848 THE WONDERS OF GEOLOGY. Lect. VIII. 



One specimen was twenty feet long and twelve in dia- 

 meter." * 



It would appear, therefore, that silex and quartz may be 

 deposited in two ways : by the action of thermal waters 

 {ante, p. 94), and by the influence of intense heat. The 

 remark of Professor Keilhau here applies with equal force as 

 to the experiments which called it forth : — " The greater 

 part of the crystalline rocks have an entirely hidden origin 

 and development. Chemistry alone cannot decide this 

 question, for the same minerals can be composed in nature 

 by different processes. By the side of the celebrated fact 

 which showed the possibility of the formation of felspar by 

 heat, we can now place experiments which prove the pos- 

 sibility of producing felspar in the moist way." 



The rocks of this system are widely expanded over 

 Europe and America ; and everywhere abound in metal- 

 liferous veins. They are of various ages ; and their meta- 

 morphic character is proved by the occurrence of gneiss, 

 mica-schist, and talcose-schist, in the Alps and Apennines, 

 under circumstances showing that their crystalline structure 

 has been acquired since the origin of many of the fossili- 

 ferous strata ; even in some instances long after the depo- 

 sition of those which repose directly upon them. On the 

 other hand, " the gneiss of Kinnekulle in Sweden, and of 

 the Falls of Montmorenci, and many of the plutonic rocks 

 of the Aidarondach mountains west of Lake Champlain, 

 are of older date than any strata in which organic remains 

 have yet been found." f 



24. Basalt, or Trap. — The consideration of the ancient 

 volcanic rock, designated by the various names of Whin, 

 Trap, Basalt, and Clinkstone, will next engage our attention. 

 Basalt occurs in veins or dykes, which traverse rocks of all 



* Mr. Darwin, On the Geology of the Falkland Islands. Geological 

 Journal, vol. ii. p. 267. 



f Mr. Lyell's Travels in America, p. 129. 



