554 P. F. Kendall— On " Slickensides." 



the residues were then filtered off, dried, and weighed, with the 

 results given below. 



3 cubic in. of Chalk = 86-5 gv. -5 cubic in. of Marl = 19'6 gr. 

 2"92 gr. chalk yielded an insoluble residue weighing -0645 gr. 

 1-6939 gr. marl „ „ ,, -503 gr. 



The insoluble residue from the Chalk amounts to 2*2 per cent., while 

 that of the marl is almost exactly 30 per cent. ; therefore if my estima- 

 tion be correct, such a band of marl 1 in. thick yields a residue equal 

 to that from 18-277 in. of the Chalk immediately overlying it. Even 

 this is by no means a full estimate of the amount of Chalk in its 

 original purity and of its original composition which a Marl band of 

 that thickness represents ; for, apart from the question of what has 

 become of the aragonite shells, which were doubtless once present 

 in their due proportion, the process of dissolution has probably been 

 going on without interruption so long as the Surrey Chalk has been 

 exposed at the surface ; moreover, the specimen of chalk which I 

 used was by no means free from marl, one or two excessively fine 

 layers being visible on the clean scraped surface of the rod. This 

 would in a measure account for the very high percentage of insoluble 

 matter present. The marl, too, was an unusually chalky specimen. 

 Taking my figures as they stand, it would appear that this Marl-band 

 where attaining its maximum thickness (5 inches) indicates the 

 dissolution of 7 ft. 7 in. of Chalk — a really surprising result — and 

 one which, if established, may go far to explain some of the dis- 

 crepancies which are apparent when a comparison is made of the 

 thickness of the zones of the Chalk at points not very remote. 



With a view to pursuing this inquiry further, I shall be very glad 

 to receive any information as to the extent, number, and thickness of 

 the Marl-bands in the different zones at all exposures of the Chalk. 



A very interesting piece of evidence, which appears to be corro- 

 borative of the views expressed in this pajjer, was furnished mo by 

 Mr. James Clark, M.A., to whom I had communicated my ideas upon 

 this subject. This gentleman found in the " Eose and Crown " Pit 

 a slender cylindrical flint, which for about half its length was most 

 beautifully "slickensided" upon all sides, the strias being longitudinal. 

 Unfortunately Mr. Clark was unable to give me any information as 

 to the original position of the flint, as when he saw it it was embedded 

 in a block of chalk which was just about to be thrown into the lime- 

 kiln ; but on a careful examination of chalk I found a very thin seam 

 of marl scarcely exceeding in thickness the paper on which this 

 communication is printed, which traversed the block in a plane at 

 right angles to the axis of the flint, and placing it almost beyond a 

 doubt that, like so many of the flints from this part of the Chalk, the 

 one in question had stood in a vertical position. 



Coal-balls I have observed to be " slickensided " in a very similar 

 manner, and containing as they do uncrushed vegetable tissues, it 

 appears to be a legitimate inference that an infiltration of carbonate 

 of lime has taken place very soon after the deposition of the Coal 

 (as peat), and that the incompressible mass has been marked by the 

 movement of compression of the surrounding Coal. The Septaria of 



