48 GEOLOGY OF THE BLACK HILLS. 



epidotic and cliloritic rocks, steatites, serpentines, and talcose (or more prob- 

 ably hydrous mica) schists. 



Hunt furtlier distinguishes the White Mountain or Montalban series of 

 rocks, which he regards as newer than the Huronian but Pre-Silurian and 

 Pre-Cambrian, and he traces tliem, in a series of de})Osits parallel to the 

 Green Mountain rocks, far southward into Pennsylvania. The rocks of this 

 series in the White Mountains are characterized by mica-schists passing 

 on one hand into micaceous gneisses and on the other into argillite. They 

 include beds of micaceous quartzite and concretionary granitic veins with 

 beryl an<l tourmaline. 



West of the Missouri the Archaean is exposed over large areas in the 

 axial lines of the Rocky Mountains arid its various subordinate ranges, and 

 includes most of the rocks already enumerated, tliough the granites, syenites, 

 and highly feldspathic rocks are more largely develoyied in man}- regions, to 

 the exclusion often of the schists and slates. U n til lately no attempt has been 

 made to draw any line among these Archcean rocks and establish the divis- 

 ions recognized in the East. Such a separation would be based exclusively 

 upon the lithological structure of the rocks and the presence or absence of 

 particular mineral species, for no fossil has been found; and by the majority 

 of sreoloffists such evidence is not admitted in the identification of jfeoloofi- 

 cal formations when, as here, the points of observation and comparison are 

 widely separated. Mr. King, of the 40th Parallel Survey, has, however, 

 noticed diflferences in the rocks of certain of the ranges, which lead him to 

 regard a separation into Laurentian and Huronian as possible. 



The Archaean rocks of the Black Hills, as may be seen on the geologi- 

 cal map, occupy the axial or nucleal area, and their exposure is somewhat 

 to the east of the center of the uplift. The extent of country occupied by 

 them is about sixty miles long north and south, and twenty-five miles in its 

 greatest width east and west, with an area of about 8n0 square miles. 



Rugged and broken as is the entire region of the Hills, the interior 

 area underlaid by the Archaean schists and slates is particularly mountain- 

 ous and rough. Cut up into peaks, ridges, and valleys, and watered 

 by numerous brooklets, it has the aspect of many of the picturesque 



