GEOLOGY. 495 



primitive. The mountain ranges and cliffs in this series, less 

 rugged and softer in their outline than the primitive rocks, and 

 with wider valleys and sides less abrupt, present, however, a 

 bolder outline than those of what are termed the secondary for- 

 mation. Most of the Transition rocks are distinctly stratified ; 

 the strata are frequently vertical, and, like those of the primi- 

 tive class, exhibit the same general direction through large 

 tracts of country. Thus the strata in the great high land 

 which ranges from St AbFs Head to the Irish sea, and which 

 is almost composed of transition rocks, range everywhere nearly 

 from N. E. to S. W. 



The crystallization of the transition rocks appears to be less per- 

 fect than that of the primitive rocks, because the parts of which 

 they are composed have a lower degree of lustre, inferior hard- 

 ness, less translucency, and colours of less purity. They are, 

 besides, distinguished by the important circumstance of con- 

 taining fossil organic remains. These remains are of animals 

 low in the zoological scale, as corals, shells ; and of vege- 

 tables belonging to the class Cryptogamia. Hence it has 

 been concluded that the primitive class of rocks existed prior to 

 the creation of animals ; and that those classes of living beings 

 whose remains are found in the transition series had existed pre- 

 viously or contemporaneously with them. Transition rocks fre- 

 quently abound in ores of various descriptions. The mining 

 districts of Leadhills and Wanlockhead in Scotland are in 

 transition rocks ; the lead and silver mines in the Hartz, and 

 many of those in Mexico, are in rocks of the same description. 

 Gems are comparatively rare in this class of rocks. The rocks 

 comprehended in this division are greywacke, clay-slate, lime- 

 stone, trap, granite, syenite, porphyry, serpentine, gneiss, mica- 

 slate, and quartz-rock, thus characterized by Professor Jameson. 



" 1. Greyrvacke is a conglomerated looking rock, with a basis of 

 clay-slate, including angular and various shaped portions (by many 

 considered as fragments) of clay-slate, flinty-slate, quartz, felspar, 

 &c. and occasionally scales of mica. When the imbedded masses be- 

 come small, and the mass slaty, it is named greyrvacke-slate. 



" 2. Clay-slate This rock is of the same general nature with 

 primitive clay-slate, but differs from it in having less lustre, and in 

 sometimes containing fossil plants and fossil shells. 



" 3. Limestone. It is more compact, and much smaller granular, 



