is called Zwitter-rock. If the felspar has to a great extent disappeared, and the mica is Lepidolite (Lithia Mica), the rock is called Greissen. Claystone is a more or less decomposed form of Felstone. Some of the old glassy rocks have become devitrified, and so properly belong to the Metamorphic series; but, as already mentioned, it is difficult to separate them, and no separate names have yet been assigned to these altered forms. Dolerite Group. No very conspicuous representatives exist of the intervening groups, but of this there are several. Euphotides. In gabbro the felspar has often been converted into a hard white mineral allied to saussurite, and the diallage into hornblende (in some cases the pretty green variety called Smaragdite). The rock thus altered is called Euphotide, Hornblendic, or Smaragdite-gabbro. A variety where the felspar appears to be anorthite, the pyroxenic constituent is rather rare, and the abundant olivine is more or less converted into serpentine, is called Troktolite or Forellen-stein. Diabase. Similar, though less definite changes have converted dolerites and basalts into Diabase, by the development of a chloritic mineral, commonly with a little. calcite. If the augite is converted into uralite, the rock is termed Uralite-diabase.' Basalt sometimes becomes so much decomposed, that water and carbonic acid constitute about a fifth part of the rock, and the alkalies are almost lost: the rock assuming an earthy texture and a pale cream-coloured tint. This is called White-trap, and is far from rare among the carboniferous rocks of Staffordshire and central Scotland. Peridotite Group. Serpentine. This term has been used with great vagueness, and has been applied to a large group of rocks of various origins which have absolutely nothing in com 1 Hornblende does sometimes appear to be an accessory mineral in true gabbro or dolerite. mon except the presence of a fair proportion of silicates of magnesia. Properly, however, the term should be restricted to the well-defined group of rocks, of which the Serpentine of the Lizard, in Cornwall, is an excellent type-a compact massive rock, of various dull brown, red, and green tints, in which glittering crystals of a certain variety of enstatite are frequently conspicuous. It may be considered as proved, that these rocks are altered Peridotites; chemically, they are mainly composed of silicate of magnesia, with iron oxides, and about 12 per cent. of water. Conditions of Temperature and Pressure under which Igneous Rocks were formed.-In an important paper on "The Microscopical Structure of Crystals," ↑ Mr. H. C. Sorby has shown that it is possible to arrive at some interesting and remarkable conclusions as to the temperature and pressure under which the crystalline particles of granite and other igneous rocks were formed. The following is an abstract of his argument : When artificial crystals are examined with the microscope, it is seen that they have often caught up and enclosed within their solid substance portions of the material surrounding them at the same time when they are being formed. Thus, if they are produced by sublimation, small portions of air or vapour are caught up, so as to form apparently empty cavities; or if they are deposited from solution in water, small quantities of water are enclosed, so as to form fluid-cavities. In a similar manner, if crystals are formed from a state of igneous fusion, crystallizing out from a fused-stone solvent, portions of this fused stone became entangled, which, on cooling, remain in a glassy condition, or become stony, so as to produce what may be called glass- or stone-cavities. All these kinds of cavities can readily be seen with suitable magnifying powers, and distinguished from each other by various definite peculiarities. From these and other facts the following conclusions are deduced: 1. Crystals containing only cavities with water were formed from solution. "Quart. Journ. Geol. Soc." vol. xiv. p. 453. 2. Crystals containing only stone- or glass-cavities were formed from a state of igneous fusion. 3. Crystals containing both water- and stone- or glasscavities were formed under great pressure by the combined influence of highly heated water and melted rock. 4. That the amount of water present in the cavities may, in some cases, be employed to deduce the temperature at which the crystals were formed. Applying these principles to the examination of igneous rocks, Mr. Sorby proves from them the igneous origin of all, with this remarkable result, that the fluidity of the more superficial lavas was a more purely igneous one than that of the deeper seated trappean and plutonic rocks. The minerals of erupted lavas contain stone and glass cavities like the crystals formed in the slags of furnaces; but the blocks ejected from volcanoes contain watercavities as well, the amount of water indicating that they were formed under great pressure and at a dull red heat, when both liquid water and melted rock were present. The crystals of the Cornish elvans and the Cornish and Scotch granites contain both fluid and stone cavities, proving the presence of water, and perhaps also of gas, as well as the conditions of great heat and pressure. "On the whole, then," says Mr. Sorby, "the microscopical structure of the constituent minerals of granite is in every respect analogous to that of those formed at great depths and ejected from modern volcanoes, or that of the quartz in the trachyte of Ponza, as though granite had been formed under similar physical conditions, combining at once both igneous fusion, aqueous solution, and gaseous sublimation. The proof of the operation of water is quite as strong as that of heat." Mr. Sorby arrives at the conclusion, that if granite and elvan finally consolidated at a temperature not exceeding 680° F., the elvans of Cornwall must have been formed under a pressure equal to that which would have been exerted by a thickness of about 40,000 feet of rock, those of the Highlands of Scotland one of 69,000, that of the granites of Cornwall indicate a pressure of about 50,000 feet, and those of the Highlands indicate one of 76,000 feet. If the temperature of consolidation was higher, the pressure must have been greater. Mr. Sorby does not mean in his conclusions to point out the absolute depths at which the rocks consolidated, since the pressure they were subjected to might arise in part from the impelling force acting from below against the superincumbent mass. THE CHAPTER IV. DERIVATIVE OR STRATIFIED ROCKS. 'HEIR Origin and Composition.-In Chapter III. we have examined the igneous rocks which come from the interior of the earth, and we have seen that they are essentially siliceous, some of them, especially the granites, containing crystals of pure silica, and all of them containing silicate of alumina. Now pure sand consists of grains of quartz, and pure clay consists of silicate of alumina combined with water. It is obvious, therefore, that clay and sand may be formed by the detrition and destruction of an igneous rock, such as granite, and this process may be actually watched wherever granite rocks are exposed to the erosive action of water. All the sandy (arenaceous) and all the clayey (argillaceous) deposits, therefore, which are met with in the earth's crust, have been derived from some previously existing rocks, and these older rocks must either have been igneous rocks, or must originally have been derived from some igneous rock. Similarly the siliceous and calcareous rocks of organic origin have been formed out of the silica and carbonate of lime held in solution by the sea-water; these materials having been primarily derived from the decomposition of the soluble silicates contained in igneous rocks. The varieties of rock produced by the intermixture of the aluminous, quartzose, and calcareous elements are very numerous. The proportions of these ingredients in any deposit will depend partly on the character of the rocks exposed to erosion in the area whence the chief supply is derived, and partly on the amount of matter contributed by organisms to the sediment which is accumulating below. Thus the chemical composition of any stratified rock is |