68 BOOKS FORMED THEOUaH IGNEOUS AGENCIES 



5. Transitions from compact obsidian into pumiceons forms, 

 due to expansion of included moisture, are common. 



Classification and Nomenclature. — The following varieties 

 are now generally recognized, the distinctions being based mainly 

 on structural features, as with the quartz porphyries. We thus 

 have the granitic-appearing variety nevadite^ the less markedly 

 granular and porphyritic variety rhyolite, and the glassy forms 

 hyaloUpmite, hyaline rhyolite, or obsidian as it is variously 

 called. Hydrous varieties of the glassy rock with a dull pitch- 

 like lustre are sometimes called rhyolite pitchstone. 



The name rhyolite^ from the Greek word />£«>, to flow, it may 

 be stated, was applied by Richtofen as early as 1860 to this 

 class of rocks as occurring on the southern slopes of the Carpa- 

 thians. Subsequently Roth applied the name Liparite to similar 

 rocks occurring on the Lipari Islands. The first name, owing 

 to its priority, is the more generally used for the group, though 

 Professor Rosenbusch in his latest work has adopted the latter. 

 The name Nevadite is from the state of Nevada, and was also 

 proposed by Richtofen. The name Ohsidian as applied to the 

 glassy variety is stated to have been given in honor of Obsid- 

 ius, its discoverer, who brought fragments of the rock from 

 Ethiopia to Rome. The name pantellerite has been given by 

 Rosenbusch to a liparite in which the porphyritic constituent 

 is anorthoclase. 



Rocks of these types occur, in the United States, only in 

 the regions west of the front range of the Rocky Mountains. 

 Apo-rhyolite is the name proposed by Dr. Williams for the 

 devitrified and otherwise altered pre-Cambrian rhyolite found 

 at South Mountain in Pennsylvania. 



2. THE SYENITE-TRACHYTE aROXJP 



This group stands next to that of the granites in point of 

 acidity, from which it differs mainly in the lack of free silica 

 (quartz) as an essential constituent. On chemical grounds this 

 and the next group to be described belong to the intermediate 

 series, standing midway between the acid granites and the basic 

 basalts. As with the last, there are plutonie and effusive forms. 

 These may be described as below: 



