252 G-EOLOGY OF THE EUREKA DISTRICT. 



covering an extensive area and of great thickness, has no outlying 

 exposures. The Cliff Hills which lie beyond the limits of the Eureka 

 Mountains, present a grand exposure of pyroxene-andesite, but as they stand 

 alone afford no evidence as to the relations of the different lava flows to 

 each other. Along the base of the escarpment, which forms the south side 

 of Richmond Mountain, occurs a contact over a mile in length, between 

 pyroxene-andesite and rhyolitic pumices, yet nowhere along this line has 

 the sequence of eruption been definitely determined by actual contact. 

 Evidence fails to show whether the pyroxene-andesite broke through the 

 pumices, which, on account of their friable nature have suffered more or less 

 erosion, or whether the latter banked up against a preexisting wall of the 

 former. For the greater part of the distance the junction of the two rocks 

 is completely obscured by both large and small blocks of. andesite, which 

 have fallen from the cliff above, and wherever these are wanting the con- 

 tact is hidden by fine friable pumice and ash, which has accumulated in 

 considerable thickness along the base of the escarpment, piled up by the 

 prevailing westerly winds. Although no actual superposition is seen, all 

 indirect evidences point so strongly to the true order of succession that the 

 fact seems well established that the pyroxene-andesite followed the rhyo- 

 lite. 



Between the pyroxene-andesites and basalts there exists the closest 

 possible relationship, so much so that it is by no means an easy matter to 

 establish a sharp line between them, either in mineral composition or field 

 occurrence. Unlike pyroxene-andesite, however,- the outbursts of basalt 

 present a considerable diversity in their mode of occurrence and distribu- 

 tion, forming broad table-like masses and numerous small extrusions in dikes 

 and rounded knolls. Although the two rocks are closely related by transi- 

 tion products, extreme typical forms may easily be distinguished from each 

 other by both geological and petrographical features of rock masses, and 

 as to their order of succession there exists, fortunately, abundant proof to 

 show that the pyroxene-andesite preceded the basalt. Evidences of their 

 relative age may be seen on the summit of Richmond Mountain, where 

 several dikes of dense glassy basalt cut the andesite in sharply defined lines 



