--------------------+------------------------------------ Chlorite Schist | Chlorite. --------------------+------------------------------------ Serpentine | Serpentine. --------------------+------------------------------------ Greensand | Glauconite. ====================+==================================== 2. Eruptive or Unstratified Rocks. The rocks of this great class are formed by the cooling and solidification of materials that have come up from a great depth in the earth’s crust in a melted and highly heated condition. When the fissures in the earth’s crust reach down to the great reservoirs of liquid rock, and the latter wells up and overflows on the surface, forming a volcano, then we may, as was pointed out on page 33, divide the eruptive mass into two parts: first, that which has actually flowed out on the surface, and cooled and solidified in contact with the air, forming a lava flow; second, that which has failed to reach the surface, but cooled and solidified in the fissure, forming a dike. Lava flows or volcanic rocks and dikes or plutonic rocks are identical in composition; but there is a vast difference in texture, due to the widely different conditions under which the rocks have solidified. The dike or fissure rocks solidify under enormous pressure, and this makes them heavy and solid—free from pores. They are surrounded on all sides by warm rocks: this causes them to cool very slowly, and allows the various minerals time to crystallize. Other things being equal, the slower the cooling the coarser the crystallization; and hence, the greater the depth below the surface at which the cooling takes place, the coarser the crystallization. The volcanic rock, on the other hand, cools under very slight pressure; and the steam, which exists abundantly in nearly all igneous rocks at the time of their eruption, is able to expand, forming innumerable small vesicles or bubbles in the liquid lava; and these remain when it has become solid. Cooling in contact with the air, the lava cools quickly, and has but little chance for crystallization. Hence, to sum up the matter, we say: plutonic rocks are solid and crystalline; and volcanic rocks are usually porous or vesicular, and uncrystalline. As we descend into the earth’s crust, it is perfectly manifest that the volcanic must shade off insensibly into the dike rocks, and we find it impossible to draw any but an arbitrary plane of division between them; but this is no argument against this classification, for, as already stated, all is gradation in geology, and we experience just the same difficulty in drawing a line between conglomerate and sandstone, or between gneiss and mica schist, as between the dike rocks and volcanic rocks.