basaltic dikes. 



121 



and other gaseous fluids. The lava sometimes flows 

 over the edge of the crater, and thus thickens and 

 strengthens the sides of the cone; but sometimes it 

 breaks down on one side, and often it flows out from 

 a fissure at the base of the hill." 



Of existing volcanoes, none exhibit volcanic ac- 

 tion on a more sublime scale than that of Kirauea, 

 in Hawaii, in the Pacific Ocean. This whole island, 

 which covers an area of 4000 square miles, is a com- 

 plete mass of volcanic matter, in different states of 

 decomposition, perforated by innumerable craters, 

 and rising to an altitude of 16,000 feet. "It is, in 

 fact," says Dr. Mantell, " a hollow cone, with nu- 

 merous vents, over a vast incandescent mass, which 

 doubtless extends beneath the bed of the ocean ; the 

 island forming a pyramidal funnel from the furnace 

 beneath to the atmosphere." 



Trap Dikes. — Having briefly noticed the mineral 

 characters and composition of volcanic rocks, we are 

 now prepared to consider the manner and position 

 in which they occur in the earth's crust. 



We have already remarked that basalt occurs 

 sometimes in veins or dikes, which traverse rocks 

 of all ages, filling up fissures or crevices ; and at 

 others, spread over the surface of the strata, or in- 

 terposed between them, thus : 



Fig. 32. 



J. Granite. 2. Gneiss. 3. Mica Slate. 1. Argillaceous 

 Slate. 5. Basalt. 



Basalt often has a columnar structure, occurring 

 in the fo.rm of regular pillars clustered together, as 

 in the following out, which represents Fingal's Cave, 

 fa the island of Staffa, one of the Hebr: les. 

 K 



