196 



ME. E. WETHEEED O^T IXSOLUELE EESIDUES FE03I 



rous in the residues from the Middle Limestone. As to the source 

 whence the secondary silica has come, there may be some room 

 for speculation. Has it been deposited from solution in the sea- 

 water, or, as I should prefer to express it, extracted from solution 

 by the molecular affinity between the silica of the detrital quartz 

 and the silica in solution ? This appears to be the probable expla- 

 nation of the matter ; but there is the fact that in the secondary 

 silica, and especially around the nuclei, are frequently to be seen 

 remnants of amorphous silica ; this feature is well shown in the 

 crystal represented in fig. 3, Plate YIII., and in some of those in 

 fig. 4. The fact tends to show that in some instances the secon- 

 dary silica may have been derived by contact direct from amorphous 

 silica associated with the detrital quartz. 



So far I have referred only to those micro -crystals which show 

 detrital quartz-nuclei. There are, however, others in which no 

 such nuclei can be detected, and these I believe to have originated 

 from the crystallization of chalcedonic silica. These crystals can be 

 seen in process of formation around the edges of chalcedonic in 

 the woodcut (p. 194). We have only to imagine such crystals to 

 become detached to account for those which do not show detrital 

 quartz-nuclei. The facts which I have produced, in connexion with 

 the origin of the micro-crystals, seem to show that the theory 

 advocated by Professor Sollas must at least be modified. 



Origin of the Amorplious and Clicdcedonic Silica in the Limestone. 



In dealing with the origin of the amorphous and chalcedonic 

 silica, I am led up to a very interesting controversy between Dr. 

 Hinde and Professor Hull as to the origin of chert. The essence of 

 the controversy is as to whether the Carboniferous chert in Ireland 

 is of organic or inorganic origin. 



In 1878 a paper was published* in which Professor Hull and 

 Mr. Hardman speak of chert as a pseudomorphic deposit which has 

 resulted from the sea-water becoming highly charged with silica. 

 This water percolated through the limestone before it became con- 

 solidated, and the calcareous structure of the organisms became re- 

 placed by the silica in the sea-water before the overlying Yoredale 

 beds were deposited. " It does not appear," says Professor HuU f, 

 in his fifth general conclusion as to the origin of chert, " that the 

 case of silicious sea- bottoms, such as that of the great depths 

 discovered by the soundings of the ' Challenger ' in the Southern 

 Ocean, affords an example of the phenomena here described — the 

 sea-bottoms referred to being directly due to animal organisms 

 secreting silica, such as Diatomaceae, Poljxystineae, and the spiculaB 

 or skeletons of sponges. The silicious material here described can 

 only be considered as a secondary product, due to the replacement 

 of lime carbonate by silica."' From these remarks it appears that 

 Professor HuU regards the pseudomorphous silica as of inorganic, 



* Scientific Trans. Eojal Soc. of DubHn, vol. i. N. S. pp. 71-94 (1878). 

 t Loc. cit. p. 84. 



