importance than their structure, or the relations of the parts of which they consist to one another. Regarded from this standpoint rocks may be divided into the crystalline and the frag mental. Inorganic matter, if free to take that physical state in which it is most stable, always tends to Crystalline Rock. crystallize. Crystalline rock masses have consolidated from solution or from fusion. The vast majority of igneous rocks belong to this group and the degree of perfection in which they have attained the crystalline state depends primarily on the conditions under which they solidified Such rocks as granite, which have cooled very slowly and under great pressures, have completely crystallized, but many lavas were poured out at the surface and cooled very rapidly; in this latter group a small amount of non-crystalline or glassy matter is frequent. Other crystalline rocks such as rock-salt, gypsum and anhydrite have been deposited from solution in water, mostly owing to evaporation on exposure to the air. Still another group, which includes the marbles, mica-schists and quartzite's, are recrystallized, that is to say, they were at first frag mental rocks, like limestone, clay and sandstone and have never been in a molten condition nor entirely in solution. Certain agencies however, acting on them, have effaced their primitive structures, and induced crystallization. This is a kind of metamorphism.
The fragmental structure needs little explanation; wherever rocks disintegrate fragments are produced which are suitable for the formation of new rocks of this group. The Fragmental Rocks. original materials may be organic (shells, corals, plants) or vitreous (volcanic glasses) or crystalline (granite, marble, &c); the pulverizing agent may be frost, rain, running water, or the steam explosions which shatter the lava within a volcanic crater and produce the frag mental rocks known as volcanic ash, tutfs and agglomerates. The materials may be loose and incoherent (sand, clay, gravel) or compacted by pressure and the deposit of cementing substances by percolating water (sandstone, shale, conglomerate) The grains of which frag mental rocks are composed may be coarse or fine, fresh or decayed, uniform or diverse in their composition; the one feature which gives unity to the class is the fact that they are all derived from pre-existing rocks or organisms. Because they are made up of broken pieces these rocks are often said to be “ clastic ”
Origin of Rocks.—The study of the structure of rocks evidently leads us to another method of regarding them, which is more fundamental than those enumerated above, as the structure depends on the mode of origin. Rocks are divided into three great classes, the Igneous, the Sedimentary and the Metamorphic. Igneous Rocks. The igneous (Lat. ignis, fire) rocks have all consolidated from a state of fus1on. Some of them are crystalline or “massive”; others are fragmental. The massive igneous rocks include a few which'are nearly completely vitreous, and still more which contain a small amount of amorphous matter, but the majority are completely crystallized Among the best known examples are obsidian, pumice, basalt, trachyte, granite, diorite. The frag mental igneous rocks consist of volcanic ashes more or less firmly compacted. The sedimentary rocks form a second group the Sedimentary Rocks.have all been laid down as deposits on the earth's surface subject to the conditions of temperature, moisture and pressure which obtain there. They include frag mental and crystalline varieties. The former consist of the débris of pre-existing rocks, accumulated in seas, lakes or dry land and more or less indurated by pressure and cementing substances. Gravel, sand and clay, conglomerate, sandstone, shale are well-known examples. Many of them are fossiliferous as they contain fragments of organisms. Some are very largely made up of remains of animals or plants, more or less altered by mineralization. These are sometimes placed into a special group as rocks of organic origin; limestone, peat and coal are typical of this class The crystalline sediments are such as rock-salt and gypsum, deposits of saline lakes or isolated portions of the sea They were formed under conditions unfavourable to life and hence rarely contain fossils. The metamorphic rocks are known to be almost entirely altered igneous or sedimentary masses. Metamorphism consists in the destruction of the original structures Metamorphic Rocks. and the development of new minerals. The chemical composition of the rocks however suffers little change. The rock becomes as a rule more crystalline; but all stages in the process may be found and in a metamorphosed sediment, e.g. a sandstone, remains of the original sand grains and primary frag mental structure may be observed, although extensive recrystallization has taken place. The agencies which produce metamorphism are high temperatures, pressure, interstitial moisture and in many cases movement. The effects of high temperatures are seen best in the rocks surrounding great outcrops of intrusive granite, for they have been baked and crystallized by the heat of the igneous rock (thermo-metamorphism). In folded mountain chains where the strata have been greatly compressed and their particles have been forced to move over one another a different type of metamorphism prevails (regional or dynamic metamorphism).
Methods of Investigation.—The macroscopic (Gr. μακρός, large) characters of rocks, those visible in hand-specimens Without the aid of the microscope, are very varied and difficult to describe accurately and fully. The Macroscopic Characters. 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 themselves to determine the true nature of a rock, they usually serve for a preliminary classification and often give all the information which is 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 to What, group a rock belongs. The fine grained species are often indeterminable in this Way, and the minute mineral components of all rocks can usually be ascertained 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 not infrequently contain 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 fossils. 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 plagioclase.
But when dealing with unfamiliar types or with rocks so fine grained that their component minerals cannot be determined with the aid of a lens, the geologist is obliged to have recourse to more delicate and searching methods of gfargcfgf investigation. With the aid of the blowpipe (to test the fusibility of detached crystals), the goniometer, the magnet, the magnifying glass and the specific gravity balance, the earlier travellers attained surprisingly accurate results. Examples of these may be found in the works of von Buch, Scrope, Darwin and many others. About the end of the 18th century, Dolomieu examined crushed rock powders under the microscope and Cordier in ISIS crushed, levitated and investigated the Hner ground-mass of igneous rocks. His researches are models of scrupulous accuracy, and he was able to announce that they consisted essentially of such minerals as felspar, augite, iron ores and volcanic glass, and did not differ in nature from the coarser grained rocks. Nicol, whose name is associated with the discovery of the Nicol's prism, seems to have been the first to prepare thin slices of mineral substances, and his methods were applied by Witham (1831) to the study of plant petrifaction's. This method, of such far-reaching importance in petrology, was not at once made use of for the systematic
