446 Mr. W. J. Sollas on the Flint 



lime, alumina, silica, carbonaceous matter, and traces of iron. 

 Out of 30 lbs. of limestone dissolved in hydrochloric acid, 

 there was left a residue which, when well washed with dis- 

 tilled water and dried, was found to contain 680 grains of 

 mud (consisting of alumina for the most part and carbonaceous 

 matter), and 1260 grains (or nearly 3 ounces) of silica, which, 

 when we put it under the microscope, we were delighted to 

 find consisted entirely of microscopic crystals, of six-sided 

 prisms terminated by six-sided pyramids, the usual form of 

 rock crystal. It may be accepted as a fact that in the Moun- 

 tain Limestone these beautiful crystals are prevalent. Mr. 

 Woodcroft has dissolved many pieces, and always found them. 

 In the Buxton Limestone they occur in larger crystals and a 

 little worn or corroded ; but in that of this locality (except in 

 the hydraulic Mountain Limestone of Waterhouse near Leek, 

 in which the silica occurs in an amorphous form) they are 

 always perfect in form, transparent, and very interesting 

 ' objects,' averaging in measurement about the 400th of an 

 inch in length, by the 1000th of an inch in breadth. The 

 smallest are less than 1000th of an inch long. They are 

 beautiful polariscopic objects. The encrinital slabs, which 

 seem wholly composed of fossils, also contain these crystals. 

 They do not appear to be present in the Liassic, Oolitic, or 

 Silurian Limestones." 



Prof. A. Renard also mentions precisely similar forms as 

 occurring in the Carboniferous Limestone (assise V. f) of 

 Belgium (loc. cit. p. 15, footnote). I have myself seen them 

 in the Silurian limestone of Hamilton, Ontario, in the Devo- 

 nian limestone of Newton Abbot, the Carboniferous of North 

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

 the Devonian limestone they were obviously associated with 

 the remains of siliceous organisms. 



In these crystals 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. 



(b) A similar case to the preceding occurs in some flints, 

 where quartz crystals, with their apices directed inwards, line 

 a cavity in the interior; but these crystals are macroscopic. 



(c) The siliceous casts in the interior of some Foraminiferal 

 shells appear to offer a case of the simple deposition of silica. 



(d) The last case is that presented by various forms of 

 chalcedonic incrustation. The fossil Lithistids of Blackdown 

 and Haldon afford a good illustration of this. The reticulate 

 skeletons of these sponges are now reduced to the condition 



