38 
THE GEOLOGIST. 
This is confirmed by the occurrence of slickensides at different angles in shdes 
of the rock willun the faults, and also by highly ])olishcd surfaces, occurring in 
very slight faults, wliich dis])lacc the strata only a foot or eiglitcen inches. 
I bcheve it is tlic compact sandstone within 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\k Sir, — The subject of " shekensides" 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 1 possibly could. I have read 
with much interest in the last number of the " Geologist" the queries on 
"slickensides" submitted to tlie 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 
would, in the first instance, be the pulverization more or less of the two 
opposmg rock-surfaces, and when this crushing action ceased, tlie 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-" strife 
are on the surface of the beds, and their direction frequently parallel to that of 
the dip. In my geological notes I find many references Uke tlie following : 
" Slickcnsides-strise parallel to the dip of the beds, showing vertical displace- 
ment in the mass ;" but when the striee are transverse to the dip, and m the 
direction of the strike of the beds, I say that " horizontal displacement is 
indicated." In either instance of course there is no fault produced in 
the strata, though a displacement of them en masxe is clearly iudicated, the 
direction of which being pointed out by that of the strise of compression. 
This is the only way iu which rock-masses can be displaced without being 
faulted. 
The slickenside-strise are frequently oblique to the dip of the beds, the angle 
of obliquity being of course variable, but always indicating the direction of 
the displacement. 
The thickness of the sHckenside, or striated substance, is very variable ; 
sometimes it is as thin as card-paper, at others nearly an inch 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, which 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 structure is only 
partial, and confined to the surfaces ; and again, when the slickenside is tolera- 
bly thick, it is homogeneous throughout, the surfaces 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 white calcareous 
material resembling opaque carbonate of lime, but coated wdth a carbonaceous- 
looking glaze, wliich 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 didl-looking quartz, or quartzite, having both sur- 
faces highly 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 iu 
the carboniferous limestone of L'elaud ; it exhibits two distinct sets of striae. 
