THE PLUTONIC ROCKS. 



4G3 



summits, instead of presenting needle-like points, are usually 

 rounded. Grneiss is one of the most metalliferous of the 

 primary rocks, its ores occurring both in beds and veins, but 

 more frequently in the latter form. The localities in which 

 gneiss occurs in this island are the north and north-western 

 parts, and the "Western Isles of Scotland. 



THE PLUTONIC, OR MELTED BOCKS. These are usually 

 divided into two classes, the trap-rocks, comprising the 

 basalts, porphyries, and their associate substances, and the 

 granites and their kindred rocks. The trap-rocks commonly 

 occur in the shape of veins or dikes, as shown in fig. 301, 

 which represents a vein of porphyry, traversing argillaceous 

 schist, at St. Agnes, Cornwall. 



Their columnar arrangement is developed at the cave of 

 Fingal, in the Isle of Mull. (Fig. 302.) 



FIG. 302. Basaltic Columns at the Entrance to Fingal's Cave . 



The following illustration (fig. 303), portrays an instance 

 occurring in the vicinity of Carlsbad, of a vein of granite 

 traversing a mass of granite older than itself, and proving 

 the different ages of this rock. 



For more minute details we refer to the works of Hutton, 



