72 REPORT UNITED STATES GEOLOGICAL SURVEY. 



It is not an easy matter to make subdivisions in an extensive series 

 of limestone strata unless material aid is aiforded by palceontological 

 remains. 



Eesting directly upon the red quartzites of tbe Potsdam Group we 

 find a thin series of sharply-bedded strata. They are composed of blue 

 limestones, blue and yellow dolomites, and, locally, of oolitic dolomites. 

 Siliceous inclusions of irregular form apj)ear within these rocks, as well 

 as higher up. About 250 feet may be regarded as the maximum thick- 

 ness reached within our district. A definite thinning out and eventual 

 disappearance of the beds may be observed to the southeast. We have, 

 in this appearance of the Calciferous Group, evidently the southern 

 edge of what aiDi)ears farther north as an extensive series.* While the 

 beds in that direction are very considerably amplified, we find but the 

 termination both of horizontal and vertical dimensions of this group in 

 our district. • 



In stratigraphical relations the Calciferous series of this region is en- 

 tirely conformable with the underlying Potsdam quartzites. Both dip 

 off from the mountains, and both are exposed along a line following the 

 general trend of the range. It may be observed that the angle of dip 

 decreases very slightly as we approach the lower country to the east. 

 Some fossils were observed within the beds belonging to this group, but 

 they were so poorly preserved that they could not be identified. Pro- 

 fessor Comstock mentions Ortliistritonia, Dicellocephalus, and Theca (I) 

 as occurring there. As his investigations extended farther northward 

 than mine, his opportunity for collecting palseontological remains was 

 much more favorable. 



While the Potsdam sandstones are the result — ^in that region — of a 

 long, shallow shore deposit, extending in a line approximately parallel 

 with the elevation of archaean rocks, the waters depositing the material 

 which now composes the Calciferous Group approached from the north 

 and northeast, extending but a short distance into our district. It is for 

 this reason that we find, firstly, the increased horizontal and vertical 

 dimensions of the group elsewhere, and, secondly, the direct superposi- 

 tion of the lowest Carboniferous strata upon the Potsdam sandstone 

 within the remaining portions of our district. At the period during 

 which the later Silurian fauna appeared in that section of coimtry, the 

 region where we now find the red quartzites exposed had already attained, 

 considerable elevation, thus rendering it impossible that any extensive 

 accumulation of Calciferous beds should take place. ISTo evidence was 

 obtained indicating a direct removal of Calciferous strata prior to the 

 deposition of Carboniferous beds. For this reason the explanation above 

 offered suffices for the occurrences noted in this instance. 



Professor Comstock separates a number of strata overlying the last- 

 described beds as the Niagara Group. The section which he furnishes 

 is obtained from a region west of Camj) Brown. To this group he 

 ascribes a thickness of 150 feet. I have failed to find any decisive evi- 

 dence that would warrant such a division. Palseontological evidence, 

 as furnished by the Silurian limestones, is so extremely meagre and un- 

 satisfactory that we cannot appropriately apply to the series the same 

 rules for subdivision that elsewhere may be definitely exjiressed. While 

 examining the eastern slope of the range I observed the same beds which 

 he has designated as Quebec and Niagara, but I am not prepared to ad- 

 mit of any subdivision at those localities. We have red quartzites rest- 

 ing directly upon the granites, and a series of limestones and dolomites 



* Compare Dana, Report of Reconnaissance of YelloAvstone Park, 1875, Ludlow, 

 ■Wasliingt(.n, 1876, p. 133. 



