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. 
3. Granite. 2. Gneiss. 3. Mica Slate. 4. Argillaceous 
Slate. 5. Basalt. 
Basalt often has a columnar structure, occurring 
in the form of regular pillars clustered together, as 
in the following cut, which represents FingaPs Cave, 
in the island of Staffa, one of the Hebrides. 
L 
