SLATE; SCHIST; GNEISS 771 



etc., in which the other mineral was supposed to be quartz. If 

 feldspar was present, the rock was called a gneiss. (Rosenbusch- 

 27.) The structural use of the term seems, however, to be the 

 preferable one. 



Gneiss was originally defined as a banded or foliated rock hav- 

 ing essentially the composition of granite, i. c, quartz, feldspar and 

 mica or hornblende. The quartz and feldspar were taken as dis- 

 tinctive minerals and the others added as qualifying prefixes. Thus, 

 mica gneiss meant a schistose or foliated rock, consisting of quartz, 

 feldspar and mica ; and hornblende gneiss, the same combination 

 with hornblende added or replacing the mica. Such gneisses were 

 supposed to be metamorphic derivatives of granites. Since, how- 

 ever, many rocks, to which the name gneiss has been commonly 

 applied, prove not to have the composition above given, the petro- 

 graphic use of the name must be abandoned in favor of the struc- 

 tural use, or the use of the term gneiss as well as schist must be 

 very much restricted. Van Hise's proposition to use the term as 

 a purely structural one seems to be the most satisfactory solution, 

 and accordingly we may define gneiss as a banded metamorphic 

 rock in which crystalline structure has been developed and in which 

 the bands are petrographically unlike one another and consist of 

 interlocking mineral particles. The bands in different gneisses are 

 of variable thickness, ranging from a fraction of a centimeter to 

 many centimeters (Van Hisej. There may also be a similar varia- 

 tion and range in thickness of the different bands of the same gneiss. 

 Thus the fundamental distinction between gneiss and schist is the 

 banded character of the former as compared with the homogeneous 

 character of the latter. This homogeneous character is still more 

 strongly expressed in the slates in which the cleavable mineral par- 

 ticles are not visible as they are in the schists. 



Gneiss may be derived either from igneous or from sedimentary 

 rocks. When derived from igneous rocks the parallel arrangement 

 of the mineral particles which results in cleavage is more often 

 lacking. 



General Terms for Metamorphic Rocks. 



Two terms have come into use for general designation of meta- 

 morphism in rocks. These are iiieta and apo. Meta is used as a 

 prefix to any rock name and designates that the rock has been 

 altered without stating how or to what degree. Thus we may say 

 meta-arenytc, meta-shale, meta-granite, meta-diorite, etc. Rocks 

 already metamorphosed to a certain degree may undergo a second 



