Illustrations of the Huttonian Theory of the Earth

by John Playfair

[184] Barrow's Travels into Southern Africa, p. 245.

It is evident, therefore, that the form of this land has been determined by the slow working of the streams. The causes which produced the effects here described, began their action from the line of greatest elevation, and extended it from thence on both sides, in opposite directions. This is the most precise character that can mark the alluvial operations, and distinguish them from the overwhelming power of a great debacle.

367. Lastly, if there were any where a hill, or any large mass composed of broken and shapeless stones, thrown together like rubbish, and neither worked into gravel nor disposed with any regularity, we must ascribe it to some other cause than the ordinary detritus and wasting of the land. This, however, has never yet occurred; and it seems best to wait till the phenomenon is observed, before we seek for the explanation of it.

368. These arguments appear to me conclusive against the necessity of supposing the action of sudden and irregular causes on the surface of the earth. In this, however, I am perhaps deceived: neither Pallas, nor Saussure, nor Dolomieu, nor any other author who has espoused the hypothesis of such causes, has explained his notions with any precision; on the contrary, they have all spoken with such reserve and mystery, as seemed to betray the weakness, but may have concealed the strength of their cause. I have therefore been combating an enemy, that was in some respects unknown; and I may have supposed him dislodged, only because I could not penetrate to his strongholds. The question, however, is likely soon to assume a more determinate form. A zealous friend of Dr Hutton's theory, has lately[185] declared his approbation of the hypothesis which has here been represented as so adverse to that theory; and, from his ability and vigour of research, it is likely to receive every improvement of which it is susceptible.

[185] Trans. Royal Society Edin. vol. v. p 68.

Note xix. § 117.

Transportation of Materials by the Sea.

369. The existence of the great and extensive operations, by which the spoils of the land are carried all over the ocean, and spread out on the bottom of it, may be supposed to require some further elucidation. We must attend, therefore, to the following circumstances.

When the detritus of the land is delivered by the rivers into the sea, the heaviest parts are deposited first, and the lighter are carried to a greater distance from the shore. The accumulation of matter which would be made in this manner on the coast, is prevented by the farther operation of the tides and currents, in consequence of which the substances deposited continue to be worn away, and are gradually removed farther from the land. The reality of this operation is certain; for otherwise we should have on the sea shore a constant and unlimited accumulation of sand and gravel, which, being perpetually brought down from the land, would continually increase on the shore, if nature did not employ some machinery for removing the advanced part into the sea, in proportion to the supply from behind.

The constant agitation of the waters, and the declivity of the bottom, are no doubt the causes of this gradual and widely extended deposition. A soft mass of alluvial deposit, having its pores filled with water, and being subject to the vibrations of a superincumbent fluid, will yield to the pressure of that fluid on the side of the least resistance, that is, on the side toward the sea, and thus will be gradually extended more and more over the bottom. This will happen not only to the finer parts of the detritus, but even to the grosser, such as sand and gravel. For suppose that a body of gravel rests on a plane somewhat inclined, at the same time that it is covered with water to a considerable depth, that water being subject not only to moderate reciprocations, but also to such violent agitation as we see occasionally communicated to the waters of the ocean; the gravel, being rendered lighter by its immersion in the water, and on that account more moveable, will, when the undulations are considerable, be alternately heaved up and let down again. Now, at each time that it is heaved up, however small the space may be, it must be somewhat accelerated in its descent, and will hardly settle on the same point where it rested before. Thus it will gain a little ground at each undulation, and will slowly make its way towards the depths of the ocean, or to the lowest situation it can reach. This, as far as we may presume to follow a progress which is not the subject of immediate observation, is one of the great means by which loose materials of every kind are transported to a great distance, and spread out in beds at the bottom of the ocean.