194 MR. E. WETHEEED OiST INSOLUBLE EESIDTJES FEOM 



quartz, micro-crystals of quartz, amorphous and chalcedonic silica, 

 and, less frequently, p3Tites, tourmaline, and zircon. From some 

 beds the residues obtained were little else than micro-crystals of 

 quartz. Among the fragments of amorphous and chalcedonic silica 

 may be seen sponge-spicules, casts and spherical bodies. Towards 

 the top of the Middle Limestones the proportion of detrital quartz 

 in the residues increases, and the deposition of secondary silica on 

 the surfaces of the grains becomes less marked, until, as before 

 stated, the calcareous beds become replaced by the Millstone Grit. 



Form in ivJiich the Silica occurs oilier than that of the Detrital 

 Quartz. 



The silica exists in three forms — (1) Amorphous ; (2) Chalce- 

 donic : (3) Crystalline. These three stages are frequently seen in 

 the same fragment, as represented in the woodcut. The crystal- 



A fragttient of amorphous silica passing first into the clialcedonic or crypto- 

 crystalliue condition, and the latter into the crystalline. From the 

 Middle Limestone. X40 diam. A. Amorphous, B. Chaledonic, C. Crys- 

 talline Silica. 



lizing process commences around the edges and proceeds towards 

 the centre, a process wbicb is well illustrated in the figure, and 

 also in one of tbe objects (B.) in fig. 3, Plate YIII. At times we 

 get masses of crystals, as though tlie crystallizing process had been 

 completed. In the amorphous silica we appear to have a practical 

 proof of the tendency of an unstable substance to return to the 

 stable, or original condition, after having served a purpose for which 

 it is no longer required. There are also the micro-crystals with 

 nuclei which might be treated under this head ; but I prefer to 

 take them separately, and I shall also make further reference to 

 those without nuclei. 



Formation and Origin of tlie Micro-Crystals of Quartz. 



The finding of quartz-crystals in limestone rock is no new dis- 

 covery^, but previous observers do not mention the nuclei. Mr. 

 Thomas Wardle records them* as occurring in the Mountain Lime- 

 stone of Caldon ; Professor A. Eenard speaks f of them in the same 



* Presidential Address to the North Staffordshire Field-Club, 1873, p. 42. 

 t "Eecherches lithologiques sur les Phthanites du Galea ire Carbonifere de 

 Belgique," footnote, p. 15. Bulletin de lAcademie Eoyale de Belgique. 



