588 GKEISS — HORNBLENDE-SCHIST. [Ch. XXXV 



or superposition in the members of this family ; clay- slate, for example, 

 having been often supposed to hold invariably a higher geological posi- 

 tion than mica-schist, and mica-schist always to overlie gneiss. But 

 although such an order may prevail throughout limited districts, it is 

 by no means universal. To this subject, however, I shall again revert, in 

 the 37th chapter, when the chronological relations of the metamorphic 

 rocks are pointed out. 



The following may be enumerated as the principal members of the 

 metamorphic class : — gneiss, mica-schist, hornblende-schist, clay-slate, 

 chlorite-schist, hypogene or metamorphic limestone, and certain kinds of 

 quartz-rock or quartzite. 



Gneiss. — The first of these, gneiss, may be called stratified, or, by those 

 who object to that term, foliated, granite, being formed of the same ma- 

 terials as granite, rnmely, felspar, quartz, and mica. In the specimen 

 here figuied, the white layers consist almost exclusively of granular fel- 

 spar, with here and there a speck of mica and grain of quartz. The dark 

 layers are composed of gray quartz and black mica, with occasionally a 



Fig. T04. 



Fragment of gneiss, natural dize: section made at right angles to 

 the planes of foliation. 



grain of felspar intermixed. The rock splits most easily in the plane of 

 these darker layers, and the surface thus exposed is almost entirely cov- 

 ered with shining spangles of mica. The accompanying quartz, however, 

 greatly predominates in quantity, but the most ready cleavage is deter- 

 mined by the abundance of mica in certain parts of the dark layer. 



Instead of consisting of these thin laminse, gneiss is sometimes simply 

 divided into thick beds, in which the mica has only a slight degree of 

 parallelism to the planes of stratification. 



The term " gneiss," however, in geology is commonly used in a wider 

 sense, to designate a formation in which the above-mentioned rock pre- 

 vails, but with which any one of the other metamorphic rocks, and more 

 especially hornblende-schist, may alternate. These other members of the 

 metamorphic series are, in this case, considered as subordinate to the true 

 gneiss. 



The different varieties of rock allied to gneiss, into which felspar enters 

 as an essential ingredient, will be understood by referring to what was said 

 of granite. Thus, for exaniple, hornblende may be superadded to mica, 

 quartz, and felspar, forming a syenitic gneiss ; or talc may be substituted 

 for mica, constituting talcose gneiss, a rock composed of felspar, quartz, 

 and talc, in distinct crystals or grains (stratified protogine of the French). 



