DISCUSSION. 



Mr. C. D. Walcott: In northern Vermont a somewhat similar over- 

 thrust fault occurs, by which the basal strata of the Cambrian have been 

 thrust over upon the Trenton limestone, Hudson shales, etc. It was in this 

 area that Professor Jules Marcou noted the presence of Lower Silurian fos- 

 sils, and the theory of "colonies" was introduced by him to explain the 

 occurrence of the Primordial fauna above them. A misinterpretation of 

 the geology east of the Hudson river arose from a misunderstanding of the 

 overthrust fault. Professor Amos Eaton regarded the shales (which are 

 now known to be of lower Cambrian age), where they were thrust over upon 

 the Chazy limestone, as having been unconformably deposited upon the lime- 

 stone. Dr. Ebenezer Emmons, a pupil of Eaton, adopted the view of Eaton 

 as to the unconformity between the shales and the limestones and, also, con- 

 sidered a band of calciferous sandrock above as resting unconformably upon 

 the slates. It was upon this evidence that he first based the view that the 

 Taconic slates were pre-Silurian in age and that the fossils in them belonged 

 to a fauna inferior to that of the Silurian. I studied the typical locality at 

 Bald mountain, Washington county. New York, and found that the superior 

 beds were those of the Olenellus zone and that they had been forced by an 

 overthrust fault upon the Calciferous and Chazy. This sytem of over- 

 thrust faults, extending with more or less continuity from Alabama to the 

 St. Lawrence river at Quebec, has added immensely to the intricacy of 

 the geology and led many geologists into error, owing to their misinterpreta- 

 tion of the phenomena. 



Professor W. M. Davis : If I have correctly understood Mr. Hayes, he 

 finds reason to think that the overthrusts which he has so ably worked out 

 were produced at a later date than that of the general folding of the region, 

 and that these two disturbances were separated by a period of time long 

 enough for the accomplishment of a considerable amount of erosion. If this 

 be the case, I would suggest the following reasons for regarding the over- 

 thrusts as of post-Triassic date : In southeastern Pennsylvania, in the neigh- 

 borhood of the Triassic belt, whose monoclinal dip is northwestward, the 

 strongly compressed folds of the underlying Paleozoic formations are some- 

 what overthrown, causing a prevailing and steep dip to the southeast. It 

 is physically necessary that the underlying formations suffered an angular 

 movement along with the overlying Triassic beds, when the latter were 

 given their monoclinal dip ; it is reasonable, therefore, to regard the prevail- 

 ingly overthrown dips of the Paleozoic formations not as a feature given to 

 them at the time of the general Permian folding, but at the later time of 



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