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Parr IL § vi] CRYSTALLINE ROCKS—MASSIVE. 143 
3 ie MS 
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\. 3 c = 
_ they are basic compounds, though in a few of them free quartz, as an 
original constituent, can be detected with or without the microscope. 
In structure they present a range similar to that of the orthoclase 
rocks. Some of them are thoroughly crystalline (diorite), thongh they 
never attain the coarseness of texture which is often reached by granite. 
Many of them are characteristically porphyritic (porphyrite), while 
in some cases they assume a completely vitreous texture (tachylite). 
They may be arranged in groups, according as the predominant 
mineral after the felspar is hornblende, mica, augite, or diallage. 
Diorite (Greenstone in part).—A crystalline-granular aggregate 
of a triclinic felspar and hornblende, usually with magnetite and 
apatite. The proportions between the felspar and hornblende vary so 
greatly as to give rise to considerable differences in the colour and 
composition of the rock. The felspar when fresh shows its twin 
lamellations, but is frequently tinted green (from decomposition 
of the hornblende), and more or less decayed. The hornblende is 
- dark green or black with vitreous lustre on the cleavage planes when 
fresh, but apt to decompose and to give rise to secondary products, 
such as epidote and chlorite. The apatite occurs in fine needles, 
usually only discernible under the microscope. There is commonly 
no trace of any base between the ingredients of the rock, which thus 
presents a thoroughly crystalline or granitoid structure. Average 
chemical composition :—silica, 54; alumina, 16°0—18; potash, 1:5 
—2:5; soda, 2—8; lime, 6—7°5; magnesia, 6°0; oxides of iron and 
manganese, 10—14 ; mean specific gravity about 2:95. 
Among the varieties of diorite the following may be enumerated. 
Quartz-diorite, containing free quartz, usually only to be detected by 
microscopic examination. Aphanite (aphanitic-diorite) an exceedingly 
compact rock, in which the component minerals are not macroscopl- 
cally distinguishable. A variety containing dispersed crystals of 
felspar or hornblende is termed diorite-porphyry. Corsite, a granitoid 
mixture of greyish-white anorthite, blackish-green hornblende and 
some quartz, which here and there have grouped themselves into 
globular aggregations (orbicular diorite, kugel-diorite, Napoleonite). 
Mica-diorite, containing abundant dark mica, which may even replace 
the hornblende. 
Diorite occurs as an eruptive rock under conditions similar to 
those of quartz-porphyry and syenite. It is found among Palxozoic 
voleanic regions, as in North Wales, in “neck ”-like masses which 
may mark the position of some of the volcanic orifices of eruption. 
It occurs also in association with granite and the crystalline schists in 
such a manner as to suggest a community of origin with these rocks.’ 
1 On diorite, its structure and geological relations, consult the memoir on Belgian 
plutonic rocks by De la Vallée Poussin and A. Renard, Mem. Acad. Royale Belg. 1876 ; 
Behrens, Neues Jahrb. Min. 1871, p. 460; Zirkel, Microscopical Petrog. p. 83. : 
Phillips, Q. J. Geol. Soc. xxxii. p. 155 and xxxiv. p. 471—valuable papers in which the 
constitution of some of the “greenstones ” of the older geologists is clearly worked out. 
Many of these ancient rocks are there shown to be forms of doleritic lava, and the 
change of their original augite into hornblende is traced. 
