10(5 GEOLOGY OF THE BLACK HILLS. 



metiunori)hic rocks wo have undoubted evidence. Tliere is nowhere observed 

 any thiiniing- out of the foniiation as we approach the center of the Hills, 

 but it everywhere maintains within naiTOw limits the same thickness. 



In accounting for the origin of the sands forming the upper part of the 

 formation, which once concealed the metamorphic rocks entirely, we 

 start an old problem that has never yet been fully settled by geologists. 

 What was the derivation of the mechanical deposits, covering now such 

 wide extents of territory in the West, near which no original land surface 

 has yet been revealed ? The coarse silicious sand of the Potsdam could not 

 have been derived from a distant shore, for it is not fine enough to be cairied 

 by ocean currents. There was undoubtedly at that time a great area of 

 exposed land surface, much of which is now buried beneath the land 

 or the sea, some primordial Archaea w'hich having been w'orn away to 

 form our present land surface has vanished from view. The great 

 extent of Laurentian and Huronian rocks, stretching from Lake Superior 

 to the Lake of the Woods and thence northwesterly to the Arctic Ocean, 

 furnished in all probability much of the material; but for the sandstone of 

 the Potsdam and later formation of the Rockv Mountains there must have 

 been a land surface farther w^est, possibly in Pacific regions. The problem 

 has occurred to Dr. Newberry, who says in his report to Lieutenant Ives*: 



The outlines of the western part of. the Xorth American continent were ap- 

 proximately marked out from the earliest Paleozoic times; not simply by areas of 

 shallower water in an almost boundless ocean, but by groups of islands and broa<l 

 continental surfaces of dry land. 



Since the erosion of rocks is always subaerial, or at least never takes place more 

 than 40 feet below the ocean surface, it follows that to form the stratified rocks of only 

 that i)ortion of the great central plateau which borders the Colorado, an island three 

 Imnded miles in diameter and at least 0,000 feet high, or, what is more probable, a 

 continent of six times that area and 1,000 feet high was worn down by the action of 

 waves and rains, and in the form of sediments, sand, gravel, clay, or lime, deposited ou 

 the sea bottom. 



When we reflect that, with the exception of narrow wedges of erupted material 

 in the mountains, an area having, on the 3Gth parallel, the breadth of the entire dis- 

 taiu-e between the great bend of the Colorado and the Mississii)pi (twelve hundred 

 miles), and a great though yet unmeasured extension north and south, is occupied by 

 several thousand feet of Paleozoic and secondary strata, we must conclude that these 

 sediments have not been derived from the erosion of emerged surfaces east of the Mis- 



• Colorado Exploriug Expedition, Li«-ut. J, C. Ives, IHGl, Gcol. Rept., p. 47. 



