TACONIC QUESTION IN GEOLOGY. 151 



ings County, Ontario, have been noticed as probably worm-burrows by Sir J. W. Dawson, 

 who has also described the Eozoon Canadense found in the associated limestones, while the 

 argillites which I have referred to this series, from the western end of Lake Superior, have 

 afforded the remains of a sponge. The Taconian, as I have suggested, may constitute a 

 link between the older eozoic groups and those of paleozoic time. 



§ 199. The Upper Taconic group, the First G-ray wacke of Eaton, the Potsdam and 

 Quebec groups of Logan, (which include a large part of what was described by Mather 

 and by Logan as Hudson River group,) we have seen to be the Appalachian representative 

 of the Cambrian period. It sometimes overlies the Taconian, but, in the absence of this, 

 rests directly upon the older crystalline groups along the eastern border of the great Ap- 

 palachian basin. Unlike the Taconian, however, it does not, so far as known, extend east- 

 ward of this limit, while to the west, as we recede from this border, it is soon replaced 

 by the Adirondack type of the Cambrian. 



This Appalachian Cambrian series is wholly uncrj'stalline, and is separated from the 

 Taconian by a stratigraphical break, and probably by a great interval of time. From the 

 distribution of the Cambrian and the Ordovician in eastern North America, there was evi- 

 dently another great stratigraphical break, with erosion, followed by a considerable conti- 

 nental depression, which preceded the deposition of the Ordovician limestones. Similar 

 disturbances seem to have intervened at the beginning of the Silurian period in this east- 

 ern region, for we find the Silurian limestones resting directly upon somewhat inclined 

 and eroded Ordovician strata near Montreal, and, apparentlj^, also in the valley of the Hud- 

 son, while throughout this eastern border the great mechanical sediments of the Oneida, 

 Medina and Clinton, which to the west of the Eiver Hudson constitute the chief part of 

 the Second Graywacke of Eaton, at the base of these limestones, are apparently absent, — 

 a fact pointing to the emergence of this eastern region during the early part of Silvirian time. 

 The local disturbances which at this period prevailed in the eastern part of the great basin, 

 are farther shown in the conglomerate character of these Silurian sandstones in parts of 

 New York and Pennsylvania, though it should be noted that in these regions, as well as 

 in Ontario, there appears to be an unbroken succession from the Loraiue shales to the 

 Oneida, Medina and Clinton subdivisions. 



§ 200. As a result of all these various movements which affected the eastern border 

 of the Appalachian basin, we find that the Taconian is there in some parts directly over- 

 laid by Cambrian, in others by Ordovician strata, and in parts, it woirld seem, by Ifme- 

 stones belonging to the upper portion of the Silurian, or to Devonian time. The strata 

 of all of these periods are more or less involved with each other, and with still older crys- 

 talline groups, by the successive movements of folding and dislocation which continued 

 to affect the Atlantic belt at intervals until after the close of paleozoic time. From the 

 complex stratigraphical relations which have thus resulted, various observei's have, during 

 the past forty years, conjectured that the Taconian limestones are strata of Cambrian, of 

 Ordovician, of Silurian, or even of Devonian age, which, by a process of so-called meta- 

 morphism, have been changed into granular non-fossiliferoiis marbles, often holding 

 crystalline silicates. 



§ 201. These various conjectures are not only in contradiction with each other, but, as 

 we have seen, are in direct conflict with the facts of stratigraphy, and are, moreover, based 

 upon the unproved and now generally discredited hypothesis of progressive and regional 



