p. F. Kendall— On " Slickensules." 553 



but in this case the agent which effected the movement necessary 

 to produce " slickensiding " was not, I believe, compression, but 

 the action of a solvent, which, by removing some portion of the 

 rock, allowed a subsidence of the remainder, and the dissolution 

 being unequal, caused bosses to be left standing up, which were then 

 " slickensided " by the downward movement of the chalk or of the 

 insoluble residuum around them. 



This explanation will account for the whole of the peculiarities 

 detailed above. The solvent agent was in all probability water 

 charged with CO.2, which, percolating through, attacked the chalk 

 and converted the carbonate of lime (CO Cao") into the dihydrio 

 carbonate [(CO Ho).3 Cao"], which, being readily soluhle in water, 

 would be carried away, leaving the insoluble constituents to form 

 a layer of marl. 



This has taken place at certain definite levels, determined possibly 

 by the occurx-ence of a bed of chalk having a denser texture or 

 fewer joints (both of which characters appear in the chalk under 

 the marl bed I am describing). 



So soon as a layer of marl of sufficient thickness and impermea- 

 bility had been produced, all further dissolution below would be 

 arrested — hence the relatively sharp definition of the lower boundary 

 of the band — and the dissolving process would then encroach upon 

 the upper part of the chalk, its completeness being in inverse ratio 

 to the distance from the impervious layer. 



The solubility of the chalk must necessarily be very unequal, 

 depending as it does upon the character of the preponderating 

 organism at a particular spot, and thus fragments or nodules may 

 escape dissolution through a very slight retardation of the process 

 allowing time for the formation of a protecting film of marl. 

 Joint-fissnres would be filled with marl by the flow of water through 

 them acting upon their sides, only, however, to an infinitesimal 

 extent, as will be seen from the sequel, but mainly by actual 

 downwash. 



The abundance of fossils will be readily understood when it is 

 considered that all the shells preserved in the Surrey chalk are 

 composed of calcite, and each layer of marl represents a great 

 thickness of chalk. 



The question now presents itself: — What amount of chalk is 

 required to yield a layer of marl of a given thickness ? This is 

 a question to which, I think, it is impossible to give an answer 

 which is more than a very rough approximation ; but the results of 

 two analyses which I have made furnish some data of value in such 

 an estimation. 



Taking a specimen of chalk from a point one foot above the marl- 

 band, which I may mention is about forty feet below the lowest 

 layer of flints in the " Eose and Crown " pit, I cut a rod of three 

 cubic inches, which, after thoroughly drying at 100° C, I weighed, 

 and the same process was gone through with the marl, only "5 of 

 an inch, however, being available. From each of these a weighed 

 sample was taken and digested with acetic acid for eighteen hours; 



