364 NEW YORK STATE MUSEUM 



Spraker. The upper beds contain Ophileta and seem of about the 

 same age as the basal part of division D of Brainard and Seeley's 

 section. The upper Cassin beds are wholly lacking in the Mohawk 

 valley, and apparently the latter region was uplifted while they 

 were being deposited on the east. Certainly, the connection be- 

 tween the two areas was broken during this time, confining the 

 Oassin fauna to the lOhamplain basin, and this seems to the 

 writer an added reason for the separation of the Cassin beds from 

 the normal Beekmantown, 



On the north side of the Adirondacks the exposures of the for- 

 mation are poor and infrequent, the dips are flat, and the breadth 

 of outcrop considerable, with the full thickness not showing on 

 the New York side of the international boundary. Nothing 

 definite is known concerning the thickness in this area, but, as 

 its western limits are reached, in the Thousand islands region, 

 it becomes evident that the formation has greatly thinned. On 

 the western side of the region it is wholly absent, the later Black 

 Eiver and Trenton limestones resting directly on the Precam- 

 bric, making it perfectly evident that the Beekmantown 

 shore line there lay farther to the west than the present Pre- 

 oambric boundary. Moreover, the imperfect records of the gas 

 wells of Oswego and Jefferson counties, as given by Orton, indi- 

 cate a thickness of only 200 feet of Beekmantown rocks under 

 cover in the former county, and none at all in the latter, the 

 former wells being 35 miles, and the latter 15 miles distant from 

 the present Precambric boundary.^ It seems therefore that the 

 Beekmantown sea covered by no means all of the present Pre- 

 cambric region of northern New York, and that the main area 

 left unsubmerged by its waters was on the west and south. On 

 the west the shore was several miles west of the present Pre- 

 cambric edge; on the south it did not extend in more than from 

 10 to 30 miles beyond the present edge, as the writer has else- 

 where shown ;2 much less is known about the rate of overlap on 

 the northeast, but the great thickness of both the Potsdam and 

 Beekmantown formations there would indicate that, by the close 

 of the Beekmantown, the sea must have widely encroached on 

 that portion of the region. 



^N. Y. State Mus. Bui. 30, p.442, 458. 

 -N. Y. State Mus. Bui. p.77. 



