GEOLOGY OF THE NORTHERN ADIRONDACK REGION * 375 



half of its thickness, is formed of blue black, brittle, somewhat 

 muddy limestones, carrying a trilobite, cephalopod, lamellibranch 

 fauna, as distinguished from the abundant brachiopod fauna of 

 the gray limestone. These rocks are nearly on the strike of the 

 section north of Bluff point, and there is unquestionably some 

 overlap in the two; but, as the larger part of the Crab island 

 section is of higher beds than any shown in the other, these three 

 sections can only be fitted together by the most painstaking 

 paleontologic work, perhaps not even by that. The writer is con- 

 fident that the upper beds on Crab island are from 300 feet to 

 350 feet above the base of the formation. These are the three 

 best sections of the Trenton which the writer has seen toward 

 the lower end of the lake. They indicate a large thickness for 

 the formation, but give no clue to the amount that may be lacking 

 above the upper beds of the Crab island^ section. 



In a recent report on the geology of Grand isle, Vermont, Pro- 

 fessor Perkins has described an interestinig section which shows 

 that there is no sharp, lithologic break between the limestones 

 of the Trenton and the overlying black shales of Utica age, but 

 rather an imperceptible gradation from the one into the other, 

 forming a series of passage beds.^ These consist of rapidly alter- 

 nating shales and limestones, with a comparatively steady in- 

 crease upward in amount of shaly matter. The thickness of these 

 beds is not stated, possibly because the section is not sufficiently 

 complete, possibly because their recognition as a separate litho- 

 logic unit simply increases the difficulty of constituting bound- 

 aries by making two vague horizons instead of one. The beds 

 of distinctively intermediate character seem however to be v^f 

 considerable thickness. 



Along the shores of Cumberland head, on the New York side 

 of the lake, is an excellent, though much disturbed, section, con- 

 sisting of blue black slaty limestones and calcareous shales, with 

 some firmer limestone bands. These rocks are much faulted and 

 squeezed, and somewhat folded, with much development of slaty 

 cleavage at a high angle with the bedding planes. These rocks 

 extend along shore northward to beyond Point au Koche, in 

 Beekmantown. Dr White states that similar rocks occur on 



iRep't Vt. State Geol. 1901-3, p. 167-68. 



