GEOLOGY OP THE NORTHERN ADIRONDACK REGION 371 



a wholly different basin. On the west its waters encroached 

 farther on the Precambric land than the Beekmantown waters 

 did. On the south it wias very irregularlj^ deposited, owing to 

 slight unevenniess of the floor. It is not certain whether it was 

 contemporaneons with the later Chazy or followed that in time. 

 Its fluctuations in thickness, its occasional absence, and its sharp 

 contact line with the Beekmantown through most of the Mohawk 

 valley give physical evidence of a considerable time gap between 

 the two formiations, during which some slight flexing and erosion 

 of the Beekmantown rocks took place, and during which the 

 Cassin and Chazy rocks were being deposited in the Chazy basin. 

 This evidence is so clear that it would seem that the apparent 

 passage beds in the West Canada creels district can not be actually 

 such, but belong with the Lowville. The Lowville depression 

 seems to have invaded the district from the southwest, and these 

 beds rejiresent an older stage of the formation than do any of 

 those found to the eastward in the Mohawk valley. 



Black River limestone. This formation is found on all sides 

 of the Adirondacks, though locally lacking in the Mohawk val- 

 ley. Throughout the Champlain valley region it shows a mas- 

 sive, basal layer of pure, dove-colored limestone which much 

 resembles the Lowville except that it lacks the characteristic 

 calcite tubes. Above this layer the entire formation consists 

 of massive layers of solid, brittle, pure, black limestone, break- 

 ing with conchoidal fracture. About the lower end of Lake 

 Champlain it ranges from 30 feet to 50 feet in thickness. South- 

 ward along the lake the writer has not seen the -sections, but 

 White reports a. measured thickness of 71 feet, 3 inches on 

 Crown Point peninsula, the greatest observed thickness reached 

 in the State.^ From this point it thins toward the south, as do 

 all the Paleozoic formations of the region. In Saratoga county 

 it has an intermediate character, containing layers which re- 

 semble the Lowville, Prosser's sections at Glens Falls showing 

 a thickness of 27 feet for the two combined. This seems how- 

 ever an exceptional tlnickness, most sections in the vicinity being 

 vastly thinner. Coming around into the Mohawk valley, the 

 formation ranges from .5 to 9 feet in thickness about Anister- 



^White, T. G. Geol. Soc. Am. Bui. 10 :4.-)7. 



