stones for their generally crystalline character. Even the fossiliferous rocks have much of this feature, and all the older beds are really crystallized. "The case of limestone is soon settled. It is known, that, in contact with igneous rocks, the chalk of Ireland and the limestone of Teesdale, are turned to crystallized carbonate of lime; and experiments in the laboratory have left no doubt of the propriety of referring this crystallization of the limestone to the mere agency of heat and pressure. This high temperature must have pervaded, of course, all the rocks with which the altered limestone is associated. But it occurs with nearly all members of the mica-slate and gneiss systems. All these rocks, then, have suffered the influence of heat. In like manner, experimental proof has been offered by the chemist that quartz-rock is merely sandstone altered by heat; and thus we find reason to believe that some of the characters by which gneiss and mica-slate approach to granite, are owing to their having experienced considerable influence of heat." And in conclusion from this geologist, we give the following summary, adduced by him when referring to some late theories, which endeavour to account for the transformation into crystalline material of so great a proportion of the earth's crust: "Hence❞—says he—" as a consequence, we infer the consolidation and many other characters of primary strata, to be the effect of heat; but this falls short of the proof required, which must be to the extent of showing, not the changes of secondary to primary strata, but the changes of these into granite, and other crystalline rocks generally. Satisfactory proof of this nature and to this extent, is, we believe, nowhere afforded."* The crystalline structure of the igneous rocks is so well known and admitted by all; and the assumption of a transformation having taken place in producing that texture is so general and implicit, that we need hardly multiply proofs on the subject; we shall, therefore, conclude with a brief observation from Mr. Lyell: which we do the more readily, not only because he has dedicated so much attention to this parti * Treatise on Geology, pp. 69, 70, 75, 76, 259. cular branch of geology, but in this instance he identifies his own opinion-the result of his labours-with that of another distinguished writer. "Sir John Herschel," says he, in allusion to slaty cleavage, "has suggested, that if rocks have been so heated as to allow a commencement of crystallization; that is to say, if they have been heated to a point at which the particles can begin to move amongst themselves, or at least on their own axes, some general law must then determine the position in which these particles will rest on cooling. Probably that position will have some relation to the direction in which the heat escapes. Now, when all, or a majority of particles of the same nature have a general tendency to one position, that must, of course, determine a cleavage plane. Thus we see the infinitissimal crystals of fresh precipitated sulphate of barytes, and some other such bodies, arrange themselves alike in a fluid in which they float; . . . . . and what occurs in our experiments on a minute scale, may occur in nature on a great one." We come now to apply the information which we have acquired to the more direct chain of the general argument, with a view to discover whether the mineralogical structure of the older rocks affords those internal symptoms of having been subjected to the fusing heats which this Theory pre-supposes. We find it stated in the twenty-fourth Theorem, "That there exists an essential mineralogical difference between the older crystalline rocks, such as granite, trap, porphyry, serpentine, and others of that age and denomination, considered to be of igneous origin, and those which have been ejected from modern volcanoes, distinguished by the name of lavas, a difference attributed to the greater pressure under which the older masses were formed, to the non-action of the atmosphere and consequent retention of their gaseous or volatile parts, and to the more gradual manner in which they have cooled down." This being an important point, which requires to be well authenticated, we shall go into its evidences somewhat in detail. Elements, vol. ii. pp. 399, 400, taken from a letter from the Cape of Good Hope, of 20th February, 1836. M. de la Beche enters on the subject at once, but with his characteristic caution, when he says "If this opinion, of the greater prevalence of the granitic rocks over the trappean at the earliest periods be correct, it would seem to point to a certain condition of things at such periods, which subsequently became so modified that the igneous eruptions became altered. What that condition of things may have been, we do not as yet appear to have any very definite ideas; and we obtain little help on the subject from the phenomena of modern volcanoes, granite never having been known to flow from them. We, however, learn from this circumstance, that igneous eruptions into the atmosphere are not favourable to the production of granites, and we may consequently infer, that the conditions under which granite was produced were not similar to those which we now observe on the surface of the earth; at least so far as relates to those phenomena which occur in the atmosphere. What igneous matter ejected beneath a great pressure of sea may form we are unable to determine, but that it would be greatly modified by such pressure cannot be doubted. "It has been, indeed, generally considered that the mineralogical character of igneous rocks has been changed during the deposit of the stratified rocks, through which they have more or less forced their way; that is, we do not find granite and serpentine flowing from modern volcanoes, nor trachite, nor leucitic lavas intimately associated with the oldest strata in such a manner, that their relative differences of age could not be very considerable. . . . . . We are compelled, therefore, to admit, that the conditions under which the two kinds of igneous rocks have been formed have not been the same. What these conditions may have been is a separate question, and one, as observed above, requiring investigation; but it will be at once obvious, that the ejection of a mass, in a state of igneous fusion, into the atmosphere, would be likely to have its constituent parts arranged differently from those in a similar manner forced out beneath great pressure, such as we may consider to exist beneath deep seas. Independently, however, of this consideration, there appears to have been something in the condition of the world at the earliest times, causing certain compounds to be formed in great abundance, which does not now continue in such force as to permit the production of similar compounds."* * Manual, pp. 493, 501. "If," says Mr. Lyell," we examine a large portion of a continent, especially if it contains within it a lofty mountain range, we rarely fail to discover two other classes of rocks, very distinct from either of those above alluded to, and which we can neither assimilate to deposits such as are now accumulated in lakes or seas, nor to those generated by ordinary volcanic action. The members of both these divisions of rocks agree in being highly crystalline and destitute of organic remains. The rocks of one division have been called plutonic, comprehending all the granites and certain porphyries, which are nearly allied in some of their characters to volcanic formations. "The members of the other class are stratified and often slaty, and have been called by some the crystalline schists, in which groups are included gneiss, micaceous-schist, or (mica-slate), hornblendeschist, statuary marble, the finer kinds of roofing slate, and other rocks afterwards to be described. As it is admitted that nothing strictly analogous to these crystalline productions are now to be seen in the progress of formation upon the earth's surface, it will naturally be asked, on what data can we class them as to origin." And again "To a certain extent, however, there is a real distinction between the trappean formations and those to which the term volcanic is almost exclusively confined. The trappean rocks first studied in the North of Germany, and in Norway, France, Scotland, and other countries, were either such as had been formed entirely under deep water, or had been injected into fissures and intruded between strata, and which had never flowed out in the air, or over the bottom of a shallow sea. When these products, therefore, of sub-marine or subterranean igneous action were contrasted with loose cones of scoriæ, tuff, and lava, or with narrow streams of lava in great part scoriacious and porous, such as were observed to have proceeded from Vesuvius and Etna, the resemblance seemed remote and equivocal. It was, in truth, like comparing the roots of a tree with its leaves and branches, which, although they belong to the same plant, differ in form, texture, colour, mode of growth, and position. The external cone, with its loose ashes and porous lava may be likened to the light foliage and branches, and the rocks concealed far below, to the roots."* Elements, vol i. p. 14, and vol. ii. pp. 232, 233. Professor Phillips appears to take a similar view of this branch of geological research, and to confirm it, when he says, "But there is yet another form of modern volcanic aggregates which it is of great importance to distinguish from the preceding, because of its bearing upon points of great importance in old geology. There are subterranean volcanic products which neither are poured into the sea nor thrown into the air, but secretly elaborated under the pressure of a solid covering, and effused into the fissures of the rocks. "Although it may reasonably be allowed that the great variety of productions ejected by sub-aerial volcanoes affords a good indication of the principal mineral structures generated by volcanic action, we must be cautious not to limit our notions of their combinations in the deep parts of the earth to those which are suggested by the compounds which are determined at the surface. "The degree of pressure, rate of cooling, and mass of ingredients which are known to be important modifying conditions of molecular aggregation, are wholly different at the roots and about the surface of the immense volcanic chimnies which, like Etna and the Peak of Teneriffe, become filled with the liquid rocks whenever the subterranean pressure amounts to a particular degree. "At the base of a volcanic vent, deep in the earth or under the sea, particular mineral aggregates, slowly cooled, under great pressure, and in great masses, may, and probably do, at this day assume the large crystalline texture, and distinctness of ingredients of granite. On the bed of the sea they may flow in the state of phyry or basalt; on the surface of the land appear as porous lava, and be blown into the air in disintegrated scoriæ, ashes, and dust."* * Treatise, pp. 238-240. por |