the most accurate sections have been made, and where the most regular succession of these strata takes place, the primary rocks are found associated on the north-western side of the island, the coal measures and new red sandstone in the middle, while the lias, oolite, chalk, and newer formations, form the eastern and southern boundaries.* It is also sufficiently ascertained that the aggregate thickness above mentioned does not extend throughout the whole space occupied by those strata. Indeed, it may be assumed as the maximum rather than the average thickness, for many of the strata are very unequal, deepening in some places so as to form hollow troughs, and thinning out in others so as entirely to disappear. Yet, taking the most moderate computation, the mass of accumulated matter appears so enormous, that, in the opinion of many geologists, its gradual deposition, in layer after layer, must have occupied a vast period of time, which has been so far divided into epochs or eras from indications of change and reyolution which the strata seem to have undergone.

Now, it must be observed, that in many of the sedimentary formations, but especially in the primary strata, it is exceedingly difficult to ascertain their actual thickness. A succession of stratified rocks may, on a general view, appear to

* See Section, Plate IV.

have a continuous inclination to each other for miles in extent, and thus seem of enormous thickness, whereas it may only be a bed of very moderate dimensions, broken up by repeated wave-like eruptions of igneous rocks from below, which may not always make their appearance on the surface. The sedimentary matter may have originally been deposited by a current of water flowing over a sloping channel, by which means a succession of inclined strata may have been formed, extending for a long space horizontally, although of no very considerable depth, - a mode of deposition which may be witnessed daily in many river currents, and which has been so well illustrated by M. de la Beche.*

In the schistose strata, too, it is exceedingly diffi. cult to form a probable guess at their thickness, from the extreme confusion and irregularity of their position, and from the lines of cleavage being often mistaken for those of stratification. If these slatey masses, too, owe their laminar, and perhaps their stratified, appearance, in a considerable degree, to a process of crystallization,+ this will add to the difficulty of ascertaining the actual depth of the original deposition.

Theoretical Geology.

+ Vide Boase on Primary Strata.

20

SECTION II.

PROOFS OF THE RAPID ACCUMULATION OF SEDIMENTARY MATTER AT PRESENT FORMING.

As it is universally agreed, that almost all the sedimentary strata have been formed from the wearing down of older rocks, in a manner similar to the processes of disintegration, which are taking place in the present land and ocean, we must first examine the amount of this, in order to compare it with the operations of preceding ages.

All our rivers are continually carrying down to the sea the matter of the rocks and soil over which their currents flow. This process is daily taking place, but in seasons of flood it is prodigiously augmented; so that, the mouths of our smallest streams, basins, and harbours, are year after year filled up with the accumulated silt; and in the larger rivers of the world, extensive deltas of solid matter are heaped up for hundreds of miles into the sea.

Pliny long ago remarked the extraordinary accumulation of detritus in the Mediterranean Sea and

Persian Gulf, and the rapid advance of the land upon the sea. From more recent observations, it

has been calculated that not less than one thousand square miles of solid land have been accumulated there within the last two thousand years. Celsius has also given details of the filling up of the Gulf of Bothnia, from which it appears that in many parts of this gulf, the process of accumulation had shallowed the water three feet in the course of fifty years. According to Linnæus, the annual increase of land on the eastern side of Gothland, near Hoburgh, had been for ninety years at the rate of from twelve to eighteen feet in extent.

Sir George Staunton estimated the waters of the Yellow River in China to contain one part of earthy matter in two hundred; and that the river carried down hourly two millions of cubic feet, or forty-eight millions daily. From some recent experiments on the waters of the Ganges, Mr Everest has ascertained that, during the rainy season, the river holds suspended upwards of one four-hundredth part by weight of earthy matter, and he has calculated that, taking the average of the whole year, the enormous quantity of six thousand three hundred million cubic feet are discharged annually, —a mass equal in bulk and weight to sixty of the great Pyramids of Egypt. The delta forming at the mouth of this river, extends to a length of two hundred and twenty miles.

Major Colebrook mentions the rapid filling up of some of the branches of the Ganges, and the excavation of new channels, where the mass of soil removed in the course of a few years amounted to forty square miles, or 25,600 acres. The immense transportation of earthy matter by the Ganges and Megna, is proved by the great magnitude of the islands formed in their channels, during a period far short of a man's life.*

Similar accumulations are in the process of formation at the mouths of all the great rivers of the world. The Amazon carries the disintegrated rocks and soil of the South American continent into the Atlantic Ocean. Captain Sabine could distinguish the current of this immense river three hundred miles out at sea; and, aided by an oceanic current flowing with the rapidity of four miles an hour, from south to north, the sedimentary mass is carried on as far as the mouth of the Orinoco, forming an extensive swamp along the coast of Guiana, and a long range of muddy shoals bordering the marshes. The sediment of the Orinoco is partly detained, and settles near its mouth, causing the shores of Trinidad to extend rapidly, and is partly swept into the Caribbean Sea by the equatorial current. The rivers which flow from the high lands of Mexico, especially when swollen by the tropical

* Lyell's Geology. Cuvier's Ossmens Fossiles. Playfair's Illustrations of Hutton.