THE TRAPPEAN ROCKS. 



29 



overlying masses, capping the sandstone of the southern 

 plateau, and rising into the highest hills of this division of 

 the island. They are interposed amid the sedimentary 

 deposits, in huge sheets or beds conformable to the stratifi- 

 cation, and cut through all the rocks alike, from the lowest 

 to the highest, in vertical or slightly inclined dikes, which 

 range continuously across great horizontal distances. These 

 dikes are never observed to wedge out downwards; and no 

 doubt they descend to the level of cooled down lakes of 

 once molten matter, concentric with the crust the common 

 ' source whence they all proceeded, and whose vents or outlets 

 these dikes once formed, in past stages of the earth's history, 

 when the various rocky materials were elaborating. The 

 pressure of an ocean of great depth, or that of other strata, 

 amid which they were poured out, gave these various igneous 



Fig. 8. 

 Relation of the Igneous and Sedimentary Rocks. 



products that density and compactness, which constitute 

 almost the sole differences between them and the modern 

 products of fire thrown out under the pressure of the atmos- 

 phere only. Their differences among themselves are due, less 

 to a difference in mineral or chemical composition than to 

 the arrangement of component parts which would be given 

 by different rates of cooling from a state of fusion. 



The trap rocks of Arran may be arranged in three classes, 

 according to their composition, 



The Felspathic, comprising porphyry, claystone, compact 

 felspar, and pitchstone. 



The Hornblendic, as diorite and amygdaloid. 



The Augitic, as basalt and dolerite. 



