26 PROOFS OF THE RAPID ACCUMULATION

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But the waters of the ocean having an infinitely more extended surface to act upon, are also accumulating detritus to an enormous extent. Not only do the tidal waves incessantly wear down and encroach upon the shores, but oceanic currents flowing over half the circumference of the globe, with considerable impetuosity, abrade the rocky depths and channels, and heap up and accumulate masses which, in many situations, are of very great magnitude. "So great," says Mr Lyell, is the quantity of matter held in suspension by the tidal current on our shores, that the waters are in some places artificially introduced into certain lands below the level of the sea; and by repeating this operation for two or three years, considerable tracts have been raised in the estuary of the Humber, to the height of about six feet. Large quantities of coarse sand and pebbles are also drifted along at the bottom, and where such a current meets with any deep depression in the bed of the ocean, it must necessarily fill it up, just as a river when it meets with a lake in its course, fills it gradually with sediment." The bed of the German Ocean, according to the accurate survey of Mr Stevenson,* is traversed by several enormous banks, one of which, occupying a central position, extends from the Firth of Forth in a north-easterly direction, to a distance of one

Edin. Phil. Journal, vol. v.

hundred and ten miles. Others run from Denmark and Jutland, upwards of one hundred and five miles to the north-west; while the greatest of all, the Dogger Bank, stretches for upwards of three hundred and fifty-four miles from north to south. The whole superfices of these shoals is equal to about one-fifth of the entire area of the German Ocean, or to about one-third of the whole extent of England and Scotland. The average height of the banks measures about seventy-eight feet; the upper portion consisting of fine and coarse silicious sand, mixed with shells and corals.

If such, then, be the ascertained rate at which detrital matter is constantly accumulating by the action of rivers and the ocean, it ought to afford some data for ascertaining the period within which the ancient and now consolidated strata of the globe were formed. Thus, suppose that our largest rivers accumulated one foot in depth of solid strata annually, we would have, in two thousand years, a deposit of the same amount over the space which their waters covered. And suppose that the ocean, on an average, accumulated one-fifth of this quantity over its whole superfices annually, and that this matter was collected by tides and currents into numerous beds of unequal thickness, we can easily conceive, that over particular regions, the accumulated collection of oceanic and fluviatile debris would amount, in the same period of time, to masses of

28

PROOFS OF THE RAPID ACCUMULATION

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.