All crystalline rocks have consolidated from solution or from fusion. A granite arises by the consolidation of a liquid at high temperatures and great pressures; a metamorphic rock like mica schist, on the other hand represents the recrystallization-product of a clay rock under the influence of heat and stress. Throughout this conversion the rock has been essentially solid, recrystalliza tion taking place in the presence of minor amounts of interstitial liquid. Other crystalline rocks, like rock-salt, gypsum, anhydrite, etc., have been deposited from solution in water, mostly owing to evaporation on exposure to the air. The majority of sedi mentary rocks, however, consist of a mechanical aggregate of crystalline particles ; such are sandstone, clay, etc. The structure of these sedimentary and all pyroclastic rocks is referred to as fragmental.
All igneous rocks have solidified from a state of liquidity, the liquid that finally consolidates as rock being technically referred to as magma (Gr. µa-yya from
to knead). Rock-magma is a complex silicate solution carrying gases, the most important of which is water, which may constitute 5-6% of the mass. The greater part of the clouds given off by active volcanoes consists of steam, with which is associated minor amounts of other gases, the chief being CO2, CO, HCI, S vapour, oxides of sulphur,
HF, NH3, CH4, Cl, F, H and N. When the magma consolidates the major part of these volatile constituents usually escapes. With very rapid cooling, however, the magma may solidify without crystallization as a volcanic glass, at the same time retaining much of its water. Analyses of pitchstone show not infrequently a content of 5-6% of water, and even higher values are recorded; on the other hand unaltered crystalline igneous rocks seldom show a water content greater than 2%. Here we have almost direct confirmation of the con clusion that the water-content of the magma before crystalliza tion is greater than the water-content of the rock formed from it.
From what source is igneous magma derived? In the older conception of the earth a volcano was pictured as a safety valve of a molten interior, but a close study of igneous action, past and present, clearly negatives any such interpretation. The active vents of Mauna Loa and Kilauea (Hawaii) though only some 20 m. apart, have the summits of their lava columns at a difference of level of io,000 ft. Yet liquid lava is erupted more frequently and in greater quantity from the higher column. Clearly these two outlets are not directly connected with a common liquid reservoir. Again, neighbouring vents, as those of the Lipari Isles,
erupt widely different products, and each may maintain its inde pendent behaviour regardless of what is happening to its neigh bour. The conclusion is enforced that each distinct eruptive centre possesses its own reservoir, and we must conceive a num ber of independent magma chambers underlying an eruptive region. At greater depth these are no doubt connected with some magmatic reservoir of much larger dimensions. In some such common source the close community of petrographic char acter revealed in the succession of liquids erupted or intruded throughout any given igneous cycle is to be traced.
The macroscopic (Gr. ,uaKpos, large) characters of rocks, those visible in hand-specimens with out the aid of the microscope, are very varied and difficult to describe accurately and fully. The geologist in the field depends principally on them and on a few rough chemical and physical tests; and to the practical engineer, architect and quarry-master they are all-important. Although frequently insufficient in them selves to determine the true nature of a rock, they usually serve for a preliminary classification and often give all the information really needed. With a small bottle of acid to test for carbonate of lime, a knife to ascertain the hardness of rocks and minerals, and a pocket lens to magnify their structure, the field geologist is rarely at a loss in deciding to what group a rock belongs. The fine grained species may be indeterminable in this way, and the minute mineral components of all rocks can usually be ascer tained only by microscopic examination; but it is easy to see that a sandstone or grit consists of more or less rounded, water worn sand-grains and if it contains dull, weathered particles of felspar, shining scales of mica or small crystals of calcite these also rarely escape observation. Shales and clay rocks generally are soft, fine grained, often laminated and they not infrequently con tain minute organisms or fragments of plants. Limestones are easily marked with a knife-blade, effervesce readily with weak cold acid and often contain entire or broken shells or other fos sils. The crystalline nature of a granite or basalt is obvious at a glance, and while the former contains white or pink felspar, .clear vitreous quartz and glancing flakes of mica, the other will show yellow-green olivine, black augite and grey striated plagi oclase.