FELSITIC AND RHYOLITIC ROCKS. 



37 



The Felsite and Rhyolite Series oj Imperfectly Crystallised Granitic 



Mocks. 



When the materials which would otherwise have constituted a 

 granitic rock solidify under moderate pressure or quickly, then a 

 large part of the rock remains in the form of a paste, which to the 

 unaided eye appears to be uncrystalline. This paste, in old rocks, is 

 termed felsite. In some felsites crystals of quartz, felspar, and mica, 

 &c., occur, but in others crystals have hardly ever or never been 

 formed. Quartz-felsite is an ancient Rhyolite. 



NAME. 



Granite- 

 porphyry 



Elvanite or 

 quartz - 

 porphyry 



Felstone or 

 felsite-rock 

 Petrosilex 

 or Eurite 



Pitchstone 



Rhyolite, or 

 quartz- 

 trachyte 



Perlite 



Obsidian 



COMPOSITION. 



Compact felsitic matrix with crys- 

 tals of quartz, felspar, mica, or 

 chlorite. 



Compact, felsitic matrix with crys- 

 tals of quartz and felspar. 



Compact felsitic matrix sometimes 

 with crystals of quartz and fel- 

 spar. An ancient rhyolite or 

 trachyte. Silica 70-80 p. c. 



A vitreous condition of any of 

 the preceding rocks, but contains 

 more combined water. Often 

 contain s balls of felsite, and some- 

 times crystals of sanidine, quartz, 

 and mica. Silica, 63-70 p. c. 



A compact felsitic matrix, with 

 crystals of quartz, sanidine, and 

 mica. Hence formerly named 

 trachyte-porphyry. 



An enamel - like grey rhyolitic 

 rock,containing concentric grains 

 like pearls, with occasional crys- 

 tals of quartz, sanidine, and mica. 



Dark volcanic glass, often with 

 crystals of sanidine, and some- 

 times balls of felsite. 



Pumice-stone! Obsidian converted into a froth by 

 the multitude of steam cavities 

 which it contains. It is always 

 white. 



LOCALITIES. 



Drusenthal and Schmiede- 

 feld in Thuringia. 



Cornwall, at Penzance, near 

 Marazion, &c. 



North Wales, &c. ; Skye, 

 Mull. On the flanks of 

 granite it is termed felsite 

 schist or Halleflinta. 



Arran, Rum, Mull ; Mexico, 

 Peru, Iceland, Auvergne. 



Lipari Islands, Ponza Is- 

 lands, Euganean Hills, 

 Schemnitz in Hungary. 



Lipari Islands, Schemnitz 

 in Hungary, Tokay, &c. 



Lipari Islands, Peak of Tene- 

 riffe, Iceland, Mexico. 



Generally with obsidian but 

 sometimes without it, as at 

 the Laacher See, near An- 

 dernach, on the Rhine. 



Like the granites these rocks are of all geological ages. Felstone 

 appears to be a form in which granite consolidates when, having been 

 mred out as a lava- stream, it cooled under water. Felstone rocks 

 )m North Wales are commonly used for paving the roadways in 

 my parts of London. 



