484 THE GEOLOGIST. 



22. I have heard them called "trap dykes." Is this name justified m any 

 localities ? 



23. In what relation do they stand to fossils ? 



Here (Birkenhead, &c.), where they abound, nothing ever intervenes between 

 the two pairs of " slicks" but sandstone (sometimes altered) ; elsewhere the 

 coal, though highly polished, is simply coal. Sir C. Lyell mentions polished sur- 

 faces produced by molten lava passing through fissures in old lava. And this 

 reminded me of a " slick," as there would, of course, be four surfaces (two 

 vis-a-vis) in those dykes of Somma. 



I have more hopes of this curious subject being pressed, since Mr. Cunniag- 

 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 3Ir. Price's Queries. — 1. SKckensides 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 similar, 

 but they vary greatly with the nature of the rock. 



3. Soft sands, uncrystaUi^ed 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 question 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. Yery 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 character, 

 the appearance of the sHck is so different that it could hardly be identified. 



10. Yes. 



11. It makes a clear cut through the limestone 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 believe 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 s?(i generis. 



22. No. 



23. They may occasionally intersect fossils, but the suiface change has, as 

 far as I have seen, always obliterated aU organic chai-acter. — D. J. Ajs'sted. 



