THE NONMETALLIC MINEKALS. 285 



Although the basal cleavage which permits of the ready splitting of 

 the mica into thin sheets is the only one sufficiently developed to be 

 of economic importance, the mica as found is often traversed by sharp 

 lines of separation, called gliding planes, which may, by their abun- 

 dance, be disastrous to the interests of the miner. Such partings, or 

 gliding planes, supposed to be induced by pressure, are developed at 

 angles of about 66i with the cleavage, and may cut entirely through 

 a block or extend inward from the margin only a short distance and 

 come to an abrupt stop. In many cases the mica is divided up into 

 long narrow strips, from the breadth of a line to several inches in 

 thickness, with sides parallel, and as sharply cut as though done with 

 shears. (Specimens Nos. 62517, 63134, U.S.N.M.) 



The imperfections in mica are due to inclosures of foreign minerals, 

 as flattened garnets, to the presence of free iron oxides, often with a 

 most beautiful dendritic structure, to the partings or gliding planes 

 noted above, and to crumplings and V-like striations which destroy its 

 homogenity. (Specimens Nos. 63139, 44450, U.S.N.M.) 



Occurrence. Mica in quantity and sizes to be of economic impor- 

 tance is found only among the older rocks of the earth's crust, par- 

 ticularly those of the granite and gneissoid groups. Muscovite and 

 biotite are among the commonest constituents of siliceous rocks of all 

 kinds and ages, while phlogopite is more characteristic of calcareous 

 rocks. It is, however, only when developed in crystals of consider- 

 able size in pegmatitic and coarsely feldspathic veins, or, in the case 

 of phlogopite, in gneissic and calcareous rocks associated with erup- 

 tive pyroxenites, that it becomes available for economic purposes. 

 The associated minerals are almost too numerous to mention. The 

 more common for muscovite are quartz and potash feldspar, which 

 form the chief gangue materials in crystals and crystalline masses, 

 sometimes a foot or more in diameter. With these are almost inva- 

 riably associated garnets, beryls, and tourmalines, with more rarely 

 cassiterite, columbite, apatite, fluorite, topaz, spodumene, etc. In- 

 deed, so abundant are, at times, the accessory minerals in the granitic 

 veins, and so perfect their crystalline development, that they furnish 

 by far the richest collecting grounds for the mineralogists. Of these 

 minerals the quartz and feldspars are not infrequently contemporane- 

 ously with the mica and utilized in the manufacture of pottery and 

 abrasives. 



The origin of these pegmatitic veins is a matter of considerable 

 doubt. The finer grained pegmatites are, in certain cases, undoubted 

 intrusives, though to some authorities it seems scarcely possible that 

 the extremely coarse aggregates of quartz, feldspar, and mica, with 

 large garnets, beryls, and tourmalines, can be a direct result of cooling 

 from an igneous magma. To such it seems "more probable that they 

 are portions of an original rock mass altered by exhalations of fluor- 



