GEOLOGY. 85 



mica, with a small portion of felspar ; and, like gneiss, 

 is very extensively diffused : it is of a laminar structure, 

 and splits into parallel layers. This rock is also rich in 

 metals, but they are found rather in beds and masses 

 than in veins. 



CLAY SLATE is composed of indurated clay, with 

 particles of quartz and mica. Beds of this slate are 

 sometimes many hundred feet in thickness. Clay 

 slate forms extensive mountain ranges in many parts of 

 the globe, and the soil which is formed from its decom- 

 position is of a fertile nature. Wales furnishes a very 

 large quantity of clay slate for roofing and for writing 

 slates. Hones are clay slates with an an admixture of 

 quartz. 



CHLORITE SLATE is composed of quartz and chlorite, 

 a mineral of a greenish colour. It contains mica, gar- 

 net, &c. 



QUARTZ ROCK is either pure quartz, or quartz mixed 

 with felspar. It forms extensive strata in the Western 

 islands, and is most probably universally diffused. 



PRIMARY MARBLE, or LIMESTONE. This rock is 

 pure carbonate of lime ; and often contains crystals of 

 mica, quartz, felspar, &c. Its colour varies from pure 

 white to a yellowish grey. This limestone comprises 

 some of the finest marble used in sculpture. 



The Rocks that have been already noticed are called 

 Primary, because they are supposed to have been the 

 first formed, and from which the other superincumbent 

 strata were produced. As no organic remains have 

 been discovered in the primary rocks, it has been in- 

 ferred that neither vegetable nor animal life had been 

 then called into being; that the world was then but in 

 an incipient state, and that thousands and tens of thou- 

 sands of years must have rolled on before it became a 

 suitable habitation for man. 



TRANSITIVE SERIES. 



The Transitive Series is divided into two groups, the 

 CAMBRIAN, and the SILURIAN. 



The Cambrian group is so called from Cambria, the 

 ancient name of Wales, because this formation is 



