



232 Mr. Webster on the Strata lying over the Chalk, 



larly interesting, since it enables us to trace back, in some degree, 

 the great changes which have taken place upon the surface of the 

 earth. 



In that part of our island which we are now considering, this 

 alluvium or covering is of a nature peculiar to it. Besides the 

 vegetable earth, clays, marls, and sands, which it possesses in com- 

 mon with other places, it is distinguished by a vast quantity of 

 rounded siliceous pebbles of various kinds and sizes, which lie dis- 

 tributed in a very unequal manner, sometimes forming thick beds 

 intermingled with clay, sand, and small sharp fragments of flints, at 

 other places mixed with shells of various kinds, and sometimes 

 almost without any other substance. This compound is termed 

 Fli?2t graveL 



When we observe a heap of these pebbles, we easily see that they 

 consist of a great variety of kinds, and upon attentively examining 

 them we are able to reduce this variety to several classes. 



Some are evidently fragments of the flinty nodules originally be- 

 longing to the chalk strata. This is evinced by their mineralogical 

 characters, their sharp conchoidal fracture, peculiar black colour, and 

 by portions of the white crusts with which they were invested while 

 in the chalk beds still remaining attached to them. 

 . . In others this origin is not so evident, the crusts having been 

 entirely worn off, and the fragments themselves rounded by attrition. 

 Yet their fracture, colour, and other circumstances, oblige us to sup- 

 pose that these also were derived from the chalk. In many places 

 the whole or the greater part of the gravel consists of these rounded 

 chalk flints ; and hence probably some have been induced to sup- 

 pose that all the pebbles of the London gravel have proceeded from 

 the same source. 



But besides these other siliceous nodules occur, the origin of 



