CONSTRUCTION OF THE EARTH. 



147 



under the pressure of rocky strata, which it had lifted 

 up, and which was covered with water probably of con- 

 siderable depth. This could not happen without crea- 

 ting great commotion, especially if there were fractures in 

 the superincumbent strata, letting down the water upon 

 the surface of the red molten mass. Hugh Miller, in his 

 magnificent description of what may be imagined to have 

 occurred in the emergence of the trap rocks^ speaks of 

 steam and flame issuing from the fissures up through the 

 boiling waters. This may all be correct ; but when he 

 speaks also of the heavens being dark with ashes, as if a 

 real volcano were in action, the facts, which the rocks now 

 reveal to us hardly wan-ant such imaginings. Though fire 

 is the agent in both cases, the formation of trap rocks dif- 

 fers very decidedly from the building up of a volcanic cone. 

 The open throat or chimney, through which materials of 

 various kinds are thrown up from the volcano, is wanting 

 when the trap rocks are elevated into their position. 



It is in the* cooling and solidification of the molten 

 mass that the prismatic, pillar-like form is sometimes so 

 perfectly assumed, as exhibited in the Giant's Causeway, 

 Fingal's Cave, etc. The igneous origin of these prisms 

 fairly entitle them to the name of "furnaced pillars," 

 which the Ettrick Shepherd has given them. 



231. Veins. There are found in the rocks what are 

 called veins that is, fissures filled with rocky material, 



or with metallic ores, or with 

 both. When they are filled 

 with rocky material, this is 

 sometimes the same with the 

 rock itself in which the veins 

 are, and sometimes it is of a 

 different composition. Some- 

 times one vein traverses an- 

 other, as is shown in Fig. 

 77. This is often the case 

 with veins of granite. Such 



