This enigma is explained, however, when it is observed, that the basis of the whole tract just described is a red sandstone, often containing in it a hard quartzy gravel, perfectly similar to that which has just been mentioned. From the dissolution of beds of this sandstone, which formerly covered the present, there can be no doubt that this gravel is derived. But, as the gravel is in general thinly dispersed through the sandstone, and abounds only in some of its layers, it should therefore seem, that a vast body of strata must have been worn away and decomposed, before such quantities of gravel as now exist in the soil could have been let loose. 338. I have said, that a rock capable of affording such gravel as this, is not to be found in the tract of country just mentioned. This however, is not strictly true; for in Worcestershire, between Bromesgrove and Birmingham, about seven miles from the latter, a rock is found consisting of indurated strata, greatly elevated, and without doubt primitive, from the detritus of which such gravel as we are now speaking of might be produced. These strata seem to rise up from under the secondary, where they are intersected by the road; and, for as much as appears, are not of great thickness, so that they cannot have afforded the materials of this gravel directly, though they may have done so indirectly, or through the medium of the red sandstone; that is to say, a primary rock of which they are the remains, may have afforded materials for the gravel in the sandstone; and this sandstone may in its turn have afforded the materials of the present soil, and particularly the gravel contained in it. 339. Pudding-stones being very liable to decomposition, have probably, in most countries, afforded large proportion of the loose gravel now found in the soil The mountains, or at least hills, of this rock, which are found in many places, prove the great extent of such decomposition. Mount Rigi, for instance, on the side of the Lake of Lucerne, is entirely of pudding-stone, and is 742 toises in height, measured from the level of the lake. By the descriptions given of it, as well as of other hills of the same kind in Switzerland, we may, without due attention, be led to suppose that they are entirely formed of loose gravel. Even M. Saussure's description is chargeable with this fault, though, when attended to, it will be found to contain a sufficient proof, that this hill is composed of real pudding-stone.[168] The nature of the thing also, would be sufficient to convince us, that a hill, more than 4000 feet in height, could not consist of loose and unconsolidated materials. If, then, we regard Mount Rigi as the remains of a body of pudding-stone strata, we must conclude, that these strata were originally more extensive, and the adjacent valleys and plains will serve, in some degree, to measure the quantity of them which time has destroyed. [Footnote 168: Voyages aux Alpes, tom. iv. § 1941.]