264 J. H. coLLiirs on the geolo&t 



of Cornwall, but of course without their refreshing green turf-sur- 

 rounding. 



In the immediate neighbourhood of the mines the natural 

 barrenness of the mining part of the Andevallo is greatly height- 

 ened by 



(1) The destruction of vegetation by the sulphur-smoke from, at 

 Eio Tinto alone, the volatilization of about 200,000 tons of sulphur 

 per annum ; 



(2) The destruction of brushwood and timber for the use of the 

 mines ; 



(3) The removal of the unprotected soil by the occasional heavy 

 rains. 



The climatal conditions have led to a much more rapid denudation 

 than is observable in countries like England, and it will be readily 

 understood that they have been greatly aided by the temporary 

 conditions noted above. 



In the height of summer the surface -rocks of this region are often 

 heated to 160° F., or even more during the day, while the coming of 

 a wind from the north will sometimes reduce the temperature of the 

 same rocks to 70° during the night — a range of at least 90° in perhaps 

 eight or nine hours. These alternations of temperature have, in 

 some cases, caused the scaling-off of thin layers of porphyTy, diabase, 

 ironstone-breccia, and other rocks from the more exposed masses on 

 the hilltops and in the broader valleys in a most peculiar manner. I 

 have picked up pieces of the ironstone-breccia measuring at least 

 8 inches across, with a thickness not exceeding three quarters of an 

 inch. In some spots the rocks have acquired, from this cause alone, 

 rounded forms, which might easily be supposed to result from glacia- 

 tion, while the flakes of rock cover the ground around to a depth of 

 more than a foot*. 



It will be readily believed that disintegration, especially of the 

 schistose rocks, proceeds very rapidly under such conditions as these. 

 Furthermore, in the neighbourhood of the rock-junctions, a great part 

 of the surface-slate, and even the porphyry, is so decomposed as to be 

 commonly used as a kind of fire-clay (harro). The occasional heav}" 

 rains of spring and autumn, falling upon a ground so prepared, act 

 with extreme energy and produce very marked effects, a single storm 

 sometimes cutting out channels several yards in depth. These facts 

 should be taken into account in forming an estimate of the time 

 required to produce a given amount of denudation, as, for instance, 



* The finest example of this " pseudo-glaciation " which I ever saw occurred 

 in the ironstone-breccia lying on the top of the hill a little to the S.E. of No. 4 

 shaft on the North Lode ; and at first I felt pretty sure that here, at least, I 

 had met with a real glacial polishing— the angular pieces of quartz, with their 

 ferruginous cement, were smoothed off" so wonderfully. But I soon saw here, 

 as I had seen on other occasions, that there were no ice-scratches and that the 

 apparently polished surfaces met each other in sharp re-entrant angles in such 

 a manner that they could not possibly hare been produced by friction. More- 

 over, the stones were surrounded with bushels of thin flakes of precisely similar 

 character which had peeled off from time to time, some of them being nearly a 

 foot across, with a thickness of considerably less than one inch. 



