236 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 contem- 

 poraneous agency of similar causes in different places. Prodigious 

 banks of such pebbles are thrown up on our shores at the present 

 lime, 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, 1 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. 



