ON THE FELSITES AND ASSOCIATED ROCKS OF 

 MOUNT READ AND VICINITY. 



By W. H. Twelvetrees, F.G.S., and W, F. Petterd^ 



C.M.Z.S. 



Associated with the schists of Mount Read and district 

 are some obscure igneous rocks, siliceous in nature, com- 

 pact in grain, often slightly schistose, which are what the 

 field geologist calls felsites or felstones. These terms,., 

 however convenient, need explanation, for in different 

 countries they carry different meanings. In Germany the 

 w^ord felsite (or micro-felsite) is applied to the compact 

 homogeneous-looking groundmass of quartz porphyries.. 

 These porphyries are the acid volcanics of pretertiary age. 

 In England, on the other hand, felsite designates the rock,, 

 not its groundmass merely. If the petrographers of each 

 countr}^ could throw over the historical significance of 

 their terms and come to some international agreement as 

 to rock nomenclature and the meaning to be attached to 

 terms, much of the present deplorable confusion would 

 disappear. At present the discordance is considerable.. 

 The following extracts serve to indicate the English 

 usage : — 



Cole.* — Quartz-felsite=Eurite=" the fine-grained and compact 

 forms of granite." '' Felsite is so differently used by different 

 writers that its reputation as a rock name is lost." 



HatchA — Felsite= "' the acid quartzo-felspathic lavas — the devi- 

 trified rhyolites and obsidians." 



Ihitley.l — Felsite= " devitrified obsidians and pitchstones. Felsites 

 are not exclusively devitrified rocks ; in some cases they occur 

 as dykes and then approximate to the micro-granites or grano- 

 phyres." Quartz-telsite=" apophyre.s of deep-seated granite 

 masses (Elvan group)." 



Harher.\ — Felsites="acid intrusives. The nama 'felsite,' or, if 

 containing evident phenocrysts of quartz, ' quartz-felsite,' has 

 been applied in this country not only to these rocks but alsa- 

 to nmny volcanic rocks (acid and intermediate) , and their usage- 

 lacks precision and significance." 



* Aids in Practical Geology, 1893, p. 201. 



t An Introduction to the Study of Petrology, 1891, p. 85. 



X Granites and Greenstones, p. 15. 



^ Petrology for Students, 1897, p. 100. 



