Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/246

From Wikisource
Jump to navigation Jump to search
This page has been validated.
Mr. Webster on the Strata lying over the Chalk.

bles are almost all partly broken, and lie exactly in that confusion in which they appear when thrown up by the sea upon the shore. Water-worn pebbles are mentioned as occurring in the lower beds of the French calcaire grossier, and a number of other examples might be adduced; but these are sufficient to shew the contemporaneous agency of similar causes in different places. Prodigious banks of such pebbles are thrown up on our shores at the present time, and may serve very well to explain the origin of these ancient formations of flint gravel.

But with respect to the concentric pebbles above described, conceiving it to be improbable that they have been derived from chalk flints, I am compelled to look for their origin to some other source.

Considering them, as well as all the other bodies composing the alluvium, as detritus or ruin, a circumstance sufficiently shewn by the confused manner in which they lie, and by the water-worn appearance of a great part of them, it is unnecessary for us to confine ourselves to the chalk in seeking for the beds to which they originally belonged.

Siliceous nodules are frequent in other limestone strata; an excellent instance of which may be seen at Tillywhim quarry in the Isle of Purbeck. Trap also, as is well known, forms the matrix from which many agates are derived. Various silices, such as the carnelian, onyx and agate, invested with the usual crusts, are described by De Luc as being spread over the hills near a part of the course of the Rhine, while there are not in Europe any known natural strata that contain these stones.

One of the most remarkable circumstances in which the basin of the Isle of Wight differs from that of Paris, is the absence in the former of those siliceous formations so abundant in the latter.