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



In examining specimens of Sigillaria from the Coal-sliales, it may- 

 be seen, in many cases where the trunks have lain prone, that the 

 flutings in the shale exhibit markings exactly resembling " Slicken- 

 sides " on a small scale, and that the strige run on each side of the 

 grooves straight down to the centre. 



Now in such a case as this, clearly no faulting can have taken 

 place. It is therefore necessary to seek for some agent that will pro- 

 duce a change of position of the thin layer of coal representing the 

 tissues of the plant relatively to the shale. 



Two such offer themselves, viz. simple compression, or the 

 removal of some soluble matter. The former is no doubt the one 

 which has been operative in the present instance, as we know that 

 the process of conversion of vegetable tissues into coal is accom- 

 panied by an immense reduction in volume (it has been com- 

 puted that 8-76 feet of vegetable matter would only yield one foot 

 of coal). 



Assuming, for simplicity of illustration, that the grooves in a Sigil- 

 larian stem are -1 of an inch in depth, and -2 in. wide, then the coal 

 contained in each groove will represent a thickness of -438 in. of 

 vegetable matter which has been pressed down, and the top of the 

 layer of coal will have travelled downward -428 in., while down the 

 sides of the furrow movement to a less extent will have taken place. 

 This movement would be, I think, amply sufficient to account for 

 the slickensiding observed. 



This explanation, which suggested itself to me when studying a 

 series of Carboniferous plants at the Normal School of Science, led 

 to the elucidation of a problem having much in common with the 

 foregoing, though presenting many peculiar features. 



In the celebrated " Eose and Crown " Chalk-pit at Eiddlesdown, 

 near Croydon, numerous Marl-bands occur in the upper portion of 

 the Middle Chalk which is there exposed, and others may be seen in 

 the railway cuttings to the south of the pit: and while collecting 

 fossils there, I observed many large detached blocks of chalk, upon 

 the upper surface of which a band of marl of a thickness varying 

 from a minute fraction of an inch up to about 5 in. rested. 



Being attracted to this marl by the unusual abundance of fossils 

 contained, I was led to examine it carefully in situ, and during 

 successive visits the following features were made out : — 1. The 

 abundance of fossils (as just mentioned). 2. That while the marl 

 merged very gradually into the chalk above, it terminated quite 

 abruptly below. 3. Fragments of chalk were freely distributed 

 through and completely embedded in the marl. 4. The fossils — 

 even delicate sponges indicated merely by an iron stain — were 

 occasionally found Jilled loith ivhite cliallc. 5. Thin films of the 

 tea-green marl could be seen covering the vertical joint-planes. 

 6. The mai'1-band rested upon an extremely rough irregular surface 

 of chalk, the prominences of which were sliclcensided all round, the 

 strias being directed from apex to base. 



This last fact, when viewed by the light of the theory propounded 

 regarding the Sigillarise, gave the clue to the solution of the problem ; 



