THE CARBONIFEKO US-LIMESTONE SERIES AT CLIFTON. 195 



formation in Belgium ; and Professor Sollas figures * two micro- 

 quartz-crystals from the Caldon Lower Limestone of Derbyshire. On 

 another occasion Professor Sollas states t that he has seen them 

 " in the Silurian limestone of Hamilton, Ontario, in the Devonian 

 limestone of Newton Abbot, the Carboniferous of North Wales, and 

 the Lias of Sutton, South Wales ; and in all but the Devonian lime- 

 stone they were obviously associated with remains of siliceous 

 organisms," a statement which quite corresponds with my observa- 

 tions at Clifton. Speaking of the origin and formation of the 

 crystals, Professor Sollas considers that the silica became dissolved 

 and then crystallized out. " In these crystals," he continues, " we 

 have an instance, disentangled from all complication, of the simple 

 crystallization of quartz from a siliceous solution ; and the notion 

 that deposition of silica from diffused solutions could not take place 

 without the presence of an organic nidus is thus completely disposed 

 of." Before further considering Professor SoUas's theory, let us 

 look at the possible origin of the crystals which I have found in the 

 Carboniferous Limestone at Clifton. The possible origins may be 

 reduced to four : — 



i. Are they detrital ? 



ii. Are they the result of simple crystallization of silica from a 

 siliceous solution? 



iii. Have they originated from secondary crystallization around 

 rolled or broken fragments of an original quartz-crystal ? 



iv. Are they the result of the crystallization of amorphous and 

 chalcedonic silica ? 



With regard to the first of these propositions I can find nothing to 

 justify such a conclusion. The crystals are well defined and show no 

 signs of water-action. Much the same maybe said of proposition No.ii.; 

 all the evidence is against crystallization from a siliceous solution. 

 As I have several times, in describing the residues, remarked, many 

 of the crystals show nuclei of detrital quartz (Plate VIII. figs. 3 

 and 4), and thus the third proposition becomes possible. Dr. Sorby 

 has referred + to the deposition of crystalline quartz around the 

 broken angles of quartz-grains in sandstone rocks, the silica being 

 deposited continuously with the nuclei. Dr. Sorby's observations 

 have received confirmation from other observers, most notably from 

 Mr. E. D. Irving §, of the United States Geological Survey. An 

 examination of the crystals with nuclei, which I have obtained from 

 the limestone at Clifton, can lead to no other conclusion than that 

 they have originated from the deposition of secondary silica around 

 the broken angles of quartz-grains. In the limestone, however, 

 the process appears to have been carried to greater perfection than 

 in the instances investigated by Dr. Sorby and Mr. Irving, for, as in 

 fig. 3 and in fig. 4, Plate YIIL, perfect crystals have been con- 

 structed around quartz-grains ; moreover such instances are nume- 



* Ann. & Mag. Nat. Hist. ser. 5, vol. ii. p. 361 (1878). 



t Loc. cit. ser. 5, vol. vi. p. 446 (1880). 



\ Quart. Journ. Geol. Soc. vol. xxxvi., Proe. pp. 62, 63 (1880). 



§ Fifth Annual Eeport United States Greol. Survey, pp. 219-226. 



