KINDS OF ROCKS. 79 



(9.) Diabase. — A massive hornblende rock, for the most part fine-grained in 

 texture, having a grayish-green to greenish-black color. It is like diorite in 

 composition, except that the feldspar is less abundant and is either labradorite 

 or oligoclase. 



(10.) Aphanite. — Aphanite consists mainly of hornblende, with some feld- 

 spar, and differs from the above in having a very compact or even a flinty 

 appearance. Color, grayish green, greenish white, or gray. It has been called 

 horn-rock. 



Aphanite is sometimes slaty in structure, making an aphanitic slate. This rock 

 in the hornblendic series corresponds to argillite in the mica-bearing series. It 

 is distinguished from argillite by its greater specific gravity. 



3. Feldspathic, Epidotic, and Garnet Rocks having the mass or base compact 

 [cryptocrystalline] . 

 85. These feldspathic rocks may be simply feldspathic, or the base may be 

 partly hornblendic or quartzose. When they contain hornblende, garnet, or 

 epidote, it is apparent in the higher specific gravity. (1.) Some of the light- 

 colored rocks included are translucent and very tough, and contain grass-green 

 diallage (called also smaragdite) in laminae ; these are called euphotides : they 

 consist of feldspar, hornblende, epidote, or garnet. (2.) Others are opaque 

 and often dark-colored, and usually contain crystals of feldspar disseminated 

 through the mass; these are porphyries, or porphyroid rocks: they consist 

 mainly of feldspar. (3.) The rocks consisting of the base of the euphotides 

 without the diallage are called petrosilex. 



a. Porphyroid Rocks. 



(1.) Porphyrine. — Opaque, or nearly so. Colors, white, brown, red. In lustre 

 and fracture much resembling jasper, from which it differs in being fusible 

 before the blowpipe. Consists of feldspar : sometimes quartzose. 



(2.) Feldspar-Porphyry. — Same as the last, with disseminated crystals of 

 feldspar. 



A quartzose variety of feldspar-porphyry has the base consisting of feldspar 

 with some quartz. A variety of this kind occurs in eastern Massachusetts near 

 Lynn. Crystals of quartz as well as feldspar are distributed through the mass. 

 Colors, brownish-red and purplish. Specific gravity, 2.606. 



(3.) Horxblendic Porphyry (Diabase Porphyry). — The antique green por- 

 phyry of Greece (southern Morea) is here included. Specific gravity, 2.91-2.932. 

 Color, dark green; disseminated feldspar crystals, large, greenish white. Com- 

 position of the base: Silica, 53.55, alumina, 19.43, protoxyd of iron, 7.55, prot- 

 oxyd of manganese, 0.85, lime, 8.02, magnesia and alkali, 7.93, water, 2.67. The 

 iron and magnesia indicate the presence of hornblende or pyroxene. 



Porcelanite, or Porcelain- Jasper . — A baked clay, having the fracture of flint 

 and a gray to red color : it is somewhat fusible before the blowpipe, and thus 

 differs from jasper. Formed by the baking of clay-beds through the heat from 

 intrusive igneous dikes. Such clay-beds are sometimes baked to a distance 

 of thirty or forty rods from the dike. 



