364 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 3, 



joints being coated with green. In external appearance these beds 

 strongly resemble the adjoining porphyry, but a stroke of the hammer 

 reveals their true nature*. 



Still farther east, these same hard and nearly horizontal sandstone 

 beds are cut by a dyke of porphyry (a) six feet wide, but unaccom- 

 panied by any disturbance of the stratification. This porphyritic 

 rock has a schistose structure f, which near the sides becomes more 

 distinct J. Sir Henry de la Beche, in his Report before referred to, 

 when speaking of the porphyry in Cawsand Bay, says — " The por- 

 phyry has a coarsely laminated structure towards the east, becoming 

 as much laminated as the slates among which it runs in veins, where 

 the latter are small ; in such cases changing its common red colour 

 for a light brown stained by oxide of iron§." 



Not many yards distant, a second dyke (b), about five feet wide, 

 cuts across the same beds, and immediately adjoining there is a great 

 disturbance of the stratification. This dyke || resembles in mineral 

 structure very closely not only the dyke a, but even more so the rock 

 (Specimen No. 15) which I have described as interrupting the con- 

 tinuity of the strata of slate and as resembling a dyke. 



Again, farther eastward, another dyke (c) of somewhat wider 

 dimensions cuts the same sandstones, and in one part of it presents 

 a most remarkable schistose structure^, at right angles to the sides 

 of the dyke, so as to make it hardly distinguishable from the quartz- 

 ose sandstone beds, Nos. 19, 20, &21**, and the long-continued 

 action of the blowpipe on a minute fragment does not produce even a 

 partial fusion ff. This is probably the dyke figured in Sir Henry De 

 la Beche's Report, p. 279, describing the laminated structure as 

 cleavage planes. 



The dyke c is shortly afterwards succeeded by another (d) about 

 five feet wide, having a very distinct slaty cleavage |J. A suite of 

 specimens, from the great porphyry mass to this fissile rock, might 

 easily be made, showing the passage from the one structure into the 

 other, a gradation which may indeed be traced even by the specimens 

 that accompany this paper, although they were not collected with 

 that view§§. It is more than probable that the smaller veins are 



* Specimens Nos. 19, 20, & 21. f Specimens Nos. 22 & 23. 



t Specimen No. 24. § Report, p. 211. 



II Specimens Nos. 25 & 26. ^ Specimens Nos. 27 & 28. 



** " At Newham quarrj', near Truro, at the termination of a long dyke, extending 

 about nine miles from the westward of Penstruthal, where it cuts through granite, 

 even the porphyritic character becomes lost, and we have a suhstance not unlike 

 some arenaceous rocks. It is white and rather friable — a fine-grained compound 

 of quartz and felspar. A short distance west, however, the elvan is a well-cha- 

 racterized porphyry." — De la Beche, Report on Cornwall, &c., p. 177. 



tf " In some experiments on the fusibiUty of rocks, made jointly with Mr. L. 

 Llewelyn Dillwyu, at the Cambrian Potteiy, Swansea, we found that the Cornish 

 granites and elvans were melted at a temperature about equal to that required for 

 fusing malleable iron. The heat required for the fusion of malleable iron is the 

 greatest which can be obtained in a smith's forge." — De la Beche, Report on 

 Cornwall, &c., p. 191. %% Specimen No. 29. 



§§ Compare the Specimens in the following gradation of their respective 

 numbers, 1, 13. 31, 30, 22, 23, 2J, 20, 25. 18, and 26. 



