Oh. XXXV.] GNEISS. 733 



of other rocks, whether rounded or angular. They sometimes break 

 out in the central parts of narrow mountain chains, but in other cases 

 extend over areas of vast dimensions, occupying, for example, nearly 

 the whole of Norway and Sweden, where, as in Brazil, they appear 

 alike in the lower and higher grounds. In Great Britain, those mem- 

 bers of the series which approach most nearly to granite in their com- 

 position, as gneiss, mica-schist, and hornblende-schist, are confined to 

 the country north of the rivers Forth and Clyde. 



However crystalline these rocks may become in certain regions, 

 they never, like granite or trap, send veins into contiguous formations, 

 whether into an older schist or granite or into a set of newer fossil- 

 iferous strata. 



Many attempts have been made to trace a general order of suc- 

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

 example, having been often supposed to hold invariably a higher geo- 

 logical position 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 rela- 

 tions 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 materials as granite, namely, felspar, quartz, and mica. In the 

 specimen here figured, the white layers consist almost exclusively of 

 granular felspar, with here and there a speck of mica and grain of 

 quartz. The dark layers are composed of gray quartz and black mica, 



Fig. 756. 



Fragment of gneiss, natural size ; section made at right angles to the planes of foliation. 



with occasionally a grain of felspar intermixed. The rock splits most 

 easily in the plane of these darker layers, and the surface thus exposed 

 is almost entirely covered with shining spangles of mica. The accom- 

 panying quartz, however, greatly predominates in quantity, but the 

 most ready cleavage is determined by the abundance of mica in cer- 

 tain parts of the dark layer. 



