78 BOCKS FOEMED THBOUGH IGNEOUS AGENCIES 



iThe presence of quartz gives rise to the varieties, quartz, qxiartz- 

 augite, and quartz-mica diorites. The name tonalite was given 

 by Vom Eath to a quartz diorite containing the feldspar 

 andesine and very rich in black mica. Kersantite is a dioritic 

 rock consisting essentially of black mica and plagioclase, with 

 accessory apatite and augite, or more rarely hornblende, qnartz, 

 and orthoclase. Professor Eosenbusch has placed the kersantites, 

 together with the porphyritic diorites (camptonites), under the 

 head of diorite lamprophyrs in the class of dike rocks. The 

 name, it should be stated, is from Kersanton, a small hamlet in 

 the Brest Eoads, department of Finistere, France. 



The diorites were formerly, before their exact mineralogieal 

 nature was well understood, included with the diabases and 

 melaphyrs under the general name greenstone. They are rocks 

 of wide geographic distribution, but apparently less abundant 

 in the United States than are the diabases. The lamprophyr 

 varieties are still less abundant, so far as now known. 



(2) THE POBPHYBITES 



Mineral and Chemical Composition. — The essential constitu- 

 ents of the porphyrites are the same as of the diorites, from 

 which they differ mainly in structure. 



Structure. — The porphyrites, as a rule, show a felsitic or 

 glassy ground-mass, in which are embedded quite perfectly 

 developed porphyritic plagioclases, with or without hornblende 

 or black mica. At times, as in the well-known ''porfido rosso 

 antico," or antique porphyries of Egypt, the ground-mass is 

 microcrystalline, forming thus connecting links between the true 

 diorites and diorite porphyrites. Indeed, the rocks of the group 

 may be said to bear the same relation to the diorites in the 

 plagiocla^e series as do the quartz porphyries to the granites in 

 the orthoclase series, or better yet, they may be compared with 

 the hornblende andesites, of which they are apparently the 

 Palaeozoic equivalents. 



Colors. — The prevailing colors are dark brown, gray, or 

 greenish. 



Classification. — According to the character of prevailing 

 accessory mineral, we have hornllende porphyrite, or diorite 

 porphyrite, as it is sometimes called, and mica porpliyrite. 

 When neither of the above minerals are developed in recognizable 



