916 A TREATISE ON METAMORPHISM. 



mechanical sediments. The alterations of these sediments have already 

 been traced out, and it has been seen that from them the common charac- 

 teristic rocks produced are quartzites, quartz-schists, mica- schists, and mica- 

 gneisses, which are occasionally more or less pyroxenic or amphibolitic, 

 but with these latter materials there is apt to be a considerable quantity 

 of calcareous material. Also, staurolite and andalusite, sillimanite and 

 cyanite are very characteristic minerals. Therefore, where certain single 

 minerals are dominant in the schists and gneisses it seems to be a fairly 

 safe conclusion that the rocks are sedimentarjr in origin. Thus, where 

 great rock formations are composed chiefly of quartz, chiefly of calcite or 

 other carbonate, or, with these, silicates which may readily develop by the 

 process of silication, it is comparatively safe to assume that the materials 

 from which the schists are derived are of sedimentary origin. Also, schists 

 composed almost wholly of mica and quartz are likely to be of sedimen- 

 tary origin. 



Just as there are mineral combinations which are generally charac- 

 teristic of sedimentary rocks, so there are others which are generally 

 characteristic of the igneous rocks. From the tuffs and other rocks which 

 develop structures similar to those in the metamorphosed sediments, and 

 are therefore likely to be mistaken for metamorphosed sedimentary rocks, 

 hornblende is likely to develop instead of biotite and muscovite, and thus 

 hornblende-schists and hornblende-gneisses are produced from the metamor- 

 phism of the tuffs instead of mica-schists and mica-gneisses. In the igneous 

 rocks with the hornblende, feldspar is apt to be abundantly present, and 

 frequently this feldspar maj be near the orthosilicate end of the series. 

 In such rocks, also, there is likely to be present a considerable quantity of 

 titanium, which passes into menaccanite, leucoxene, or titanite. In the 

 igneous schists and gneisses there is likely also to be formed large amounts 

 of epidote and zoisite. The prominence of the various minerals mentioned 

 is due to the fact that the igneous rocks have not been depleted in alkalies 

 and alkaline earths. 



But it can not be positively asserted that a metamorphic rock containing 

 metamorphic minerals especially characteristic of igneous rocks may not 

 be of sedimentary origin, for, as already noted, sediments exceptionally 

 contain the unsorted constituents of the original igneous rocks. 



In discriminating between the metamorphosed sedimentary and meta- 



