38 



THE GEOLOGIST. 



TMs is confirmed by the occurrence of slickensides at different angles in slides 

 of the rock within the faults, and also by highly polished surfaces, occurring in 

 very slight faults, which displace the strata only a foot or eighteen inches. 



I believe it is the compact sandstone ^vithin the faults, and not the slicken- 

 sides, that is considered to act as a barrier to subterranean water. — I remain, 

 dear sir, yours etc., George Hilloston, F.G.S., Liverpool. 



Slickensides." — De^r Sie, — The subject of "slickensides" is one to which 

 I have paid some attention, and I have always noted as many of the facts 

 relating to this appearance on rock-surfaces as I possibly could. I have read 

 wdth much interest in the last number of the " Geologist" the queries on 

 "slickensides" submitted to the Geological Section of the British Association 

 during the meeting at Aberdeen, by Mr. Price, and the replies by Professor 

 Ansted. Permit me to offer a few remarks on this really curious and interest- 

 ing subject. 



The formation of a " slickenside" on any rock-surface is due to the sliding 

 of one rock-mass on the other, the motion very possibly having been a slow 

 one, but exerted under enormous pressure, and without the aid of more heat 

 than would have been produced by the friction. The result of this motion 

 w^ould, in the first instance, be the pulverization more or less of the two 

 opposing rock-sui-faces, and when this crushing action ceased, the re-consolida- 

 tion of that crushed material, by means of enormous pressure, accompanied by 

 motion. 



I find in limestones and sandstones that most usually the " slickenside-" striae 

 are on the surface of the beds, and theii' direction frequently parallel to that of 

 the dip. In my geological notes I find many references like the following : 

 " Slickensides-striee parallel to the dip of the beds, showing vertical displace- 

 ment in the mass ;" but when the striae are transverse to the dip, and in the 

 direction of the strike of the beds, I say that " horizontal displacement is 

 radicated." Li either iustance of course there is no fault produced iti 

 the strata, though a displacement of them en masse is clearly uidicated, the 

 direction of which being pointed out by that of the striae of compression. 

 This is the only way iu which rock-masses can be displaced without being 

 faulted. 



The slickenside-striae are frequently oblique to the dip of the beds, the angle 

 of obliquity being of course variable, but alw^ays indicatuig the direction of 

 the displacement. 



The thickness of the slickenside, or striated substance, is very variable ; 

 sometimes it is as thin as card-paper, at others nearly an iiich from one surface 

 to the other. It varies also in its internal structure : sometimes it consists 

 throughout of a series of very thin and finely striated lamellae, wliich readily 

 flake off from each other by the application of a penknife, or when struck on 

 the fractured edges with the hammer. At other times this structiu'e is only 

 partial, and confined to the surfaces ; and again, when the slickenside is tolera- 

 bly thick, it is homogeneous throughout, the siu'faces above presenting a 

 highly-polished or glazed appearance, but not such as would be the result of 

 vitrification. 



In limestones the slickenside frequently appears as a w-hite calcareous 

 material resembling opaque carbonate of lime, but coated with a carbonaceous- 

 looking glaze, which readily soils the fingers. In slate or sandstone -rocks the 

 slickenside is most usually homogeneous in its structure ; it comes away in 

 small slabs, and resembles dull-looking quartz, or quartzite, having both sur- 

 faces liighly polished, beautifully and often deeply striated, and stained of a 

 dark manganese — brown, or black colour. 



The most remarkable kind of slickenside I know of is one not unfrequent in 

 the carboniferous limestone of Ireland \ it exhibits two distinct sets of striae 



