m EOCKS POEMED THEOUGH IGNEOUS AGENCIES 



Classification and Nomenclature. — Owing to the very slight 

 development of the accessory minerals, mica, hornblende, etc., 

 it has been found impossible to adopt the system of classifica- 

 tion and nomenclature used with the granites and other rocks. 

 Vogelsang's classification as modified by Eosenbusch is based 

 upon the structure of the ground-mass as revealed by the micro- 

 scope. It is as follows : 



Ground-mass tolocrystaUine granular Micro-granite.- 



Ground-mass holocrystalline, but formed of quartz and feld- 

 spar aggregates, rather than distinct crystals Granophyr. 



Ground-mass felsitic Eelsophyr. 



Ground-mass glassy Vitrophyr. 



Intermediate forms are designated by a combination of the 

 names, as granofelsophyr, felsovitrophyr^ etc. The name felsite 

 is often given to members of this group in which the porphyritic 

 constituents are wholly lacking. The names felsfone and petro- 

 silex were once common, though now out of use. Elvaniie is a 

 Cornish miner's term and too indefinite to be of great value. 

 Ewiie, now little used, was applied to felsitic forms. The 

 name felsite pitchstone or retinite has been given to a glassy 

 form with pitch-like lustre, such as occurs in dikes cutting the 

 old red sandstone on the Isle of Arran. Kugel porphyry is a 

 name given by German writers to varieties showing spheroids 

 with a radiating or concentric structure. Micropegmatite is the 

 term not infrequently applied to such as show under the micro- 

 scope a pegmatitic structure. (Fig. 2, PL 5.) Various popular 

 names, as leopardite and toadstone, are sometimes applied to such 

 as show a spotted or spherulitic structure. 



(3) THE LIPAEITES 



Mineral Composition. — These rocks may be regarded as the 

 younger equivalents of the quartz porphyries, or the volcanic 

 equivalents of the granites, having essentially the same mineral 

 and chemical composition. The prevailing feldspar is the clear 

 glassy variety of orthoclase known as sanidin; quartz occurs in 

 quite perfect crystal forms often more or less corroded by the 

 molten magmas, as in the porphyries, and in the minute, six- 

 sided, thin platy forms known as tridymite. The accessory 

 minerals are the same as those of the granites and quartz, 

 porphyries. 



