OUYINK IN P.ASALT. 1>,V.) 



No. 10. Went of Tiill llnad, went of Dome Mountain. It ooeun ac one of the largest extniitlonB of 



basalt along the, Piuto fault. The bcoad mass lies iu contact with hornblende -andesite, and Hows 

 from the same body arc tteeii to directly overlie rhyolitie tuffs. It in exceedingly rich iii glass, "nd so 

 mottled as to present a gray color. Although the highest on the list in the percentage of silica, if 

 possesses a Strongly marked basaltic habitus, quite as characteristic under the microscope as in the 

 hand-specimen. 



It will be seen, with the exception of numbers one and two, that the 

 silica percentage in all the rocks is higher than is usually found in basalts ; 

 they show between the two extremes on the list a variation in silica of 10'88 

 per cent. 



Although olivine is not an essential constituent in the basalts, the above 

 table shows how close a relationship exists between the olivine bearing 

 and olivine free varieties, and a study of the localities and their mode of 

 occurrence demonstrates how futile an}' attempt would be to try to sep- 

 arate them on the presence or absence of this mineral. In the hill south 

 of Alhambra Hills, the silica is low, while the olivine is present in com- 

 paratively large secretions. In the dike from the summit of Richmond 

 Mountain, the second in the table, there is an increase in the amount of 

 silica of over 2'5 per cent, with a large falling off in olivine. From the 

 rocks with 58 to 59 per cent of silica, there is only a small and varying 

 quantity of olivine, while in the three specimens which gave over 59 per 

 cent of silica the microscope failed to detect its presence. 



Sufficient facts have been adduced to indicate how intricately the 

 entire series of pyroxenic rocks are related to each other throughout a wide 

 range in their composition. Throughout this entire group of extravasated 

 lavas the essential minerals remain the same, the differences consisting 

 for the most part in their relative proportions and the accompanying 

 modifications of groundmass structure. This holds true in a still more 

 striking manner if we exclude the extreme acidic end of the series where 

 the hornblende and mica play the part of accessory minerals. Some of 

 the basaltic masses determined as such by geological position and structural 

 peculiarities have been found in several instances, usually the more glassy 

 varieties, to be more acidic than the pyroxene-andesites, the two natural 

 groups overlapping each other as regards their composition. The sudden 

 changes which all these pyroxeuic lavas apparently undergo from crystalline 

 to glassy varieties is one of the marked peculiarities of the Eureka District 



