4 26 



University of California. 



[Vol. 2 



Chemical Characters. — The chemical investigation of the ande- 

 site group, while not so complete as would be desirable, is yet 

 sufficiently extensive to bring out some interesting points of corre- 

 spondence and gradation in character between its members, and to 

 establish a good basis on which the group may be differentiated 

 from the associated groups of basaltic rocks. Complete analyses 

 of the four principal types of the andesite and two silica deter- 

 minations were made and are exhibited in the accompanying table. 

 Comparing these analyses it will be seen that the range of silica is 

 not much above four per cent. It is noticeable also that of the 

 other components the alumina is uniformly high and the iron 

 oxide correspondingly low ; the alkalies show but little variation 

 in the first three analyses, but are notably small in VI while the 

 lime is exceptionally high. This analysis shows the Pie Knob rock 

 to be an exceptional type, and its classification with the andesites 

 is recognized as questionable. The specific gravity appears to 

 increase in proportion as the glassy constituent of the rock de- 

 creases. The composition of the first three confirms their classifi- 

 cation, already made on mineralogical and structural grounds, as 

 pyroxene andesites. 



Analyses of Andesites. 





I. 



II. 



III. 



IV. 



V. 



VI. 



Si 2 ... 

 Ti 2 ... 

 Al 2 3 .. 

 Fe s O s .. 

 Fe O... 

 Mu O... 

 Ca O.... 

 Mg O... 

 Na, O.. 

 K 2 O... 



p 2 o 5 ... 



H 2 O.. 



59-75 

 trace 

 19.08 



59-74 

 trace 

 19.94 

 1.81 



5-41 

 trace 



3- 74 

 1. 18 



4- 65 

 1.02 



• 35 



2.09 



56.81 

 trace 

 18.36 

 2.48 



3- 94 

 trace 



6-43 



4- 75 

 4.18 

 1.36 



•25 

 1. 17 



57-58 



60.79 



56.47 







22.55 



•95 

 1.97 



2.91 

 trace 

 6.15 

 4.72 



4-31 



.92 



■15 



2.64 











10.89 

 4.48 

 .88 



■52 







1.03 



Total... 



100.63 



99-93 



99-73 







99-74 



Sp. g... 



2.62 



2.64 



2.74 



2.66 







I. Amygdaloidal andesite, overlying Orindan formation, Frowning Ridge 

 escarpment. Analyst, Palache. 



