A Manual of Elementary Geology

igneous origin of granite - How the name of primary was still retained for granite - The term "transition," why faulty - The adherence to the old chronological nomenclature retarded the progress of g

ndary, and te

have required, says Darwin, the quick growth of green turf on a good soil; their abrupt cessation may mark the place where the soil was barren, and when a green sward formed slowly. In regard to the aqueous rocks, we have already seen that they are stratified, that some are calcareous, others arg

everal groups of plutonic, volcanic, and metamorphic rocks. This system of classification is not only recommended by its greater clearness and facility of application, but is also best fitted to strike the imagination by bringing into one view the past changes of the inorganic world, and the co

it will be desirable to say a few words on the chronology of rocks in general, although in doing so we shall be

began and ended before any members of the aqueous and volcanic orders were produced; and although this idea has long been modified, and is nearly exploded, it will be necessary to give som

measure by Steno, a century before, in Italy, formed at the time an important step in the progress of geology, and sketched out correctly some of the leading divisions into which rocks may be separated. About half a century later, Werner, so justly celebrated for his improved methods of discriminating the mineralogical characters of rocks, attempted to improve Lehman's classification, and with this view intercalated a class, called by him "the transition formations," between the primitive and secondary. Between these last he had discovered, in northern Germany, a series of strata, which in their mineral peculiarities were of an intermediate character, partaking in some degree of the crystalline nature of micaceous schist and clay-slate, and yet exhibiting here and there signs of a mechanical origin and organic remains. For this group, therefore, forming a passage between Lehman's primitive and secondary rocks, the name of übergang or transition was proposed. They consisted principally of clay-slate and an argillac

y Arduino, Fortis, Faujas, and others, and especially by Desmarest, they were all regar

of our actual seas, the transition strata were deposited. These were of a mixed character, not purely chemical, because the waves and currents had already begun to wear down solid land, and to give rise to pebbles, sand, and mud; nor entirely without fossils, because a few of the first marine animals had begun to exist. After this period, the secondary formations were accumulated in waters resembling those of the present ocean, e

mena of granitic veins, and the alterations produced by them on the invaded strata, which will be treated of in the thirty-second chapter. He, moreover, advanced the opinion, that the crystalline strata called primitive had not been precipitated from a prim?val ocean, but were sedime

y still continued to be applied to the crystalline formations in general, whether stratified, like gneiss, or unstratified, like granite. The pupil was told that granite was a primary rock, but that some granites were newer than certain secondary formations; and in confo

ed rocks occupying the same position, and inclosing similar fossils, they gave to them also the name of transition, according to rules which will be explained in the next chapter; yet, in many cases, such rocks were found not to exhibit the same mineral texture which Werner had called transition. On the contrary, many of them were not more crystalline than different members of the secondary class; while, on the other hand, these last were sometimes found to assume a semi-crystalline and almost metamorphic aspect, and thus, on lithological grounds, to deserve equally the name of transition. So remarkably was this the case in the Swiss Alps, that certain rocks, which had for years been regarded by some of the most skilful disciples of Werner to

ing over the antiquated styl

sand, our la

tide, our wo

provements in theory outrun the changes of nomenclature; and the attempt to inculcate new truths by words invented to express a different or opposite opinion

rt of the heat having been diffused into surrounding space, the surface of the fluid consolidated, and formed a crust of granite. This covering of crystalline stone, which afterwards grew thicker and thicker as it cooled, was so hot, at first, that no water could exist upon it; but as the refrigeration proceeded, the aqueous vapour in the atmosphere was conden

heat still remaining in the solid supporting substances was sufficient to increase the chemical action exerted by the water, although not so intense as to prevent the introduction and increase of some living beings. During

g the priority of all crystalline rocks to the creation of organic beings, were still preserved; and the mistaken notion that all the semi-crystalline and p

ratified rocks, has been successive, and that different portions of granite have been in a melted state at distinct and often distant periods. One mass was solid, and had been fractured, before another body of granitic matter was injected into it, or through it, in the form of veins. Some granites are more ancient than any known fossiliferous rocks; others are of secondary; and some, such as that of Mont Bl

n which certain masses belonging to each of the four classes of rocks may have originated simultaneously at every geological period, and how the earth's crust may have been continually remodelled, above and below, by aqueous and igneous causes, from times indefinitely remote. In the same manner as aqueous and fossiliferous strata are now formed in certain seas or lakes, while in other places volcani

melting of materials previously consolidated. As to the relative antiquity of the crystalline foundations of the earth's crust, when compared to the fossiliferous and volcanic rocks which they support, I have already stated, in the first chapter, that to pronounce an opinion on this matter is as difficult as at once to decide which of the two, whether the foundations or superstructure of an ancient city built on wooden piles, may be the oldest. We have seen that, to answer this question, we must

h are already ascertained to be newer than all the secondary formations. In this work I shall follow most nearly the method proposed by Mr. Boué, who has called all fossiliferous rocks older than the secondary by the name of primary. To preven

h rocks will also be primary, according to this system. Mr. Boué having with great propriety excluded the m

d that there are plutonic, volcanic, and metamorphic rocks o

iary. If, therefore, we discover any volcanic, plutonic, or metamorphic rocks, which have

e of chronology, it is by no means assumed that these columns are all of equal length; one may begin at an earlier period than the rest, and another may come down to a later point of time. In the small part of the globe hitherto examined, it is hardly to be expected that we should have discovered either the oldest or the newest me