Vol. 54.] IN THE CAKBONIFEPvOrS LIMESTONE OF DERBYSHIEE. 181 



The quartz-rock is not a sandstone or a gritty limestone altered 

 by the growth of crystalline quartz around and in optical continuity 

 with the original quartz-grains, but on the contrary is due to a 

 growth of quartz around what were originally isolated crystals of 

 the same mineral. The evidence for this opinion is the presence of 

 zonal calcite in the grains which now form a quartz-mosaic, and 

 the absence (which by itself would not be conclusive) of anything 

 like detrital quartz-grains either in the rock itself or in the 

 surrounding limestone, or in any of the residues examined. 



The quartz has not been formed from chalcedonic silica, other- 

 wise we should expect to find some traces of it in a thin section or 

 in the residues. The latter consist only of crystalline quartz. 

 Mr. Wethered found in the limestone at Clifton that amorphous 

 and chalcedonic silica and crj'stalline quartz frequently occurred in 

 the same fragment.-^ 



The presence of chert in the quartz-rock at the head of Pindale 

 and of part of a foraminifer in the similar rock of Pounder Lane, 

 and the frequent penetration of organisms by quartz-crystals in the 

 quartzose limestone, are, I think, evidence that the quartz in these 

 rocks is a replacement of the limestone. We may therefore assume 

 that, just as the separate crystals and small groups of crystals of 

 quartz have replaced parts of the organisms in the limestone and 

 also small portions of the limestone itself, so the quartz-rock has 

 replaced larger portions of limestone. 



In some places, such as Pindale and the Top Lift, the whole of 

 the limestone and quartzose limestone have been weathered away, 

 leaving the quartz-rock in bosses or loose blocks. In other places 

 patches of the softer rocks are left, which show the gradual transi- 

 tion from the quartzite to ordinary limestone. 



Y. The Oeigin of the Qfaetz. 



Mr. Wethered reduces the possible modes of origin of the quartz- 

 crystals in the Carboniferous Limestone Series at Clifton to four.^ 



The crystals in the Derbyshire rock are obviously not detrital. 

 I have already shown that they are not due to secondarj^ crystal- 

 lization around detrital grains of quartz, and that they are not the 

 result of the crystallization of chalcedonic or amorphous silica. 

 There is left only the fourth case, which is that of simple crystal- 

 lization of silica out of a siliceous solution. This is the origin 

 suggested by Prof. Sollas for the small quantity of quartz-crystals 

 often found in limestone, and it is, I consider, the origin of the 

 quartz in the quartz-rock and the quartzose limestone. 



There is room for speculation as regards the source of the silica. 

 It may have been derived from siliceous organisms in the limestone, 

 or more probably from deep-seated thermal waters containing silica 

 in solution. 



1 Quart. Journ. Geol. Soc. vol. xliv (1888) p. 19L 

 - Tbicl. p. 195. 



Q. J. Cx. S. No. 214. 



