BUILDING AND ORNAMENTAL STONES. 441 



L.— SCHISTOSE, OR FOLIATED KOCKS. 



(1) THE GNEISSES. 



The gneisses, as already noted, have essentially the same composi- 

 tion as do the granites, from which they differ mainly in their foliated 

 or schistose structure. On account of this schistosity the rocks split 

 in such a way as to give parallel flat surfaces, which render the stone 

 servicable in the construction of rough walls and for street curbing. 

 This structure, which is caused mainly by the arrangement of the mica 

 and other minerals in parallel layers, is, however, a drawback to the 

 uniform working of the stones, and hence they are more limited in their 

 application than are the granites. These rocks are frequently called 

 by quarrymen stratified or bastard granites. The name gneiss, it should 

 be stated, is of German origin, and should be pronounced as though 

 spelled nls, never as nees. For reasons already given the gneisses have 

 been included under the chapter on granites in the present work. 



(2) THE SCHISTS. 



Mica schist is a rock that consists essentially of quartz and mica. It 

 usually possesses a distinct schistose structure, due to the parallel ar- 

 rangement of these minerals, as was noted in the gneiss, from which it 

 may be said to differ only in its lack of feldspar. It is a rock which is 

 supposed to have been formed by the deposition and subsequent crystal- 

 lization of sediments, and consequently the structure of these minerals 

 and their arrangement are markedly stratified. These peculiarities of 

 the schists are not such as to render them favorites for purposes of fine 

 construction. They are, however, in most instances broken out from 

 the ledges with comparative ease, and for rough construction, such as 

 foundations and bridges, as well as for flagging, they are extensively 

 employed. 



The mica of the schists may be either muscovite or biotite, or both ; in 

 short, the schists may be characterized by one or more of the same acces- 

 sories as are the granites and gneisses, and we may have just as many 

 varieties. Through a diminution in the amount of mica these rocks pass 

 into quartz schists, and by an increase in the amount of feldspar into 

 gneisses. The relative amounts of quartz and mica in the schists varies 

 almost indefinitely, the percentage of silica, which is largely depend- 

 ent upon the amount of quartz, varying from 40 to 80 per cent. The 

 finer grained, more compact varieties of mica schist make very fair build- 

 ing material, but the coarser and more schistose varieties are not at all 

 desirable, especially if the mica be biotite and it occurs in great abun- 

 dance. 



In accessory minerals the schists are particularly rich. Some of the 

 more common of these are garnet, feldspar, epidote, cyanite, hornblende. 



