544 MANUAL OF NATUEAL HISTOEY. 



Ben-Nevis, and in the porphyry vein at St. Agnes, 

 Cornwall. The columnar Basalt is well shewn in 

 Fingal's Cave in the Isle of StaiFa, and in the 

 Giant's Causeway, in Ireland. No certain traces of 

 organized beings have been detected in these rocks. 



II. — Gneiss Geoup. 



This group is formed by layers of gneiss, syenite, 

 and quartz rocks, alternating with clay-slate, mica- 

 schist, &c, forming the lower portion of the primary 

 stratified rocks. Gneiss is composed of the same 

 elements as granite, but these are arranged in con- 

 torted or undulated layers, appearing as if produced 

 by the disintegration of granite, and then deposited 

 in water. The summits of gneiss mountains are 

 usually rounded, and numerous beds and veins of 

 metals occur in this formation. The ocean wljich 

 deposited the mica, quartz, felspar, &c, was probably 

 of too great a temperature for the support of animal 

 life, no organic remains having been found in the 

 strata of this group. 



III. — Mica-Schist Geoup. 



This group is composed of mica and quartz, in- 

 terlaminated so as to present the appearance of 

 stratification ; crystalline limestone and hornblende 

 also occur ; and the lower strata consist of greenish- 

 coloured slates, with mica and talc, schist, chlorite, 

 and quartz rock. In the mica-schist and other 

 metamorphic rocks, altered by high temperature, 

 metallic ores are found in the greatest abundance. 



