780 A TREATISE ON METAMORPH1SM. 



purely structural, and to its structural meaning the term will be rigidly 

 confined in this treatise. 



However, the term schist has been widely used both in a structural 

 sense and as the name of definite rocks; and indeed this double usage is 

 found even in Geikie, whose definition restricts the term to a structural 

 meaning. Illustrations of the use of the term schist both as the name for a 

 definite rock and with a structural signification are furnished by the terms 

 mica-schist, chlorite-schist, and hornblende-schist as generally used. In the 

 case of mica-schist, the term means that the rock is a schistose mica-quartz 

 rock. As here proposed such a rock should be called a mica-quartz-schist. 

 The omission of the term quartz from the name of this rock arose naturally, 

 since this is one of the most abundant varieties of schists, and since in it mica 

 is so conspicuous and quartz so inconspicuous. After it was discovered that 

 quartz is usually equally or more important than mica in most schists, 

 usage came to imply the presence of quartz in the rock called mica-schist. 

 But as the study of rocks continued, other schists were found in which the 

 conspicuous constituent is not mica, but chlorite or hornblende; and such 

 rocks were immediately called chlorite-schists or hornblende-schists, the 

 implication being that quartz was the remaining chief constituent How- 

 ever, when close microscopical studies were begun, they showed -that in the 

 chlorite-schists and hornblende schists quartz might or might not be an 

 abundant constituent, its place being taken by feldspar or other minerals, and 

 thus chlorite-schist or hornblende-schist, as used by most English-speaking 

 o-eologists, means a schistose rock containing chlorite or hornblende and 

 other unnamed minerals. Thus the original scheme broke down. The 

 German petrographers soon saw this, and where schistose rocks were found 

 in which feldspar was an important constituent they proposed to call the rock 

 a gneiss, since feldspar was recognized as an important constituent in the 

 rocks which had before been called gneiss. When the English and American 

 geologists ascertained that a schist, as they use the term, might contain 

 various combinations of minerals, they were inclined to return strictly to a 

 structural sense, as indicated by the quotation from Geikie already made. 

 But as yet this plan has not been carried out consistently in reference to 

 individual rocks, and the usage of combining a structural meaning with 

 mineralogical implications is still common. It seems to me that clear dis- 

 crimination can be obtained only by restricting the term schist to the 



