granites and
anorthosites alike and are evidently of much more recent age, being
separated from the Laurentian by the long interval occupied in the
upheaval and erosion of the Laurentian area.
How long before Upper Cambrian times this folding and erosion took
place cannot be determined from a study of this area, but further west
along the edge of the Protaxis in the Lake Superior district we find
that the Keweenawan, Nipigon and Animikie Series also repose in flat
undisturbed beds on the eroded remnants of a series of crystalline
rocks which have the petrographical character of the Fundamental
Gneiss. This makes it at least very probable that in this eastern area
also the erosion took place in pre-Cambrian times.
It is a very remarkable fact that the roche moutonné character
possessed by these eroded Laurentian rocks and which is usually
attributed to the glaciation which they underwent in Pleistocene
times, was really impressed upon them in the first instance in these
pre-Cambrian times, for all along the edge of the nucleus from Lake
Superior to the Saguenay, the Paleozoic strata, often in little
patches, can be seen to overlie and cover up a mammillated and roche
moutonné surface showing no traces of decay and similar to that exposed
over the uncovered part of the area. The conclusion therefore seems
inevitable that not only were these Laurentian rocks sharply folded
and subjected to enormous erosion, but that they had given to them in
pre-Cambrian times their peculiar hummocky contours so suggestive of
ice action.[4] The pre-Paleozoic surface of the Fundamental Gneiss of
Scotland, as Sir Archibald Geikie has shown, also presents the same
hummocky character.[5] On this surface the Upper Huronian, Cambrian,
and later Paleozoic rocks were deposited.
To what extent the seas of Cambrian, Silurian and Devonian times passed
over this area cannot be determined with certainty. A great series of
rocks referred to by Dr. G. M. Dawson as probably of Lower Cambrian
age and analogous in character to the Keweenawan and Animikie series
occur overlying the Laurentian in many parts of the Protaxis, not only
along its margin, but as outliers at many places in the interior. It
occurs extensively developed about the Arctic Ocean and about Hudson’s
Bay, and a large area of rocks referred to the same age also occur near
the height of land about Lake Mistassini. “Throughout the whole of
the vast northern part of the continent this characteristic Cambrian
formation, composed largely of volcanic rocks, apparently occupies the
same unconformable position with regard to the underlying Laurentian
and Huronian systems. Its present remnants serve to indicate the
position of some of the earliest geological basins, which from the
attitude of the rocks appear to have undergone comparatively little
disturbance. Its extent entitles it to be recognized as one of the
most important geological features of North America.”[6] It would,
therefore, seem that in Cambrian times a not inconsiderable part of
the Archean Nucleus was under water. Outliers of Cambro-Silurian age
are also found at several points lying well within the margin of
the Nucleus, as for instance in the Ottawa River about Pembroke at
a distance of fifty miles, and at Lake St. John at the head of the
Saguenay River at a distance of one hundred and thirty miles from its
present limit. There is reason to believe that a similar outlier exists
in the interior of the northern part of the Peninsula of Labrador, so
that the Lower Paleozoic sea must also have covered considerable areas
in the eastern half of the Protaxis, where now nothing but Laurentian
is to be seen. In that portion of the Protaxis lying to the west of
Hudson’s Bay strata of Cambro-Silurian and Devonian age extend up from
the basin of Hudson’s Bay on the east and from the great plains on the
west far over the Laurentian Plateau and probably, according to Dr.
Dawson, originally inosculated. Strata of Upper Silurian and Devonian
age are not known to exist in the eastern half of the Protaxis, of
which the typical Laurentian area forms part, with the exception of a
small outlier of Niagara age on Lake Temiscamangue at the head waters
of the Ottawa--neither do any other deposits of later age occur with
the exception of the Glacial Drift. What evidence there is, therefore,
would rather indicate that the area, during late Paleozoic, Mesozoic
and earlier Tertiary times, was out of water. If so, it must have
undergone during this great lapse of ages a process of deep seated
decay and denudation, culminating in the extensive glaciation to which
it was subjected in Pleistocene times.
During this latter period the whole area was exposed to ice action,
with the exception of the highest part of the Nucleus--the mountains
of the Labrador coast--which, except toward the base, are still
“softened, eroded and deeply decayed.”[7] This extensive denudation
served to remove all but mere remnants of any Paleozoic strata
originally deposited on the Archean of this area, while the deep decay
of the Archean rocks themselves would account for the immense numbers
of gneiss bowlders in the drift, which in all probability are but
smoothed cores of “bowlders of decomposition.” That an immense amount
of material was removed from the surface of the area during the glacial
age is shown by the immense quantities of Archean material which occurs
scattered over the surface of the Nucleus itself, as well as in the
drift to the south. The glaciation, with the depression and uplift
which succeeded it, was the last episode in the evolution of this
“original” Laurentian area and one which impressed upon it its present
surface characters and type of landscape.
It is now an immense uneven plateau, comparatively slightly accentuated
except along the Labrador coast. The surface is covered with glaciated
hills and bosses of rock with rounded, mammilated, flowing contours
interspersed with drift covered flats and studded with thousands upon
thousands of lakes great and small. A country which in the far north is
often bleak and desolate, but to the south, where it is covered with
luxuriant forest, is often of great beauty, especially when clothed
with the brilliant foliage of autumn. Even now, however, it is passing
into a further stage of its history, the smooth or polished glaciated
surfaces are becoming roughened by decay, the softer gneissic and
limestone strata are again commencing to crumble into soil, and a new
epoch has been inaugurated in which the marks of the ice age are being
gradually effaced.
FRANK D. ADAMS.
MCGILL UNIVERSITY.
FOOTNOTES
[1] Accepting the distribution of the Laurentian in the far north,
given by Dr. G. M. Dawson, as correct, the area is 2,001,250
square miles. This does not include the outlying and separated
areas occurring in Newfoundland, New York State and Michigan.
[2] See also, The Geological History of the North Atlantic, by Sir
William Dawson, Presidential Address, B. A. A. S., 1886.
[3] See FRANK D. ADAMS--_“Ueber das Norian oder Ober-Laurentian
von Canada,” Neues Jahrbuch für Mineralogie, etc., Beilageband
VIII., 1893._
[4] A. C. LAWSON.--“Notes on the Pre-Palaeozoic surface of the
Archean Terranes of Canada.” Bulletin of the Geological Society
of America. Vol. 1, 1890.
[5] “A Fragment of Primeval Europe.” Nature, August 26, 1888.
[6] G. M. DAWSON.--“Notes to accompany a geological map of the
northern portion of the Dominion of Canada.” Report of the
Geological Survey of Canada, 1886. p. 9, R.
[7] ROBERT BELL.--“Observations on the Geology etc., of the
Labrador Coast, Hudson’s Strait and Bay.” Report of the
Geological Survey of Canada. 1882–3–4, p. 14, DD.
MELILITE-NEPHELINE-BASALT AND NEPHELINE-BASANITE FROM SOUTHERN TEXAS.
These basaltic rocks were collected by Professor Dumble and Mr.
Taff, in Uvalde County, southern Texas. On the geological map of the
United States, compiled by C. H. Hitchcock, 1886, there are two of
the localities marked near the boundary of the Cretaceous and earlier
Tertiary formation, between 99° and 100° longitude, and on the 29th
degree of latitude. According to the statement of Professor Dumble,
one part of the rocks appears in dikes in the upper portion of the
lower Cretaceous formation, while the other forms hills and buttes.
Upon microscopical examination it is evident that the specimens
collected belong to two different groups of rocks. The microscope shows
that those occurring in dikes consist of typical melilite-bearing
nepheline-basalt, while those making up hills and buttes are
nepheline-basanites tending toward phonolites in composition.
The melilite-nepheline-basalts have a typical basaltic appearance. In a
dense black groundmass, the only phenocrysts seen by the naked eye are
numerous olivines. Under the microscope there appear in addition to the
olivine the following minerals: augite, nepheline, melilite, magnetite
and perovskite. As to the proportion of nepheline and melilite, it can
be said, that in nearly all the specimens examined, the two minerals
are found in about the same amount. For this reason these rocks can
be placed under the head of nepheline-basalt as well as under that
of melilite-basalt, or they may be called melilite-nepheline-basalt.
Only one of the specimens is entirely free from melilite. Feldspar is
wholly wanting. All of the specimens are in a very fresh condition, and
even the melilite shows only slight indications of decomposition. The
specimen free from melilite corresponds in structure and composition
with the other specimens, except for the absence of melilite and
perovskite, and so they may be described together.
All the rocks are porphyritic, since they bear large phenocrysts of
olivine. Under the microscope the olivine is colorless and transparent,
and only shows indications of serpentinization along the edges and
fissures. It contains rounded inclusions of glass, abundant in some
sections, besides octahedrons of magnetite, and others that are
transparent with a brownish violet color. Whether the latter are a
mineral of the spinel-group or belong to perovskite, with which they
accord in color, could not be decided.
Augite occurs in only one generation; phenocrysts of augite are
wanting. In the rather coarse-grained groundmass, it becomes the most
abundant constituent. The mineral shows a grayish-brown color, common
to basaltic augite, sometimes with a tint of violet. It generally forms
well-shaped crystals, rarely irregular grains, and bears inclusions of
magnetite and glass.
Melilite occurs in the groundmass in large and well-shaped crystals,
its dimensions never becoming as small as those of many of the augite
crystals. They may be designated as micro-porphyritical phenocrysts.
Cross sections parallel to (001) reach a diameter of 0.5 mm. The
shape of the melilite is the common one, tabular parallel to (001).
The diameter of the tables generally exceeds their thickness from
four to six times. Sections parallel to the prism-zone, therefore,
are lath-shaped and the vertical axis lies perpendicular to their
length; the axis of greatest elasticity coincides with the vertical
axis. Between crossed nicols these sections show the particular blue
interference colors characteristic of melilite and zoisite. Sections
perpendicular to the prism-zone are eight-sided by reason of the planes
(110) and (100), but frequently the outlines are rounded. In some
of the sections examined the melilite incloses minute opaque grains
arranged zonally, which present very sharply the prismatic outlines of
their host. Besides the two prismatic faces above mentioned, there is
also a ditetragonal prism, the angle of which upon the adjoining faces
of (110) and (100) was found to be nearly equal, 20°-22°. According
to this measurement the prism must have approximately the position of
(940); the angle of the latter upon (110) is 21° 2´, the angle upon
(100) = 23° 58´. A particular phenomenon in the growth of the melilite
is the fact that the base does not generally present an even plane, but
shows a conical depression. The shape of the lath-shaped sections then
resembles the profile of a biconcave lens. Sections parallel to the
base are isotropic between crossed nicols and show, when they are not
too thin, an indistinct dark cross in convergent light. The cleavage
parallel to (001), the cross-fibration of the lath-shaped sections
and the occurrence of the spear-shaped and peg-shaped inclusions
arranged parallel to the _c_ axis (the so-called _Pflockstruktur_)
are very distinct. Inclusions of pyroxene, magnetite and glass are
common; as already mentioned, these inclusions are generally arranged
in zones. In sections parallel to (001) they fill the central parts
of their host, and often make up two or three concentric zones. These
sections closely resemble leucite because of their rounded shape,
the arrangement of the inclusions and the lack of double refraction.
Melilite becomes nearly colorless and transparent, but in comparing it
with the white, colorless nepheline, it shows a feeble yellow tint.
Decomposition has taken place to only a small extent; it begins along
the cross-fibration, and greenish-yellow alteration-products result,
the fibres of which are perpendicular to the length of the lath-shaped
sections.
Nepheline is always fresh, colorless and transparent; it rarely
exhibits a regular shape, but generally forms an aggregate of irregular
grains, cementing the other components; it is evidently the latest
formed mineral in the rock.
There is abundant magnetite besides perovskite, the common associate of
melilite, which occurs in small octahedrons and irregular grains. The
perovskite becomes transparent with a brownish-violet color, and shows
in some sections a feeble, abnormal double refraction. There appears
to be no isotropic base in the normal rock, but if any is present, it
must be in a very small amount. There are coarser grained spots in
the rock, which are rich in a partly chloritized base, and in which
nepheline occurs in well-shaped crystals.
The second group of rocks, as already mentioned, falls under the
head of nepheline-basanite poor in olivine. And since the specimens
bear sanidine phenocrysts beside plagioclase, it forms a transition
to phonolite. The rock-specimens have a more andesitic than basaltic
appearance. Numerous phenocrysts of hornblende and augite are imbedded
in the dense bluish-gray groundmass. The next most abundant mineral is
nepheline in the form of phenocrysts, in part well-shaped crystals,
in part rounded, the largest of which are 0.5 cm. in diameter. The
nepheline differs from the feldspar in having a grayish color and
greasy lustre. Phenocrysts of feldspar and crystals of olivine are
scarce. Beside these components, the rocks contain apatite, some
titanite and iron ores. Under the microscope olivine is seen to be
scarce. It is fresh and shows the normal properties. It contains minute
octahedrons of picotite and in some sections abundant inclusions of a
liquid with moving bubbles.
The amphibole mineral is a typical basaltic hornblende. It becomes
transparent with a dark reddish-brown color and exhibits a strong
pleochroism according to the following scheme:
=a=, brownish yellow, =b= and =c= dark reddish brown. Absorption,
=c > b > a=.
The angle of extinction was examined in sections cut approximately
parallel to the clinopinacoid (010) and was determined to be very
small. This fact and the dark reddish-brown color are in all
probability due to a high amount of Fe₂O₃. The dependence of the angle
of extinction upon the amount of Fe₂O₃ in minerals of the amphibole
group has been recently established by Schneider and Belowsky. The
basaltic hornblende shows the well-known dark borders produced by
reabsorption by the magma in an early stage of consolidation. In
many cases nothing of the original mineral is preserved; the whole
hornblende is replaced by a fine grained aggregate of pyroxene and
magnetite, presenting clearly the outlines of the absorbed mineral.
The group of pyroxenic minerals is represented by two monoclinic
augites. One of them exhibits a violet-gray color in thin section and
belongs to the basaltic augites; the other one becomes transparent
with a dark green color. Both form numerous phenocrysts, but the first
occurs somewhat more frequently. They occur as single crystals and are
also grown together in a zonal manner, the green one always forming the
center, the gray one the outer parts of the crystals. Hence the gray
augite is the younger. The pyroxene in the groundmass shows the same
color and properties. The pleochroism of the two minerals is as follows:
Gray augite. Green augite.
=a= Brownish-yellow Light yellowish-green
=b= } Dark gray-green
=c= } Violet-gray Dark green.
The angle of extinction, _c_: =c=, is large and, as may be seen in the
zonal crystals, it is somewhat larger in the gray pyroxene than in the
green. The extinction in sections cut approximately parallel to (010)
has been observed to be about 47 degrees (gray augite) and 41 degrees
(green augite). The two pyroxenes show in addition to the cleavage
parallel to (110) another but less distinct one parallel to (010).
Inclusions of magnetite, apatite and glass are common.
Phenocrysts of feldspar are scarce. In part they show the polysynthetic
twinning lamination of plagioclase; in part the latter is wanting
and one of the latter feldspars, which was isolated and examined for
specific gravity and optical properties, was found to be sanidine.
Phenocrysts of nepheline are more frequent than those of feldspar.
The mineral appears partly in the form of short-prismatic crystals,
partly in rounded grains. It presents distinct cleavage, parallel to
(1010) and to (0001), and the usually observed optical properties.
Isolated grains are decomposed by hydrochloric acid with the separation
of gelatinous silica; the resulting solution when evaporated gives
numerous cubes of NaCl. Inclusions are scarce; there are fluid cavities
with moving bubbles, generally arranged in rows, besides some pyroxene
crystals.
Apatite forms short and stout crystals always filled with inclusions
of liquids. The opaque ore grains, judging by their ready solubility,
belong to magnetite. The groundmass of these rocks consists essentially
of pyroxene in well-shaped prisms, lath-shaped feldspar, without
twinning lamination or in single twins according to the Carlsbad law
and nepheline. The feldspar of the groundmass in all probability is
mostly sanidine. Nepheline is abundant and occurs in well-shaped
crystals. Small patches of a colorless base occur between the
crystalline components.
The structure of the rocks is hypocrystalline-porphyritic on account
of the occurrence of an isotropic base and the repetition of the
crystallization of pyroxene, nepheline and feldspar. Although the
specimens by their whole habit and structure belong under the head
of nepheline-basanite poor in olivine, the presence of sanidine
as phenocrysts causes them to form a transition to the group of
phonolites. Unfortunately, analyses of these rocks have not yet been
made.
A microscopical examination of the basaltic rock from Pilot Knob, near
Austin, Travis County, was made for the purpose of comparison with the
rocks from southern Texas just described. The rock was found to be a
nepheline-basalt porphyritic with numerous phenocrysts of olivine.
The fine grained groundmass consists essentially of augite-crystals
cemented by non-individualized nepheline in very small amount.
A. OSANN.
SOME DYNAMIC PHENOMENA SHOWN BY THE BARABOO QUARTZITE RANGES OF CENTRAL
WISCONSIN.