Page:The parochial history of Cornwall.djvu/40

xxxii therefore be again referred to the work already alluded to on "Primary Geology."

Before concluding this brief sketch, a few words must be said on the beds of clay, sand, and gravel which cover the low grounds, both on the granite and on the slate. These may be resolved into two kinds: 1st. those resulting from the decomposition of the rocks, and which are in their original position; and 2nd. those which are not in situ, affording by their appearance, composition, and position, evidence of their having been transported. These deposits belong to three distinct periods, and alternate with terrestrial and marine remains, according to their situation in respect to the sea-level at their respective epochs. Each of these transports was accompanied, or followed by, a change in the sea-level: the last corresponds with that now existing; the previous one, by the elevated ancient beaches, indicates that the sea was about fifty feet above its present level; and the earliest transport, being covered with sub-marine forests, shows that the sea was at that epoch at least fifty feet lower than it now is. (See 4th vol. Geol. Trans. of Cornwall, p. 466, et seq.) In the lowest or oldest deposit the tin-ore (stream-tin) is found in the form of sand and gravel mixed with earthy substances: it affords the purest kind of tin, known in the market by the name of grain-tin; and appears to be the source from which the ancients derived all their tin. The notices under each parish will furnish more particulars concerning these interesting deposits.