HYDRON SILICATES IN ROCKS OF CONCEPTION BAY 143 



position of the original rock from which the silicified rhyolite 

 represented by analysis No. 2 has been derived. 



Chemically this has resulted in a decrease in the percentages of 

 potash, soda, lime, and alumina, an increase in the percentage of 

 silica, and a relative decrease in the percentage of sodium with 

 respect to potassium. The process operated through the replace- 

 ment of the feldspars by quartz and a relatively more rapid 



Fig. 7. — Perlitic structure preserved in quartz replacing the groundmass of a 

 spherulitic rhyolite. Ordinary light, X 35. s = spherulite. 



replacement of the soda feldspars than of the potash feldspars. 

 The solutions which effected this alteration doubtless belonged to 

 the same general period of volcanic activity as the extrusion 

 of the lavas themselves. 



PYROPHYLLITIZATION, PINITIZATION, AND SILICIFICATION 



As has been remarked before, a later series of local alteration 

 processes has been superimposed on the already widespread slightly 

 silicified volcanics. The origins of the pyrophyllite, the pinite, and 



