Crosby.] 498 [March 7, 



large a part of the surface of New England and of the states south- 

 ward along the Appalachian crystalline belt and which Dr. Hunt 

 has called the White Mountain or Montalban system. The fact 

 that many characteristic Montalban rocks have not been observed 

 in the Black Hills does not invalidate this correlation. For this 

 older Archaean series of the Hills exhibits no types or varieties not 

 belonging to the Montalban of the east ; and it is not less diversi- 

 fied lithologically than equally large or larger areas of Montalban 

 in different parts of New England. In many ways it reminded 

 me strongly of the crystalline region between the Connecticut 

 River and the Berkshire valley, in Massachusetts. 



Origin and Age of the Granite. 



Perhaps the most characteristic Montalban rock in the Black 

 Hills is the granite. Although Newton has given us a satisfactory 

 account of the composition and texture of this rock, he has evi- 

 dently, in regarding it as eruptive, fallen into a serious error con- 

 cerning its origin and its relations to the enclosing schists. His 

 description and map appear to be at fault, also, in giving the im- 

 pression that the granite is continuous over areas several miles in 

 extent. As already explained, the granite culminates in Harney 

 Peak and the mass of mountains of which it is the summit. I as- 

 cended Harney Peak, and skirted the northern and eastern flanks of 

 the Harney Range, for the express purpose of studying the granite 

 and its relations to the schists. To my great surprise, I found no 

 broad or -continuous area of granite ; but the prevailing rock in all 

 this part of the so-called granitic area is mica schist with some mi- 

 caceous gneiss. The granite becomes relatively more abundant 

 as we penetrate the mountains ; but, so far as observed, it nowhere 

 forms more than one-half of the surface ; and it is doubtful if it 

 forms, on the average, as much as one-tenth of the surface of that 

 part of the granitic area which I traversed. 



We have already noticed Newton's important observation that 

 the granite occurs wholly, so far as known, in the form of huge 

 lenses parallel with the schists ; and his section of Raw Hide Butte, 

 one hundred miles southwest of the Black Hills, which Newton says 

 is composed of granite and schist similar to those of the Black 

 Hills, would serve admirably as an illustration of the structure of 

 almost any part of the Harney Range. The lenses and swelling 

 masses of granite splitting the schists and alternating with them 



