strata of enormous thickness, filling up large hollows and troughs in the bottom of the sea. Suppose that in some of these situations, an accumulation of five to ten feet occurred annually, we should thus have, in two thousand years, a mass of strata, varying from ten to twenty thousand feet in thickness, thus equalling the depth of the thickest ancient accumulations yet discovered. "In certain parts of the globe," says Mr Lyell," continuous formations are now accumulated over immense spaces along the bottom of the ocean. The materials undoubtedly must vary in different regions, yet for thousands of miles they may often retain some common characters, and be simultaneously in progress throughout a space, stretching thirty degrees of latitude from south-east to north-west, from the mouths of the Amazon, for example, to those of the Mississippi, as far as from the Straits of Gibraltar to Iceland. At the same time, great coral reefs are growing around the West Indian islands, and in some parts streams of lava are occasionally flowing into the sea, which become covered again in the intervals of eruptions with other beds of corals. The various rocks, therefore, stratified and unstratified, now forming in this part of the globe, may occupy, perhaps, far greater areas than any group of our ancient secondary series which has yet been traced through Europe."

* Principles of Geology, vol. ii.

Considering these circumstances, then, is it not reasonable to suppose, that, if the present ocean bed were elevated by subterranean forces above the level of its waters, and converted into dry land, we should find strata of the depth and magnitude which the present causes in action would lead us to calculate on.

Now, although it is presumed that the same causes which now operate were in action during the production of the more ancient strata, yet certain modifications may have formerly tended to aid the more rapid increase of detritus over the surface of the globe. In the first place, the primitive condition of the surface may have rendered it more liable to disintegration by aqueous action. The first igneous rocks, formed probably under less pressure, may have been of a softer and more porous nature than the later granites, which have been produced under the pressure, not only of the ocean, but of the superincumbent strata. Neither had they any covering of debris to defend their surfaces from the abrading torrents; for the greater the accumulation of detrital matter, the greater must be the protection to the subjacent rocks.

Then, there are several indications which would lead us to suppose, that, at the period of the deposition of the earlier strata, the average temperature of the globe was considerably higher than at present; that is, that a temperature approaching to tropical prevailed over the temperate and even frigid zones.

30 ACCUMULATION OF SEDIMENTARY MATTER.

May not this have influenced greatly the rate of evaporation, and consequently increased the quantity of rain, and the number and magnitude of rivers; while, at the same time, it would greatly augment vegetation, and amply supply the mass of vegetable remains which contribute so largely to swell the bulk of the carboniferous strata? We allow that these are so far hypothetical arguments, though the last statement is warranted by facts which are obvious to every geologist. The coal beds abound with numerous vegetable remains of ferns, reeds, and other cryptogamic plants, which have evidently flourished in great numbers in low marshy plains, besides trees of an immense size and of a higher class of vegetation, which must have been borne down by the impetuosity of currents from more elevated grounds in the ancient continents.

SECTION III.

DIFFERENT FORMATIONS MAY BE CONTEMPORARY.

THE next circumstance to be considered is, whether all the sedimentary strata have been deposited one after the other from the primary starta onwards to the most recent tertiary beds, in a regular sequence of time, or whether several of the groups have not had a contemporaneous formation. There can be no doubt, but that some of the formations have been regularly deposited, one after the other, which is proved by their positions, line of stratification, and from some of the newer strata, containing fragments of the older; but, then, it cannot be so easily demonstrated that this has been the case with the whole. It cannot be proved, for instance, that the great carboniferus groups may not have been partly deposited during the period when the lias and oolite were in the process of formation; or that a contemporary deposition has not taken place among some of the other members of the newer formations.

As to position in England, and the corresponding portion of the continent of Europe, the boundaries of these respective strata can be traced by well defined lines of separation.

The great coal fields of Britain lie in hollow troughs or valleys, formed by the previous disturbance of the mountain limestone and older sedimentary strata on which they rest. These coal beds have most probably been accumulated by the action of rivers carrying down vegetable and earthy matter into the basins of estuaries. The lias and oolite again seem more strictly the production of littoral oceanic currents sweeping along the shallow shores. Yet, with some trifling exceptions, the latter formations are not superimposed upon those of the coal measures, but lie in distinct beds. We do not at present take into consideration the characteristic fossils of each, because this subject will be treated of afterwards.

We think it admits of demonstration, too, that the last great change on the British strata, by which they were finally elevated above the ocean, took place at one period, or, at least, at periods nearly contemporaneous.

Thus, if the northern coal fields of England and those of Scotland were elevated above the level of the sea, at a period long antecedent to the existence of the lias and oolite beds of the south-east of England, where are the traces on the surface of the