OCCUERENCE AND SUCCESSION OK HOCKS. l9l 



portion of the Great Basin, as elucidated by the Exploration of the Fortieth 

 Parallel. In a cut on the American Flat road, just south of the Florida, 

 there occur two seams of coal-like matter half an inch in thickness. The 

 metamoi-phics extend into American Flat under the area laid down as 

 Quaternary, where the detritus is too thick to permit of tracing the con- 

 tact between the metamorphic and eruptive rocks with certainty. The 

 Rock Island shaft is inaccessible, but a careful examination of the dump and 

 the descriptions of an employti leave no doubt that it passed through meta- 

 morphics into underlying granite. There is nothing to show that any 

 eruptive rock other than granite has been met with at the Roclc Island. A 

 little coal is said to have been found well down towards the granite, and 

 was no doubt such an occurrence as that mentioned above. Metamorphics 

 of the same character appear to an insignificant extent north of American 

 Flat, and in iheXaledonia, as is shown on the section through that mine. 

 In the Gold Hill mines black slates form the foot wall of the Lode to a 

 large extent. Thin sections made across the lamination show that the dark 

 color is due to absolutely opaque particles without metallic luster, and 

 these disappear on prolonged heating in an oxidizing flame, but are not 

 affected by acids. They are therefore graphite. The rock contains pyrite, 

 which is very irregularly distributed. The slate is often confounded with 

 " black dike" (younger diabase), with which, however, it shares only the 

 black color. In a fairly good light the slaty structure serves to distinguish 

 it without difficulty The diorite at the Yellow Jacket appeal's to overlie 

 these slates, though no single mine-opening shows a contact. The masses 

 of mica-diorite shown in the Yellow Jacket section can hardly be in their 

 original position, though very likely they have been transported but a very 

 short distance; but at the surface the dioritic mass is in sight to within a few 

 hundred feet of the Yellow Jacket, where it seems to disappear under the 

 andesites, and it is almost impossible to suppose that the great exposure of 

 slates in the Yellow Jacket and the Belcher is not one surface of a body which 

 extends beneath the neighboring diorite. On the other hand, in the Cale- 

 doiiia diorite underlies the metamorphics, and it therefore seems probable 

 that the plastic diorite was forced horizontally between sedimentary masses 

 as well as vertically to the surface or, at all events, to higher points tlian 



