Crosby.] 508 [March 7, 



rectly gives the normal thickness of the Potsdam as from 200 to 

 300 feet, he is certainly in error in stating that this thickness is 

 undiminished as we approach the Archaean heights. For his map 

 shows that the Potsdam makes its nearest approach to the Harney 

 Range on the east side, between Battle and Whiskey Creeks ; and 

 he explicitly states that its thickness at this point is only fifty 

 feet. Fearing that this exposure might be simply an outlier from 

 which the upper Potsdam beds had been removed by erosion, so 

 as to make the determination of the true thickness at this point 

 impossible, I visited the locality and was agreeably surprised to 

 find a complete section. The Potsdam quartzite rests, with a gen- 

 tle inclination away from the mountains, upon the highly inclined 

 and eroded mica schists with veins of endogenous granite, and 

 encloses fragments and grains of these rocks. After an almost 

 unbroken exposure which would give a thickness not exceeding 

 fifty feet (Newton's estimate), it is overlain conformably by the 

 Carboniferous limestone. Here, then, where the Potsdam reaches 

 farthest in toward the crest of the old Archaean island it is re- 

 duced to about one-fifth of its normal thickness. We must sup- 

 pose, therefore, that the Potsdam period closed, at least so far as 

 the deposition of arenaceous sediments is concerned, when the 

 summit of the Archaean ridge sank beneath the Paleozoic sea ; and 

 that, while the Potsdam sandstone must have reached this crest, 

 it could not have crossed it with any considerable thickness. 



As already explained, the great apparent blank between the Pots- 

 dam and Carboniferous probably indicates a profound subsidence 

 and the long-continued existence of abyssal conditions. At the 

 dawn of Carboniferous time the deep sea, as indicated by the purity 

 of the limestones, still covered, not only the Black Hills, but a very 

 large part of the United States ; and in the Black Hills, as else- 

 where, the gradual shallowing of the sea during this age is proved 

 by the fact that as we pass upward the Carboniferous strata be- 

 come gradually more argillaceous and then arenaceous, the upper 

 Carboniferous or coal measures consisting largely of sandstones. 



The Appalachian revolution, which crumpled and elevated the 

 Paleozoic sediments in the Alleghany region, the disturbance grad- 

 ually dying out or changing to simple elevation farther west in the 

 central part of the Mississippi valley, was repeated in the far 

 west, in the Great Basin district, among the thick marginal sedi- 

 ments along the western shore of the Paleozoic sea, the disturb- 



