484 
THE GEOLOGIST. 
22. I have heard them called " trap dykes." Is tliis name justified in any 
localities ? 
23. In what relation do Uicy stand to fossils ? 
Here (Birkenhead, <S:c.), where they abound, nothing ever intervenes between 
the two pairs of "slicks" but sandstone (sonu'times altered) ; elsewhere the 
coal, thous>-li higidy polished, is simply coal. Sir C. Lyell mentions polished sur- 
faces produced by molten lava ])assing through fissures in old lava. And tliis 
reminded me of a " slick," as there woidd, of course, be foui- surfaces (two 
vis-a-i-is) in those dykes of Sonnna. 
I have more liopcs of this curious subject being pressed, since Mr. Cumiing- 
ham, of Liverpool, by giving it a practical turn in connection with a " water 
question," has enlisted the cui bono party in our inquiry. If they form a 
barrier to subterraneous " water-works," they want looking after indeed. 
Replies to Mr. Price's Queries. — 1. Slickcnsides occur, I believe, in all hard 
metamorphosed partly crystalline rocks, and especially in limestones, sand- 
stones, and perhaps some slates. They occur also in some clays and in coal. 
2. The general conditions being the same, the phenomena are very simiiar, 
but they vary greatly with the nature of the rock. 
3. Soft sands, uncrystallized limestones, and some clays are excepted. 
4. I never saw a case in conglomerates, but should not be surprized at find- 
ing one. The same with rock-salt. Chalk is exempt. 
5. It is not a quest ion of age. I am not aware of any difference between 
Old and New Red sandstone that could affect the question. 
6. I never saw a true slick in roofing-slate, but I think I have in indurated 
slate. It can have no relation to cleavage. 
7. There is no reason why it should not. 
8. Very similar, if not identical, phenomena are common in some clays, chiefly 
.very smooth and fine-grained varieties. It does not harden such clays, nor does 
it alter them. 
9. In passing from one rock to another of very different mineral cliaracter, 
the appearance of the slick is so different that it could hardly be identified. 
10. Yes. 
11. It makes a clear cut through the Hmestone and its contents. 
12. No, to the best of my knowledge. 
13. They vary in each case. Some striation or approximate appearance I 
have always seen. 
14. Not that I am aware of. 
15. The rock itself altered by compression, and perhaps by heat produced by 
friction. 
16. Not that I am aware of. 
17. I beheve the opposite faces where the slick is compound are not strictly 
parallel, but wedge-shaped. 
18. Not necessarily a fault affecting underlying beds, but a slide of the bed 
in which it occurs, though often to a very small extent, and locally. 
19. I think I have seen instances of it. 
20. Yes. 
21. I believe them to be sni generis. 
22. No. 
23. They may occasionally intersect fossils, but the surface change has, as 
far as I have seen, always obliterated aU organic character. — D. J. Ansted. 
