356 C. A. SUSSMILCH AND W. G. STONE. 



and probably also of ferrous carbonate. This is borne out 

 by the analysis, which shows that some magnesia and 

 probably some ferrous oxide are combined with the carbon 

 dioxide, there being insufficient lime for that purpose. 



Chlorite is a product mainly of the alteration of the 

 felspar phenocrysts. It is pale green in colour and almost 

 isotropic. 



The groundmass of the rock is microcry stall ine and com- 

 posed essentially of orthoclase felspar and its decomposition 

 products with a little quartz. In places it is much obscured 

 by alteration, mainly the kaolinization of the felspar. The 

 rock is traversed by veins of secondary quartz and much 

 of this mineral occurring in the groundmass is of secondary 

 origin. The groundmass also shows evidence of rather 

 extensive devitrification. 



The rhyolite porphyry from the eastern leg of the anti- 

 cline is seen under the microscope to be identical with that 

 described above. The phenocrysts are in general larger. 

 The felspars show a similar microperthite structure, and 

 are in a more advanced stage of alteration, being in some 

 cases completely altered to a sericitic material, or to 

 a mixture of chlorite, sericite, kaolin, and calcite. The 

 biotite is not so bleached and is consequently more strongly 

 pleochroic. The groundmass is somewhat finer grained, 

 and exhibits a well marked flow structure under the 

 microscope, which may in part, however, be really a 

 foliated structure. The groundmass also shows consider- 

 ably more alteration, mainly in the formation of sericite- 

 like minerals; these appear to have developed mainly along 

 lines of flow, although in some cases transverse to these 

 directions. 



The rhyolite porphyry from this locality also shows evid- 

 ence of having been subjected to severe dynamic action 

 subsequent to consolidation. The biotite crystals are much 



