106 GEOLOGY OF THE BLACK HILLS. 



metamorphic rocks we have undoubted evidence. There is nowhere observed 

 any thinning out of the formation as we approach the center of the Hills, 

 but it everywhere maintains within narrow 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 carried 

 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 Archsea which having been worn 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 Rocky Mountains there must have 

 been a land surface farther west, 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 Korth 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 broad 

 continental surfaces of dry land. 



Since tbe 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 portion of the great central plateau which borders the Colorado, an island three 

 handed 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 on 

 the sea bottom. 



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

 in the mountains, an area having, on the 36th parallel, the breadth of tbe entire dis- 

 tance between the great bend of the Colorado and the Mississippi (twelve hundred 

 aides), 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 Exploring Expedition, Lieut.. J. C. Ives, 1H61, Geol. Kept., p. 47. 



