388 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Juiie 19, 



The white clays of North Wales are contained in pockets in the 

 limestone, and could have had no other origin than by the gradual 

 dissolution of the latter, the cavities being complete cuts de sac to 

 which a mere mechanical abrading power could gain no approach ; 

 and the fact that vast quantities of redeposited lime, as tufa, occur 

 in the neighbourhood of the pockets proves an extensive removal of 

 the limestone by watery dissolution. That a similar process has been 

 going on with the chalk (distinct from the mere mechanical effects of 

 rain- wash) is a fact that all geological observers have been made 

 familiar with, both from the surface of the chalk being pitted with 

 sand-galls and sand-pipes, and the frequent occurrence of tufa in the 

 chalk districts ; such deposits, however, represent but a very small 

 proportion of the carbonate of lime that has been removed by solu- 

 tion, as the greater part would be carried to the sea before it could 

 be redeposited ; and, as has been shown by Mr. Sorby, lime in 

 solution can be absorbed and redeposited into the substance of the 

 chalk itself. 



All chalk contains both silica and alumina, which could not be re- 

 moved in solution ; and I beg to submit the following analyses of 

 chalk and white Tertiary clays, in proof that the clays may have 

 been composed from the insoluble matter in the chalk. 



Many of the Tertiary white clays of Devonshire and Dorsetshire 

 are almost chemically pure silicate of alumina, or impalpable silica ; 

 and it seems impossible to account for their accumulation, almost 

 entirely free from foreign admixture, as the result of the mere 

 mechanical degradation of previously existing beds. 



However effective the separating-power of water may be in sorting 

 and dividing coarse from fine matter, its mechanical operation could 

 not isola.te silicate of alumina from other materials of similar specific 

 gravity. 



Another point to be noticed is the peculiarly fine state of subdi- 

 vision of the white Tertiary clays, an impalpable condition which 

 distinguishes them from nearly all other argillaceous deposits. 



Many of the Coal-measure clays are extremely fine and pure ; but 

 their state of subdivision (for they invariably contain some propor- 

 tion of coarse particles) clearly indicates a detrital origin. The bulk 

 of the matter composing the white Tertiary clays, however, is quite 

 impalpable, though sandy particles may be occasionally associated 

 with them. 



In testing the state of division of the Bovey Tracey and "Wareham 

 clays, I found that, after mixing them with water to the consistency 

 of cream, and passing them through fine silk lawn, containing 10,000 

 perforations to the square inch, no appreciable quantity of coarse 

 matter remained behind from most of the examples, not even to the 

 weight of a grain out of several pounds of clay. I can state, from 

 the result of a number of experiments on clays and marls of various 

 ages and formations, that such a state of subdivision is peculiar to 

 these Tertiary clays. 



Mr. Sorby has demonstrated that from 90 to 95 per cent, of the 

 mass of the chalk consists of the cases of Foraminifera and com- 



