CHAPTER VIII.
OF THE MATERIALS OF MOUNTAINS:—FIRST, COMPACT CRYSTALLINES.
§ 1. In the early days of geological science, the substances which composed the crust of the earth, as far as it could be examined, were supposed to be referable to three distinct classes: the first consisting of rocks which not only supported all the rest, but from which all the rest were derived, therefore called "Primary;" the second class consisting of rocks formed of the broken fragments or altered substance of the primary ones, therefore called "Secondary;" and, thirdly, rocks or earthy deposits formed by the ruins and detritus of both primary and secondary rocks, called, therefore, "Tertiary." This classification was always, in some degree, uncertain; and has been lately superseded by more complicated systems, founded on the character of the fossils contained in the various deposits, and on the circumstances of position, by which their relative ages are more accurately ascertainable. But the original rude classification, though of little, if any, use for scientific purposes, was based on certain broad and conspicuous phenomena, which it brought clearly before the popular mind. In this way it may still be serviceable, and ought, I think, to be permitted to retain its place, as an introduction to systems more defined and authoritative.
§ 2. For the fact is, that in approaching any large mountain range, the ground over which the spectator passes, if he examine it with any intelligence, will almost always arrange itself in his mind under three great heads. There will be, first, the ground of the plains or valleys he is about to quit, composed of sand, clay, gravel, rolled stones, and variously mingled soils; which, if he has any opportunity,—at the banks of a stream, or the sides of a railway cutting,—to examine to any depth, he will find arranged in beds exactly resembling those of modern sand-banks or sea-beaches, and appearing to have been formed under such natural laws as are in operation daily around us. At the outskirts of the hill district, he may, perhaps, find considerable eminences, formed of these beds of loose gravel and sand; but, as he enters into it farther, he will soon discover the hills to be composed of some harder substance, properly deserving the name of rock, sustaining itself in picturesque forms, and appearing, at first, to owe both its hardness and its outlines to the action of laws such as do not hold at the present day. He can easily explain the nature, and account for the distribution, of the banks which overhang the lowland road, or of the dark earthy deposits which enrich the lowland pasture; but he cannot so distinctly imagine how the limestone hills of Derbyshire and Yorkshire were hardened into their stubborn whiteness, or raised into their cavernous cliffs. Still, if he carefully examines the substance of these more noble rocks, he will, in nine cases out of ten, discover them to be composed of fine calcareous dust, or closely united particles of sand; and will be ready to accept as possible, or even probable, the suggestion of their having been formed, by slow deposit, at the bottom of deep lakes and ancient seas, under such laws of Nature as are still in operation.
§ 3. But, as he advances yet farther into the hill district, he finds the rocks around him assuming a gloomier and more majestic condition. Their tint darkens; their outlines become wild and irregular; and whereas before they had only appeared at the roadside in narrow ledges among the turf, or glanced out from among the thickets above the brooks in white walls and fantastic towers, they now rear themselves up in solemn and shattered masses far and near; softened, indeed, with strange harmony of clouded colors, but possessing the whole scene with their iron spirit; and rising, in all probability, into eminences as much prouder in actual elevation than those of the intermediate rocks, as more powerful in their influence over every minor feature of the landscape.
§ 4. And when the traveller proceeds to observe closely the materials of which these noble ranges are composed, he finds also a complete change in their internal structure. They are no longer formed of delicate sand or dust—each particle of that dust the same as every other, and the whole mass depending for its hardness merely on their closely cemented unity; but they are now formed of several distinct substances, visibly unlike each other; and not pressed but crystallized into one mass,—crystallized into a unity far more perfect than that of the dusty limestone, but yet without the least mingling of their several natures with each other. Such a rock, freshly broken, has a spotty, granulated, and, in almost all instances, sparkling, appearance; it requires a much harder blow to break it than the limestone or sandstone; but, when once thoroughly shattered, it is easy to separate from each other the various substances of which it is composed, and to examine them in their individual grains or crystals; of which each variety will be found to have a different degree of hardness, a different shade of color, and a different character of form.
But this examination will not enable the observer to comprehend the method either of their formation or aggregation, at least by any process such as he now sees taking place around him; he will at once be driven to admit that some strange and powerful operation has taken place upon these rocks, different from any of which he is at present cognizant; and farther inquiry will probably induce him to admit, as more than probable, the supposition that their structure is in great part owing to the action of enormous heat prolonged for indefinite periods.
§ 5. Now, although these three great groups of rocks do indeed often pass into each other by imperceptible gradations, and although their peculiar aspect is never a severe indication of their relative ages, yet their characters are for the most part so defined as to make a strong impression on the mind of an ordinary observer, and their age is also for the most part approximately indicated by their degrees of hardness, and crystalline aspect. It does, indeed, sometimes happen that a soft and slimy clay will pass into a rock like Aberdeen granite by transitions so subtle that no point of separation can be determined; and it very often happens that rocks like Aberdeen granite are of more recent formation than certain beds of sandstone and limestone. But, in spite of all these uncertainties and exceptions, I believe that unless actual pains be taken to efface from the mind its natural impressions, the idea of three great classes of rocks and earth will maintain its ground in the thoughts of the general observer; that whether he desire it or not, he will find himself throwing the soft and loose clays and sands together under one head; placing the hard rocks, of a dull, compact, homogeneous substance, under another head; and the hardest rocks, of a crystalline, glittering, and various substance, under a third head; and having done this, he will also find that, with certain easily admissible exceptions, these three classes of rocks are, in every district which he examines, of three different ages; that the softest are the youngest, the hard and homogeneous ones are older, and the crystalline are the oldest; and he will, perhaps, in the end, find it a somewhat inconvenient piece of respect to the complexity and accuracy of modern geological science, if he refuse to the three classes, thus defined in his imagination, their ancient title of Tertiary, Secondary, and Primary.