Palache.] 



Soda-Rhyolite North of Berkeley, 



67 



portion of the sheet, being but a small fraction of the bulk of either 

 of the other types. 



Summary of Microscopic Study, — The study of the various facies 

 of rock from different parts of this formation demonstrates that we 

 have here an almost continuous series of increasing crystallographic 

 development, from a simple glass, through microlitic and spherulitic 

 forms to a microgranular porphyry. The mineralogical composi- 

 tion, where minerals have been developed, is uniform and very 

 simple, quartz, orthoclase, and acid plagioclase being the essential 

 constituents, and magnetite the only accessory. 



The entire absence of ferro-magnesian silicates is an unvarying 

 characteristic of the rock in all its phases. 



The petrographical characters are in harmony with the conclu- 

 sions arrived at in the field as to the extravasation of the formation 

 as a surface flow. 



CHEMICAL CHARACTERS. 



The interesting petrographical differentiation of the formation, 

 as revealed by studies in the field and with the microscope, sug- 

 gested an inquiry as to whether there existed a corresponding vari- 

 ation in the chemical composition. Chemical analysis was also 

 necessary to decide the position of the rock in the accepted scheme 

 of classification, as the microscopic study still left us in doubt as to 

 the dominant alkali. 



To this end three analyses were made, one of each type de- 

 scribed. The results obtained, together with two other analyses 

 for comparison, are exhibited in the following table: — 





I. 



II. 



III. 



IV. 



V. 



Si O, 



Al 2 3 



Fe 2 O a 



Fe O 



Ca O 



Mg O 



K, O 



Na, 



H„0 



Sp. g 



83-59 

 5-42 



| Trace 



3-44 

 Trace 



1- 37 

 5-33 



.76 



99.91 



2- 54 



75-46 

 13.18 



•91 



•95 

 .10 

 1.09 

 6.88 

 •93 



99-5Q 

 2.42 



69.85 

 13-34 

 •73 



•87 

 Trace 

 2.68 

 5-58 

 6.15 



99.20 

 2.32 



75-50 

 13-25 

 1.02 



•9i 



.90 



■07 

 2.85 

 4.76 



.41 



100.05 



77.70 

 12.30 

 .60 



.20 

 . 20 



.70 



.20 



7.00 



•50 



99.40 

 2.63 



