258 



GEOLOGY OF THE EUKEKA D1STKICT. 



than is ordinarily supposed to occur in normal basalt, the amount reaching 

 as high as the percentage found in many andesitic rocks, and in some 

 instances equaling the amount in the pyroxene-audesite of Richmond 

 Mountain. 



oiivine in Basalts. In order to determine the amount of silica present in 

 these rocks and its relationship to oiivine, a number of chemical analyses 

 were made from specimens which field observation and a study of thin 

 sections had shown to belong to basalt. The subjoined table gives the 

 result of ten such chemical examinations, arranged in order according to the 

 silica percentage obtained. The presence or absence of oiivine in the thin 

 sections of the same rocks, as determined by the microscope, is also given 

 in the table. 



No. 1. South of Alhambra Hills. This rock occurs as a low hill rising out of the Quaternary 

 plain, and isolated from all other volcanic outbursts. It is a highly crystalline rock. 



_Yo. 2. Dike northeast of summit of Richmond Mountain. An intrusive body penetrating the pyr- 

 oxeiie-andcsiti-. 



No. 3. East of Basalt Peak. A vesicular black basalt. 



No. 4. liasalt Cone. A compact dark rock characteristic of a large area of country. 



No. 5. Basalt Peak. A compact rock passing into vesicular varieties. 



A'o. 6. West base of Richmond Mountain, near the town of Eureka. A grayish red vesicular rock 

 lying between pyroxeuc-andcsite and earlier rhyolitic tuffs. 



No. 7. West of Basalt Peak. It occurs on the broad saddle just west of the peak, not far from 

 the groat body of Devonian limestone, and is a characteristic rock rich in glass base, black in color, 

 mottled with gray. 



No. S. West bate of Richmond Mountain, nut far from the town of Eureka. In its geological rela- 

 tions it is quite similar to No. 6. It is found breaking through rhyolitic tuffs and is a compact dark 

 rock with a characteristic basaltic habitus. 



No. 0. A dike from the summit of Richmond Mountain. Under the microscope the rock resembles 

 No. 2, which occurs not far distant, penetrating the same body of andesite under precisely similar 

 geological conditions. This rock, however, is much richer in glass and correspondingly richer in 

 silica. It is black in color, without macroscopic secretions, and has a decidedly conchoidal fracture. 



