The total absence of any trace of fossils has inclined many geologists to attribute the origin of crystalline strata to a period antecedent to the existence of organic beings. Admitting, they say, the obliteration, in some cases, of fossils by plutonic action, we might still expect that traces of them would oftener occur in certain ancient systems of slate, in which, as in Cumberland, some conglomerates occur. But in urging this argument, it seems to have been forgotten, that there are stratified formations of enormous thickness, and of various ages, and some of them very modern, all formed after the earth had become the abode of living creatures, which are nevertheless in certain districts entirely destitute of all vestiges of organic bodies. In some, the traces of fossils may have been effaced by water and acids, at many successive periods; and it is clear, that the older the stratum, the greater is the chance of its being non-fossiliferous, even if it has escaped all metamorphic action.

It has been also objected to the metamorphic theory, that the chemical composition of the secondary strata differs essentially from that of the crystalline schists, into which they are supposed to be convertible. * The "primary" schists, it is said, usually contain a considerable proportion of

Dr. Boase, Primary Geology, p. 319.

potash or of soda, which the secondary clays, shales, and slates do not, these last being the result of the decomposition of felspathic rocks, from which the alkaline matter has been abstracted during the process of decomposition. But this reasoning proceeds on insufficient and apparently mistaken data; for a large portion of what is usually called clay, marl, shale, and slate does actually contain a certain and often a considerable proportion of alkali; so that it is difficult in many countries to obtain clay or shale sufficiently free from alkaline ingredients to allow of their being burnt into bricks or used for pottery.

Thus the argillaceous shales, as they are called, and slates of the old red sandstone, in Forfarshire and other parts of Scotland, are so much charged with alkali, derived from triturated felspar, that, instead of hardening when exposed to fire, they melt readily into a glass. They contain no lime, but appear to consist of extremely minute grains of the various ingredients of granite, which are distinctly visible in the coarser-grained varieties, and in almost all the interposed sandstones. These laminated clays, marls, and shales might certainly, if crystallized, resemble in composition many of the primary strata.

There is also potash in the vegetable remains included in strata, and soda in the salts by which

they are sometimes so largely impregnated, as in Patagonia.

Another objection has been derived from the alternation of highly crystalline strata with others having a less crystalline texture. The heat, it is said, in its ascent from below, must have traversed the less altered schists before it reached a higher and more crystalline bed. In answer to this, it may be observed, that if a number of strata differing greatly in composition from each other be subjected to equal quantities of heat, there is every probability that some will be more fusible than others. Some, for example, will contain soda, potash, lime, or some other ingredient capable of acting as a flux; while others may be destitute of the same elements, and so refractory as to be very slightly affected by a degree of heat capable of reducing others to semi-fusion. Nor should it be forgotten that, as a general rule, the less crystalline rocks do really occur in the upper, and the more crystalline in the lower part of each metamorphic series.

But it will be impossible for the reader duly to appreciate the propriety of the term metamorphic, as applied to the strata hitherto called primary, until I have shown in the second part of this work, that these crystalline strata have been formed at a great variety of distinct periods.

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PART II.

CHAPTER XII.

ON THE DIFFERENT AGES OF THE FOUR GREAT CLASSES OF ROCKS.

Aqueous, plutonic, volcanic, and metamorphic rocks, considered chronologically-Lehman's division into primitive and secondary-Werner's addition of a transition class— Neptunian theory-Hutton on igneous origin of graniteHow the name of primary was still retained for graniteThe term "transition," why faulty - The adherence to the old chronological nomenclature retarded the progress of geology-New hypothesis invented to reconcile the igneous origin of granite to the notion of its high antiquity Explanation of the chronological nomenclature adopted in this work, so far as regards primary, secondary, and tertiary periods.

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In the first part of this work the four great classes of rocks, the aqueous, the volcanic, the plutonic and the metamorphic, have been considered with reference to their external characters, their mineral composition, and mode of origin; and it now remains to treat of the same classes with reference to the different periods at which they were formed. In speaking of the aqueous rocks, for example, it has been shown that they are stratified, that some are calcareous, others argillaceous, some made up

of sand, others of pebbles; that some contain freshwater, others marine fossils, and so forth; but the student has still to learn which rocks, exhibiting some or all of these characters, have originated at one period of the earth's history, and which at another.

So in regard to the volcanic and plutonic formations, we have hitherto examined their mineral peculiarities, forms, and mode of origin, but have still to inquire into their chronological history.

Lastly, a more curious question will demand our attention, when we endeavour to ascertain the relative ages of the metamorphic rocks, the chronology of which may be said to be twofold, each formation having been deposited at one period, and having assumed a crystalline texture at another.

It was for many years a received opinion, that the formation of whole classes of rocks, such as the plutonic and metamorphic, 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 some account of the ancient doctrine, in order that beginners may understand whence part of the nomenclature of geology still partially in use was derived.

About the middle of the last century, Lehman, a German miner, proposed to divide rocks into