IGNEOUS Rocks.
(Definitions depending upon external form and mode of occurrence.) Igneous rocks com prise all those portions of the lithosphere which are or were in an intensely heated and more or less fluid condition and which have subse quently solidified by cooling into massive, more or less crystalline bodies of varying sizes and shapes. They occur as intrusions into or re placements of some previously existing rocks of the lithosphere, either sedimentary, meta* morphic or igneous.' Where a huge mass of molten material slowly melts its way from some deep-seated portion of the earth up into the overlying rocks by dissolving them and incor porating them in its own mass, and subsequently cools there, slowly, without ever reaching the surface, forming a great irregular body of coarsely crystalline matter frequently scores of miles in extent,_ have what is termed a batholith. In itsmolten condition it would be termed a magma and the reservoir in which it was contained' would be called a magma basin. it might have extending from it (usually up ward) irregolar, more or less elongated, arm like processes called apophyses. Should cracks or fissures form, they would be instantly filled by the inrush of fluid substance, which would solidify with comparative rapidity to form dikes. In stratified rocks dikes usually cut at some angle across the strata. Where the molten %material intrudes itself between the strata paral lel to the bedding planes in broad sheets, it is called an intrusion-sheet or sill. Where the overlying strata •become lifted and arched up ward into a huge dome without breaking above and letting, the fluid rock escape, the solidified lenticular mass constitutes a laccolith or lacco lite. Where the molten rock actually escapes to the surface by means of a fissure or other conduit, and flows out over it in a broad sheet, it is termed a flow, lava-flow, or extrusion-sheet. The interval of time elapsing between the out break of the magma from its reservoir and its subsequent cooling to the solid state is termed the effusive period. If the conduit is rudely cylindrical the lava accumulates about the vent, forming a lava or volcanic cone. Subsequent erosion may entirely remove the cone and ex pose the cold lava in the conduit, to which the term volcanic neck is applied. In fact it is only
through the exposure of these deep-seated masses of cooled igneous material by erosion that we are enabled to study them. Over the more ancient land areas many miles of rock, vertically measured, have been removed, laying bare the underlying rocks to a corresponding depth.
The Modern Conception of Rock Mag mas.— The popularly accepted idea of lava is that it is rock which has been fused by great heat. In fact the older conception of scientific men was that magmas consisted of fused rock masses. Recent opinion, however, based on the revelations of the microscope and experiments in the synthesis of minerals and rocks, tends to regard them as solutions of one mineral sub stance in another or of several mineral sub stances mutually in each other. One of the chief reasons for thinking them such can be stated as follows: If magmas are fusions then the individual mineral constituents potentially present in them should crystallize out according to their fusibilities, beginning with the least fusible, but they do not. On the other hand, as the magma slowly cools, the first individual mineral species' makes its appearance when the point of saturation for that particular species is reached, and the others follow in the order of their solubilities. For example, in granite {which consists of quartz, feldspar and mica or hornblende, with some accessory constituent, such as apatite or zircon, and, it may be, also a small amount of one .or more of the ores), the order of crystallization is: (1) the ores with apatite and zircon; (2) mica or horn blende; (3) feldspar; (4) quartz. Quartz is the least fusible, and should aPPear first wider the fusion theory. As a matter of fact it lip, pears last. In short, all of the materials ap pear in the reverse order of what we would. expect if they are true fusions. But they do appear in the order of their solubilities. Pres sure, however, exerts an important influence upon the solvency of a substance, and as is variable the order of appearance is not ab-, solutely fixed. Moreover, heat, the most im portant factor involved, is very variable, and, still farther modifies the problem.