(H,l\ INi; IN I'.ASAI/r. '2M 



Xo. 10. Il'est of Toll Ufiid, ir. »/«/ 7toiKr .l/((«K/<(iii.— It oiciiis :im on, of t lie largest (extrusions i.C 

 1>iisalt aloQjc tlio I'iuto fault. '\'\u' lnoiiil mass lies in ciintart with liiiriil>liiiclfan(le.siti', and IIiiwh 

 from tliesamr body aro sciu to <lirmetly ovi'ilii' rliyolitic tnlVs. It is o.\(<re<linjj;ly rich iu fjlass, anil so 

 mottled as to ])rrscnt a fjray lolor. Althouj;Ii tlie hi;;hrst on tin- list in tin' perci'nta);r of silii'a. it 

 jiossessos a stroni^ly niarkced liasaltii- haliiUis, i|iiilr as cliaractcristlr unihr thr micnisro])!' as in tho 

 liand-si>fiimi II. 



It will Ix' scH'ii, with the cxccpliou ul' uuiuIkts one ;iiiil two, tliiit the 

 silii-ii peiTC'iitiiiii' iu all the rocks is hij^her than is usiiallv t'oniul in l)a.salts ; 

 tliey sliow Ix'twccn the two extremes on the list a variation in siliea of 1()".S8 

 j)er cent. 



Although oli\-ine is not an essential constituent in the basalts, the above 

 table shows how close a relationship exists between the olivine bearing 

 ami olivine tree varieties, ami a stu(l\- of the localities ami their niode of 

 occurrence demonstrates how futile any atteuij)t \\'oul(l be to try to sep- 

 arate them on the ])resence or absence of this mineral. In the hill south 

 of .Vlhanibra Hills, the silica is low, while the olixine is jiresent in com- 

 paratively larg-e secreti»ins. In the dike from the summit of Richmond 

 ^lountain, the second in the tal)le, there is an increase in the amomit of 

 silica of over "2"o per cent, with a large falling oil' in olivine. P'rom the 

 rocks with 58 to 59 j)er cent of silica, there is onl\' a small and ^•ar^■ing 

 quantity of olivine, while in the three sjieciniens which gave over 5!J per 

 cent of silica the microscope failed to tU'tect 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 groiij) of extravasated 

 lavas the essential minerals remain the same, the ditl'erences consisting 

 for the most part in their relative ])r()portions and the accompanying 

 niodilicatioiis of gi-oundmass 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 ])art of accessorv minerals. Some of 

 the basaltic, masses detei'mined as such b\' geological positionand 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 smlden 

 changes which all these })yroxenic lavas appareiitl\- undergo from cr\ .->talline 

 to glassy varieties is one of the marked peculiarities of the Ein-eka District 



