BRITISH ASSOCIATION MEETING. 



488 



same fact. I hardly ever saw it m situ without finding two contiguous 

 instances — i. e., two pairs of polished surfaces sometimes within half an inch, 

 sometimes two feet of each other, the intervening space being occupied by 

 rock more or less altered in character, and generally more compact. The planes 

 never agreeing with the stratification, and often nearly perpendicular to it. I 

 never saw any termination of the polished planes, so that I should suppose 

 they intersect whole mountains. In granite and trap the surfaces I have seen 

 were far from plane, and exhibited a cUlferent substance rather like steatite or 

 soapstone (so in serpentine). In sandstone the polish is such as could not be 

 produced by artificial means (without vitrifying) ; but, however sleek the sur- 

 face, it always exhibits stride, sometimes parallel, but often inclined at various 

 angles. The following questions seem worth following up, to obtain more 

 light on the subject : — ■ 



1. Are aU rocks found to exhibit these sui-faces ? 



2. If so, are the circumstances alike in the main ? 



3. If not, what sort of rocks seem excepted? 



4. Are conglomerates exempt ? Is chalk ? Is rock-salt ? 



5. Is it a question of age ? Is it as common in Old Red as in New ? 

 0. If in roofing slate, what is its relation to cleavage ? 



7. If in trap-dikes, does it crop the dike ? 



8. If in soft Silurian shale ("mudstone") does it harden that? X.B. It 

 does not seem to alter coal at aU, and I have seen something very like it in 

 clay, not so hard as French chalk. 



9. Is it found to pass hito contiguous rocks, that overlie — e. g., from Moun- 

 tain-limestone into Old Red sandstone. 



10. Does it ever pass into rocks lying conformably ? 



11. Does it affect the accidental minerals of the rock through which it 

 passes — e. g., Barytes in Mountain-limestone ? 



12. Does crystallization interrupt it, or vice versa ^ 



13. Are the striae universal ? 



14. Does it never correspond with stratification ? 



15. Are the surfaces coated with a different substance, or is it merely the 

 rock itself altered ? 



16. Has it been seen on the opposite sides of valleys, or of mountains ? 



17. Have the apparently parallel pairs been found to meet, and so come to 

 an end ? I have seen something like this. 



Mr. Cumiingham asserts that here they form a barrier to springs of water. 

 Is this general ? 



Allow me to commend to the notice of tourist-geologists Llysfaen, near 

 Abergele, as exhibiting within a very small compass five very remarkable and 

 perfectly distinct drifts ; also a good mountain-limestone fossil station, with a 

 good flora (silurian fossils near) between Rhyl and Llandudno. 



Additional Queries. — 18. Does the "slick" or polished surface indicate a 

 fault? 



19. Is the " slick" ever intersected by mineral veins ? 



20. Are the perfectly level "specular" surfaces and those which, though 

 glossy, are uneven, to be referred to the same agency ? 



21. Do they ever coincide with the stratification, partings, cleavage, " lamina- 

 tion," or " fohatiou" (E. Forbes), and are they to be considered as absolutely 

 sui generis, or to be classed with some of these ? 



