166 GEOLOGY OF THE BLACK HILLS. 



It will be remembered that a similar fossiliferous layer was found on 

 Red Canon Creek, resting-, as here, immediately upon the red clays of the 

 gypsiferous Red Beds. The list of species at that locality is nearly the 

 same. 



From the southern end of the Hills, where the thickness of the forma- 

 tion is about 200 feet, there is a great thickening toward the north and 

 northwest, reaching on the Belle Fourche a maximum of nearly GOO feet. 

 There seems to be no reasonable cause for suspecting that this thickening is 

 only apparent and is due to errors of observation. The possibility of 

 errors of mensuration is negatived by the fact that independent measure- 

 ments in the same neighborhood were always found to agree as closely as 

 was to be anticipated. Nor is it credible that any appreciable error was 

 made in tracing out and identifying the lines of demarkation by which the 

 formation is separated from the Dakota sandstone above and the Red Beds 

 beneath. Each of these lines represents a definite lithological change and 

 could readily be recognized at every point. There was some hesitation in 

 the selection of the point of separation between the Jura and the Creta- 

 ceous, but when it was once selected at one place there was no difficulty 

 whatever in finding it at any other place. 



The cause of this thickening toward the northwest is not fully under- 

 stood, but it is probably a normal increase of depth in the direction of the 

 source of the material which formed the sediments. East of the Black 

 Hills the formation is not known. At the southeast end of the Hills area 

 it is 200 feet thick; at the north end 410 feet; and thirty miles west of the 

 north end 580 feet. Dr. Hay den found in the Bighorn Mountains (one 

 hundred and twenty miles west of the Black Hills) a thickness of 800 to 

 1,000 feet, and Professor Comstock makes the same report of the Wind 

 River Mountains. 



North of the Uinta Mountains in the various ranges of the Rocky 

 Mountain chain the Jura is well marked by abundant and characteristic 

 fossils, from which considerable collections have already been made, and the 

 same is true of the Black Hills. But to the southward very few fossil 

 forms are known and their localities are rare. In like manner the rocks 



