HYDRON SILICATES IN ROCKS OF CONCEPTION BAY 141 



PRELIMINARY SILICLFICATTON 



The first process of alteration which operated on the Avondale 

 Volcanics was that of regional silicification. On these preliminary 

 silicified rocks a local series of alterations, those of pyrophyllitiza- 

 tion, pinitization, and further local silicification, were superimposed. 



Fig. 6. — Perlitic structure preserved in pinitized rhyolite. Ordinary light, 

 X35. w = microspherulites; /> = pinite. 



This is evidenced by the following data: (1) few later quartz 

 veins are found traversing the pinite, quartz schists, or pyrophyllite ; 

 (2) fragments of breccia in the volcanic breccias often exhibit 

 quartz veins which stop abruptly at the contact with the matrix, 

 and (3) under the microscope aggregates of sericite scales are found 

 replacing granular quartz which had previously replaced the heart 

 of a spherulite, and these sericite scales finger into and inclose 

 unreplaced fragments of quartz, proving definitely their later 

 origin. 



