KINDS OF ROCKS. 69 



(6.) Mica Slate. — Of the same constitution as mica schist, but 

 with a smoother surface, the mica being not visibly in scales, unless 

 magnified. It is intermediate between mica schist and clay slate. 



(7.) Hydromica Slate. — Like the last in general characters, but 

 containing a hydrous mica, and, therefore, feeling more or less greasy, 

 and looking pearly. On account of this peculiarity, it was formerly 

 considered a talcose or magnesian slate, and has been called also talcoid 

 slate. A chloritic variety is common. Sericite slate and Paragonite 

 slate or schist are related rocks. 



(8.) Clay Slate or Argillyte. — A fine-grained slaty rock of 

 various colors, grayish to black, and sometimes greenish, reddish, 

 purplish. The evenly splitting kinds are roofing slate and writing 

 slate. It consists usually of pulverized quartz and feldspar, with 

 sometimes a little chlorite. Another kind of the same color contains 

 much chlorite. Another, undistinguishable by the eye, contains no 

 alkalies, and hence no feldspar. The common imbedded minerals are 

 andalusite, staurolite, garnet, phyllite (chloritoid). 



2. Hornblendic Series. 



The hornblendic series commences in a granite -like species, called 

 syenyte, containing quartz and one or more feldspars, along with horn- 

 blende in place of mica. Hornblende is not so cleavable into leaves 

 as mica, and is brittle instead of elastic. It is also tough and heavy ; 

 and hence hornblende rocks are generally tough and heavy, the 

 specific gravity between 2-7 and 3-5. From syenyte the series runs 

 down through syenytic gneiss to hornblendic schist and hornblende 

 rock ; then to rocks of very even texture and compactness, called 

 diabasyte and aphanyte, the last like hornstone in fracture and sur- 

 face. Often pyroxene replaces hornblende ; and occasionally epidote. 



The species of rock depends largely on the kind of feldspar pres- 

 ent : syenyte and hyposyenyte contain chiefly orthoclase ; true dioryte 

 contains oligoclasc or albite ; hypersthenyte and diabasyte contain 

 labradorite, passing into andesite and anorthite, and are included by 

 Hunt under the general name anorthosyte. 



The hornblendic series blends laterally with the magnesian series, especially through 

 the chloritic rocks of the latter, chlorite being near hornblende and pyroxene in com- 

 position, though containing water. It also blends with the mica series, through 

 granites and schists that contain both hornblende and mica. Through the pyroxenic 

 varieties, it also passes into the igneous series. 



CI.) Syenyte. — Resembles granite, but contains, in place of mica, 

 the mineral hornblende, which is in cleavable grains and either black 

 or greenish black in color. The feldspar may be orthoclase or oligo- 

 clase, and sometimes the quartz is nearly wanting. Named from 



