conception. If for the purpose of arriving at more definite ideas we select from the pile of strata some part whose origin is clear, and whose formation was continuous, the time still comes out enormous. The Coal-formation, in South Wales, is 12,000 feet thick, and through a great part of its thickness there, gives proof after proof that this whole series of strata was deposited bed by bed at or nearly at the level of the sea; the estuarine alveus continually sinking, and continually receiving additions of fresh sediment. Many beds of Coal, amounting to one hundred and twenty feet in thickness, alternate with these sediments. Not a single bed of limestone occurs in the series; towards the bottom are marine shells, above only freshwater shells and land-plants; the whole mass agreeing with the facts of estuarine accumulation at the mouth of a great river. If the growth of the sediment were at the same rate as the waste of the land, 12,000 × 12 × 111 = the years employed in the production of the sediments = 15,984,000 years. But there are circumstances in the Coal-formation which greatly modify this result. It is evidently a deposit in quiet water-quiet as compared with the agitation of the open ocean. In such a case the addition of sediment would not follow the law of open sea-deposits but rather that of lakes, and so the strata would be for the most part accumulated in a sort of delta, or along a limited range of coastline. The Coal-field of South Wales is connected by the character and succession of the Strata with those of the Forest of Dean, Kingswood, and Somersetshire. This gives a length of 125 miles and a maximum breadth of 50, which may be assumed to have been one great deposit, on the southern side of one line of coast. If the 6250 square miles thus calculated were all deposited from the spoils of one great river equal to the Ganges, something less than half an inch would be deposited in a year, and 12,000 feet in 333,000 years. If we vary the computation by taking what may be thought a fairer basis of comparison, and suppose a stream equal to the great Indian river to have produced the Wealden deposits-3000 square miles and 1000 feet thick-twelve thousand years would accomplish the work, but the Wealden deposits probably extended to more than four times the area, though with a less average thickness. But the sea also attacks the land, often very strenuously, and along the vast line which it beats with restless energy, many parts of the coast yield much, all yield something, to the watery hammers. The rate at which the sea wastes the land depends on the nature of the coast, and the force and direction of the currents. A cliff of shale or glacial de tritus falls away very rapidly in consequence of the decay of its base. On the coasts of Yorkshire the fertile lands of Holderness have lost from this cause 2 yards per annum, during the whole period since it has been carefully noted. So the feeble shores of Norfolk, Suffolk and Essex have suffered enormous loss. The picturesque cliffs of the southern coast of England, from Dover to Devonshire included, composed of chalk, sand, sandstone, and indurated clays, are suffering rapid displacement. Look at the many arched rocks and island-peaks in the Purbeck beds about Lulworth; contemplate the Chalk Needles of the Isle of Wight; consider the vast heap of pebbles in the Chesil bank; climb the sliding Lias cliffs of Charmouth; everywhere signs of recent, daily, destruction. Stand on the high ridge of the chalk where it fronts the sea in Warburrow Bay; mark the incomplete ring of the fortifications which sheltered British or Saxon warriors,-incomplete because since that comparatively modern date the sea has reclaimed a part of his ancient domain, and you will have a true idea of the real recession of our island boundary. Measure on the lofty summit of Golden Cap, near Lyme Regis, the fissures prepared for many yards to yield at once along the precipitous face; look at the Preventive stations on the cliffs at Lulworth and in sight of Kimmeridge R. L. K which threaten to fall, and must soon be removed; see this, which is a fair specimen of the average waste of English coasts, and specially of the south coast, before adopting an estimate of the rate of annual loss. In his recent work, On the Origin of Species, Mr Darwin assumes the rate of waste for the Wealden coast to be one inch in a century; I should have preferred, and do prefer, an estimate of one inch in a year, that is to say, one hundred times as great, and I suppose that by most observers this will be thought too low an estimate for all but the most invincible coasts. Still the effect of all this violence of the waves on the production of materials for the sea to deposit in new strata is not great, compared to the powerful action of the atmosphere on the broad surfaces of the land. For example, assume an extravagantly high rate of waste, such as that on the coasts of England, which certainly cannot equal 1 foot in a year; apply it to the whole of the sea-coasts of the world; assume these to be equal to four circuits of the globe, that is to say, 100,000 miles in extent, and 100 feet high. 100000 × 100 × 1 The annual waste will be 52802 = 0.4 cubic miles of sediment. The area of the land being as before assumed equal to the area of new deposits, we have 50,000,000 sq. miles covered 20th inch deep with these spoils won by the sea from the land. The quantity of sediment derived from cliffs, although computed on exaggerated data, being so very much less than that obtained by the universal waste of the surface of the land, may be neglected in future calculations. Keeping still the same purpose in view, the attainment of some probable estimate of the time which was consumed in the production of some definite part, if not the whole, of our stratified deposits, we may take as a new basis of computation the rate of growth and duration of life of some races of fossil plants and animals. Thus at several stages in the Coal-formation of Yorkshire and Derbyshire occur beds of shale some inches or one foot in thickness which are full of Unionidæ. The area over which these shells in their peculiar beds can be traced is sometimes as much as 40 miles long in one direction, parallel to the edge of the coal-field; the breadth is considerable, may be as great, indeed, but can be proved for a few miles. They have not been spread over this area by drifting, but by natural distribution of the young, as we see happen in the lower parts of rivers approaching the sea and the beds of lakes. Modern Unionidæ live some years. Suppose only the period of one life, say five years, to have passed during the distribution of the |