CLASSIFICATION OF EECPTIVE EOCKS— RHYOLITES. 103 



culty in assigning them to their places in accordance with all their natural 

 affinities. Leucite rocks will fall readily among the basalts. Nephelin, 

 when associated with other minerals common to the basic rocks, may be 

 considered as replacing labradorite, and the rock containing it may be 

 assigned to the basaltic group. When associated with orthoclase, as in 

 phonolite, the rock will fall among those trachytes which contain notable 

 percentages of plagioclase. 



It yet remains to speak of those lavas which contain no distinct min- 

 erals, but which are wholly glassy or amorphous, like obsidian, pumice, &c. 

 Here chemical constitution becomes the sole criterion, and although the 

 external or macroscopic facies may often indicate to the trained eye the 

 approximate constitution, the only safe guide to determination is a chemical 

 analysis. 



I. RHYOLITES. The rhyolites are distinguished by their high per- 

 centage of silica and by the presence of orthoclase and free quartz. The 

 number of varieties of texture found in this group is immense. We find 

 some which have an outward semblance to granite; others containing large, 

 beautiful, and perfect crystals of glassy feldspar an inch or more in length, 

 and large grains of quartz imbedded in a compact matrix; others having 

 the coarse, irregularly granular aspect of trachyte; very many with a 

 groundmass full of elongated vesicles like drawn-out glass and holding 

 small crystals; very many which are so vitreous or slag-like that the crys- 

 tals are discernible only with the microscope, and many which exhibit no 

 determinable crystals. So protean are the forms, that the lithologist may 

 well feel discouraged in attempting to resolve the group into intelligible or 

 rational subdivisions. Richthofen has attempted it, however, but it seems 

 to me with very partial success. While he has no doubt divided the more 

 prominent sub-groups, cases are often encountered which neither of them 

 appear to satisfy, and microscopic research indicates that many of the 

 characters he has seized upon are less distinctive than the external appear- 

 ances might at first suggest, and brings to light many others which are of 

 high importance, and which the external appearance does not suggest at all. 

 Considering external characters alone, however, his subdivisions may repre- 

 sent a convenient temporary grouping of the greater part of the rhyolites. 



