If therefore we admit that the pebbles of our gravel are derived from the destruction of former strata, that many of them differ from any other siliceous bodies with which we are acquainted, and that these bear perhaps a stronger resemblance to the flints of the Paris beds than to those of the chalk (judging from some specimens of French flints in the Museum of the Geological Society), and if we have reason to believe that some of those beds formerly existed in this country, will it be considered as a conjecture rash and unwarranted, should I imagine that the substances in question have derived their origin from siliceous nodules, originally formed in strata which existed over our blue clay, but which have been disintegrated, and carried off, in one of those revolutions to which this part of the earth has been subjected?
The formation of the nodules of flint in chalk has frequently, and will probably much longer continue to excite the speculations of philosophers. But whatever was the mode in which they originated, we may fairly imagine that these siliceous masses in question now found in our gravel, (but which I have supposed to have belonged originally to regular strata) were formed in a similar manner.
The coats of the flints in chalk have been found by chemical analysis to consist of chalk mixed with the flint; and it appears that all nodules formed in a matrix have crusts or coats, composed of a combination between the matter of the nodule and the enveloping substance.
The coats of these pebbles may be supposed therefore to exhibit a similar phenomenon; and in them we may perhaps see a part of the stratum which they were at first imbedded in.
- The Egyptian pebbles are said to occur in a similar situation.