Geology of England and Wales 4"c." 216 



fragments of chalk and flint is interposed ; leading us to 

 the conclusion, that after the consolidation of the chalk, a 

 partial destruction took place by running water, and that, 

 of course, a considerable period must have elapsed between 

 the deposition of the chalk and the plastic clay.* 



Subordiiiate beds of chalk, marie, and fuller's earth, and 

 thin partings, or seams of clay, are found in the chalk stra- 

 ta. The numerous layers of nodular flints, and veins and 

 tabular masses of the same, so characteristic of this forma- 

 tion, are most abundant in the upper layers ; and hence the 

 distinction into Upper and Loioer Chalk. We extract the 

 following ingenious hypothesis of the original production of 

 these flints. 



" The chalk even yet, often contains a mixture of silex ; 

 at the period of its formation, a considerable quantity ap- 

 pears to have been precipitated with it. in a state of such 

 minute division as to allow the chemical attraction of its 

 molecules to have effect ; these (from the same causes 

 "which produced the formation of layers of calcareous con- 

 cretions in beds of clay) separating from the cretaceous 

 pulp, and uniting together, particularly where the presence 

 of any imbedded organic remains, (e. g. alcyonium, sponge, 

 or shell) offered a nucleus for them to form upon, constitu- 

 ted the layers of nodular siliceous concretions in question." 

 p. 70. 



The organic remains are not so numerous in the chalk, 

 as in some other formations ; but are very interesting. In 

 all the strata above the chalk, the genera, and some of the 

 species agree with those now existing on earth : the extinct 

 species, however increasing in number as we descend. But 

 in the chalk, many unknown genera occur, and probably 

 not one species of which we have any account in a living 

 state. Among these relics are a species of shark, several 

 genera and species of univalve, bivalve, and multivalve 

 shells, echinites, asterites, encrinites, madreporites, and 

 numerous remains of alcyonium and spongia. The aggre- 

 gate thickness of the chalk beds in England is between 600 

 and 1000 feet. 



The chalk strata, with all the strata above them, ar6 



* There are also numerous ravines on the surface of the chalk, and be- 

 neath the upper or tertiary formations, which appear to have been formei?, 

 by the action of currents of water, perhaps diluvial. — {^Ed.} 



