258 



GEOLOGY OF THE EUREKA DISTEICT. 



than is onliuarily .supi)0.sed to occur in uorinal basalt, the amount reaching 

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

 instances ec^uahng the amount in the pyroxene-audesite of Richmond 

 i\Iountain. 



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

 tliese rocks and its relationship to olivine, a number of chemical analyses 

 were made from specimens which field observation and a stu<ly 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 olivine in the thin 

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

 in the table. 



Xo. 1. South of Alhambra Hilh. — This rock occurs as a low hill rising out of the Quaternary 

 plain, and isolated from all other v<dcanic outbursts. It is a highly crystalline rock. 



Xo. 2. Dike iwrtheiist of summit of Richmond Mountain. — An intrusive body penetrating the pyr- 

 oxeno-andesitc. 



Xoi ;?. East of Basalt Peak. — A vesicular black basalt. 



No. 4. Basalt Cone.— A compact dark rock characteristic of a large area t)f country. 



Xo. o. Bamlt Peak. — A compact rock passing into vesicular varieties. 



Xo. 6. ll'est hase of Richmond Mountain, near the town of Eureka. — A grayish red vesicular rock 

 lying between pyroxenc'-andesite and earlier rhyolitic tuffs. 



Xo. 7. JVest of Basalt Peak. — It occurs on the broad saddle just west of the i>eak, not far fioiii 

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

 mottled with gray. 



^V«. .s\ ll'est base of Richmond Monntiiin, nut fur from the town of Eureka. — In its geological rela- 

 tions it is (|iute similar to No. 6. It is found lireaking through rhyolitic tuffs and is a compact dark 

 rock with a characteristic basaltic habitus. 



Xo. :i. .1 dike from the summit of Richmond Mountain. — Under the microscope the rock restunbles 

 Xo. 2, which occurs not far distant, penetrating the same body of andesite imder ))re<-isely similar 

 j;<(diit;ical conditions. This rock, however, is much richer in glass and correspondingly richer iu 

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



