KINDS OF ROCKS. 447 



albite or oligoclase. Texture granitoid to fine-grained or 

 compact. Color often grayish-white to greenish-white for 

 the coarser kinds ; olive-green to blackish-green for the 

 finer. Very tough. Gr.= 2*7-3 *0, The quartz-hearing and 

 quartz-less kinds constitute two sections having similar va- 

 rieties. Dark-red, brownish-red, and dark-green porphy 

 ritic kinds, compact in base, have been called porphyryte. 

 Metamorphic and eruptive. 



Varieties. — a. Granitoid; granite-like in texture, b. Compact 

 or fine-grained, with the feldspar grains scarcely distinguishable. 

 c. Porphyritic ; the feldspar in crystals in a compact base. d. Slaty ; 

 a dioryte slate or schist, usually chloritic. e. Micaceous, f. Apha- 

 nitic (or Aphanite) ; nearly flint-like in texture. 



An analysis of a dioryte of the Hartz afforded Silica 54'65, alumina 

 15 72, iron sesquioxide 2 00, iron protoxide 6*26, manganese protoxide 

 trace, magnesia 5 '91, lime 7'83, potash 3'79, soda 2 '90, water and ig- 

 nition 1-90 = 100-96. 



The antique red porphyry, or "rosso antico," figured on page 415, 

 is an example of porphyritic dioryte. The crystals, according to the 

 analysis of Delesse, are oligoclase, and have G. =2 67, while the base 

 has G. =2765, it consisting of an intimate mixture of oligoclase and 

 hornblende, with some grains of iron oxide. For the whole mass, accord- 

 ing to Delesse, G. =2.763, but after fusion, only 2*486. Distinct acicu- 

 lar crystals of hornblende occur in it. The rock is sometimes a brec- 

 cia, being made up of angular fragments, either quite distinct from the 

 mass or else shading off into it, but all alike porphyritic. The Mt. 

 Dokhan. in which it occurs — " Porphyrites mons" of Ptolemy — con- 

 tains also red syenyte similar to that of Syene, and a coarse red gran- 

 ite. The " porphyrite " of Ilfeld, of Schonau in Bohemia, and the 

 " quartz- porphy rite " of Koliwansk in the Altai are here referred. 



Propylyte and quartz-propylyte have the same constitution. The 

 former is the prevailing igneous rock of the Washoe district (vicinity 

 of the Comstock lode), in Nevada ; it is a grayish-green rock, yield- 

 ing, on analysis, 64 to 66 per cent, of silica, and containing, along 

 with oligoclase, hornblende, disseminated in minute points, and rarely 

 also biotite (Zirkel). 



Ophite, of the Pyrenees, is greenish black dioryte. 



2. Andesyte. Quartz-Andesyte. — Contains the feldspar an- 

 desifce along with hornblende. As in the preceding, the 

 hornblende is sometimes changed to chlorite. Quartz-an- 

 desyte, or Dacyte, is a quartz-bearing variety. Both kinds 

 occur in the Washoe district. Eruptive. Also metamorphic? 



Banatite and Tonalite are like quartz-dioryte in most characters, 

 but have the feldspar the species andesite. Each contains some bio- 

 tite, the latter much of it. Banatite is from the Banat, and Tonalite 

 from near Tonale, in the Southern Alps. 



Trachydoleryte (of Abich), a dark gray to reddish-brown rock, some- 

 what trachyte-like in aspect, is, in part, near andesyte, it consisting of 



