360 



PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. [JunO 19, 



tion, excepting as regards the relative proportions of lime and 

 alkali, resembles that which Bunsen has assigned to the ^' normal 

 trachyte " of Iceland. Some trap in contact with it (LXXVI. infra) 

 was quite unaltered ; and the two rocks were easily separated from 

 each other, leaving smooth even surfaces. The felstone must there- 

 fore have been indurated before the eruption of the trap. But some 

 dark rock, having the external character of altered shale, appeared 

 to have become partly combined with the felstone. Its constitution, 

 near the place of contact, was intermediate between the felstone 

 and trap. The two rocks must have been together in a plastic state, 

 and subsequently metamorphosed ; or the petrosilex must have been 

 in a state of fusion when brought into contact with the other rock. 

 There is, however, no evidence in favour of the conclusion that the 

 petrosiliceous rock was intrusive. 





m 



'a 



5^ 





<6 



a 

 'a 



1 



II 



s i 



XLI. 



XLII. 

 XLIII. 

 XT,TV. 



XLY. 

 XLYI. 



75-07 

 64-80 

 77-33 



76-77 

 78-92 

 75-48 



14-27 

 15-60 

 12-30 



8-75 



8-18 



13-40 



1-29 

 5-65 

 1-33 

 4-75 

 4-08 

 1-63 





0-20 

 0-20 



5-34 

 5-32 

 5-39 

 6-35 

 5-05 

 2-90 



0-42 

 2-18 

 0-91 



"'0-48" 



0-87 



1-53 

 3-06 

 1-45 

 1-03 

 1-19 

 603 



1-88 

 3-09 

 1-29 

 2-10 

 2-10 

 0-23 



0-10 





0-25 





0-30 



0-16 



XLI. Felstone or petrosilex, nearly white, from the large east 

 buttress, near the base. 



XLII. Eock resembling metamorphic shale, containing a little 

 red felspar of no distinct crystallization, in contact with XLI. In 

 composition it differs from the shales in the vicinity in containing a 

 larger proportion of alkaline earths. The fragment analyzed con- 

 tained one per cent, of hydrofluoric acid. 



XLIII. Felstone from the same place, resembling XLI., but of a 

 pink colour. G= 2-695. 



XLIV. Felstone, near the ridge south of the British camp. 



XLV. Porcellanite, north-east of the cave. 0=2-7304. 



XLYI. Greyish rock, associated with, and passing into felstone : 

 of similar composition, excepting that it contains more water. 



Dr. HoU ascribes the altered condition of these rocks to the in- 

 trusion of the trap-dykes by which they are extensively invaded*. 

 There is certainly an apparent transition from shale to felstone or 

 porcellanite, similar to that which Mr. J. Geikiet has noticed in the 

 gradual changes which the Lower Silurian greywackes and shales of 

 Carrick undergo during their passage into crystalline and pseudo- 



igneous rocks. 



* Loc. cit. p. 93. 



t Quart. Journ. Geol. Soc. vol. xxii. p. 513, &c. 



