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ART. XXX.-A Point in the Geology of Western Vermont;
by the Rev. J. B. PERRY.
PROPOSING to touch briefly, in a series of short articles, on a few prominent features in the geological structure of Western Vermont, as my first point I send for insertion in the Journal of Science, an extract from one of a course of University Lectures, which I am now delivering in the Museum of Comparative Zoology, at Harvard College, on the Geology of the Basin of Lake Champlain.
There is a series of sedimentary rocks in Western Vermont, which were many years ago described by Dr. Emmons, and by him termed Taconic. They lie, for the most part, on the east of easily-recognized Lower Silurian formations lining the shore of the Lake, in the southern part of the Champlain Basin, and themselves constitute its border in the more northerly portions of the State. The beds composing the eastern limits of this range of rocks may be found situated, to a large extent, along the base of the Green Mountains. The width of the entire range is from eight or ten to some fifteen or twenty miles, it varying somewhat in different parts of the Basin. These rocks seem to constitute several distinct groups or divisions. The series, which is apparently the oldest, lies on the east, and consists, first, of Talcose or Talcoid Slates, accompanied by Quartzites and Conglomerates ;. next, of calcareous beds known as Stockbridge Limestone ; and, finally, of Talcose or Talcoid Slates. The second series comprises at least two main sections. Starting from the west we have, first, the Black Slates, or the Swanton Group, which includes thin beds of sandstone and limestone. Proceeding eastward, in apparently ascending order, we encounter the Brown Slates, or the Georgia Group, which also contains thin beds of interstratified sandstone and limestone, and is capped on the east by a conglomerate. Along the western portion of this series there is a band of Red Sandstone running northerly and southerly which has come during the past few years to be regarded with certainty as of Potsdam age.
Now, these formations, according to a view which has been very widely prevalent, are made up of Lower Silurian rocks, which have been largely affected by heat, their structure having been more or less effaced, and their organic remains either obliterated, or, to a considerable extent, obscured. We are, accordingly, invited to look at the formations in question, for a few moments, that we may see how far this theory is sustained by facts.
It may be remarked, in a preliminary way, that traces of metamorphic action are by no means so often met with in these rocks, as the hypothesis implies. That some portions of these beds have undergone changes, under the agency of heat both dry and moist, in connection with pressure, galvanism, and the like, is undoubtedly the fact. But these instances of metamorphism are local, and by no means such as to obscure the character of the whole series of rocks, or utterly destroy the fossil remains.
With this preliminary remark we may proceed to examine the thickness of these formations.
Doing this, we shall be able to see how they correspond, in this respect, with the Champlain or Lower Silurian Series. Beginning with the inferior members of the Taconic we find the Talcoid Slates with Quartzites and Conglomerates to be, according to President Hitchcock, 1,200 feet in thickness. It should be added that the pebbles which go to make up the Conglomerate consist of Gneiss, of Schist, and fragments of other rocks, which, perhaps, belong to a pre-sedimentary age. The thickness of the Stockbridge Limestone may be well estimated in Danby Mountain, more recently known as Mount Eolus. These calcareous beds, varying little in that locality from a horizontal position, are overlaid by Lower Taconic Slate, and so situated as to preclude all probability of repetition. The thickness of this single mass of limestone is between 1,900 and 2,000 feet. The overlying Talcoid Slate may be better seen in a single mass, and its amount more correctly estimated, in Berkshire county, Massachusetts, than in Vermont. Graylock is made up of immense beds of this slate, overlying the Stockbridge Limestone, and having the successive layers only slightly inclined. Their entire thickness is 2,000 feet. We thus have more than 5,000 feet of rocks in the Lower Taconic Series.
Advancing to the next main division, we find the Black Slates with interstratified beds of Sandstone and Limestone exhibited in Swanton in great force. They dip from 30° to 90° toward the east, show no indications of any repetitions in the beds, and as thus lying, their upturned edges occupy about five miles in an east and west direction. I fail to see how they can be regarded as less than 10,000 to 15,000 feet in thickness. Frequent opportunities to study this series of rocks in various different localities have confirmed my convictions that this estimate is none too large. The Georgia Group, in other words, the whole series of Brown Slates with included Sandstones and Limestones, evidently succeeding the Swanton Group, and as well extending to the Lower Taconic, as overlying its western
limits, has been estimated by Dr. Hitchcock, and others as having a thickness of not less than 10,000 feet. We accordingly have for the thickness of the second great division of the Primordial rocks from 20,000 to 25,000 feet, and for that of the two great divisions thus far considered from 25,000 to 30,000 feet.
Leaving the Potsdam Sandstone out of view for the present, we may notice the thickness of the succeeding beds in their typical localities. That of the Calciferous Sandrock is about 300 feet; that of the Chazy Limestone is nearly the same; while that of the Birdseye is only 30 feet, and that of the La Mott, or Black River, 15 feet. The Trenton Limestone is some 400 feet thick, the Utica Slate about 75 feet, while the Lorrain Shales may reach 700 feet.* This estimate gives us less than 2,000 feet as the aggregate thickness of all these rocks.
We have now only to place the immense force of the Taconic rocks in parallelism with the thickness of the Lower Silurian formations, to see at once the striking disparity. Would we also take them up, bed by bed, and look at them in contrast, we should discover how utterly impossible it is to bring them into coördination. But time fails me now to enter into the details of any such comparison,
We may next notice the relation of the Potsdam Sandstone of Vermont to the formations under consideration.
A band of Red Sandstone runs northerly and southerly through most of Western Vermont, and may be seen in places resting on the Black or Swanton Slates. Portions of this rock were long ago described by Dr. Emmons as Potsdam Sandstone. Most geologists, however, regarded it as of Medina age, and the underlying argillaceous beds as Utica Slate and Lorrain Shale. As it is now clearly made out, by evidence which need not be here repeated, that the sandstone in question belongs to the Potsdam period, the view entertained by many in regard to the subjacent slate falls to the ground. That there may be no doubt on this matter, and as some still persist in asserting that the Black Slates are Hudson river, the Potsdam beds having been shoved, or folded over upon them, I will refer to some evidence having a bearing on the subject, and calculated,
* These figures, as should be evident, are to be regarded only as a rough ap. proximation, which may serve as a basis of comparison. I am, of course, well aware that a greater thickness has been ascribed to some of these beds, especially during the past few years. For instance, the Utica and Lorrain formations are represented as existing in immense force in Western Vermont. But this is done, by counting beds in the Hudson River formations, which really belong as I shall endeavor to indicate, to another age.
as I think when it is understood, to put the question to rest forever.
There are several localities at which the Potsdam Sandstone may be observed resting on the Black Slate very nearly as it was originally deposited. No single locality gives all the evidence ; each that I propose to mention furnishes much that is important; while a great many outcrops need to be examined, in order that the relations of the rocks may be seen in all their bearings. I will now cite in particular the southwest shore of Shelburne Bay; also, Lone Rock Point in Burlington, and the northwest side of Snake Mountain, in Addison. In some places at these localities the upper part of the slate shows that denudation took place before the deposition of the overlying sandstone, and thus evinces a want of conformity in the order of succession. Again, there are many instances serving to prove that the slate was unconsolidated at the time the Potsdam beds were laid down, and that they (and not strata of some other period) were actually deposited upon the slate, which now lies beneath them; for the upper layers of the slate contain imbedded in themselves numerous fragments of the overlying Potsdam. Moreover, the lower portion of the arenaceous deposit is exactly fitted into all the surface cavities and depressions of the slate ; so exactly fitted as to conform with them entirely, as putty conforms with the irregularities of the object upon which it is thrown down; a thing which could not have occurred had not the sandstone been laid down upon the slates very nearly as we now find them. It thus appears, in the clearest manner, that the two formations occupy to-day substantially their original position, and exhibit in the main their true relations to each other. And this leads me to remark once more, that there are places in which no break occurs between the two formations, showing that there has been no movement in such instances of the Potsdam upon the underlying slate. In other places, however, owing to the unequal pressure exerted at different points, and thus to the inequalities in the uplifting of the formations, one part being more elevated than another, there is a rupture. In some cases this may be seen a few feet below the summit of the slate, while the upper part of the slate still cleaves to the overlying arenaceous bed. There are other instances in which the break runs above the base of the sandstone, its lower portion, meanwhile continuing to adhere to the subjacent slate. In yet other localities, the rocks are slightly separated according to their natural limits. There are additional points of view, at which “slickensides” may be observed ; but there are almost always associated with them correspondencies in the two rocks,
showing that the lateral motion was one of small extent. From these facts, to mention no others, it is evident-(1) That the two series of beds are unconformable ; (2) That the sandstone was deposited, nearly as it is now found lying, upon the slate; (3) That, in some places, there has been no horizontal movement of the rocks upon each other; and (4) That there has been, in other localities, a slight, and only a slight, sliding of the upper mass upon the lower. We thus see that the underlying black, argillaceous beds cannot, by any possibility, be Útica Slate, or Lorrain Shales.*
But let us look at another phase of the matter. Some have supposed that the sandstone, while it overlies the Swanton Slate, runs under the beds of the Georgia Group, which, in many places, occur in uplifts lying to the east of the Potsdam band. Instead of this being the fact, there are localities in Swanton, Shelburne, and other townships, which render it clear that the Potsdam formation, in some places, overlies the Brown Slates, and in others rests against their ppturned edges. Meanwhile, years of search have failed to reveal a single instance of its underlying, or running beneath, the beds of the Georgia Group.
This, however, is not all the evidence. An outstanding bed of what appears to be Potsdam Sandstone occurs as an overlying mass near Franklin Centre, and not far from the eastern limits of the Brown Slate. But a still better view of these rocks, and one respecting which there seems to be no question, may be secured in the counties of Chittenden and Addison. Í refer to the fact that the Potsdam Sandstone extends in these counties broadly to the east. In order to observe this extensive exhibition of the rock, one may start from a point in Charlotte, on the west of the Red Sandstone, and where it overlies the Black Slates, and thence proceed eastward or southeastward. Advancing in either of these directions for six, eight, or ten miles, he may see the sandstone from point to point, capping the uplifts, and showing itself as an overlying rock, while he may also, from time to time, observe outcroppings of the older formations until he reaches the Lower Taconic in Monkton and Starksborough. He may thus find the Potsdam strata resting upon portions of the Swanton Slates on the west, extending across the entire limits of the Georgia Group, and also overlying a portion of the Lower Taconic beds. He may, accordingly, observe reposing unconformably beneath the Potsdam Sandstone, a series of forma
* That portions of the Utica and Lorrain formations may occur in the same neighborhood with the Potsdam Sandstone, is, of course, possible, and most freely admitted. AM. JOUR. SCI.. --Second SERIES, VOL. XLVII, No. 141.—MAY, 1869.