142 A. F. BUDDINGTON 



The spherulitic rhyolites are the rocks which exhibit most 

 clearly the manner in which the silicification has taken place. 

 The silica here is present for the greater part as a milky-white 

 chalcedonic quartz, but vitreous, granular, and white vein-like 

 quartz as well as quartz crystals are common. The chalcedony 

 usually forms the outer borders of the concentric crescent-shaped 

 areas, and of the hearts of the replaced zones of the spherulites, 

 while the inner portion may be recrystallized to form comb struc- 

 ture through the interlocking of quartz crystals, or little geodes 

 with terminated crystals projecting into a small cavity, or granular 

 vitreous quartz. When only one form of the silica is present it is 

 very generally of a chalcedonic nature. It is interesting to note 

 that while at Manuels it is the spherulites of the rhyolites which 

 are most generally replaced, at Clarenville it was the groundmass 

 which was replaced instead of the spherulites, because of the 

 perlitic structure of the former offering the most favorable surfaces 

 for attack. The banded rhyolites are often lined or streaked with 

 quartz veins parallel to the planes of flow, which in some cases are 

 a result of replacement and in others of vein filling. In thin 

 section, lenses, lines, and granules of secondary quartz are found 

 to be a common characteristic of the slightly siliciried banded 

 flows. Fig. 7 illustrates the preservation of the perlitic structure 

 in the quartz which is replacing the groundmass of a spherulitic 

 rhyolite from Clarenville. The perlitic cracks are outlined by 

 sericite. 



That the silicification of the rhyolites has been due to secondary 

 metasomatic processes and is not a primary phenomenon is indi- 

 cated by (i) the interruption of fluxion lines by the replacing 

 quartz, (2) by the presence of unsupported fragments of unreplaced 

 rhyolite in the quartz areas, and (3) by the preservation in the 

 quartz of original structures of the rhyolite, such as the perlitic 

 structure. 



The first stage then in the alteration of these volcanics has 

 consisted in the silicification under relatively static conditions by 

 hot siliceous waters of rhyolite flows which may be represented as 

 having had a similar chemical composition to the present relatively 

 unaltered gray felsites. Analysis No. 1 may be taken as the com- 



