THE CARB0NIFEJR0IJ8-HMEST0NE SERIES AT CLIFTON. 197 



and not of organic origin. This view of the matter was contested 

 by Dr. Hinde in 1885*, who argued that the irregular masses of 

 chert in the Upper and Lower Greensand had been derived from 

 the silica of sponge-remains ; and from the same source had also 

 originated the silica which in manj' deposits had replaced the tests 

 of Mollusca and other calcareous organisms. Professor Hull replied 

 to Dr. Hinde, and, in doing so, saidt : " My argument will be based 

 on the fact that the development of sponge-life in the seas of the 

 Carboniferous period was insignificant, and quite inadequate to 

 account for the existence of bands and masses of chert, sometimes 

 constituting almost a half or a third of the entire mass of the Upper 

 Limestone." Dr. Hinde replied i, brino-iug forward further proof of 

 his proposition, to which Professor Hull wrote an answer §, in 

 which he said : " Dr. Hinde's recent investigations undoubtedly 

 show that siliceous sponge-structures enter far more largely into 

 the composition of Carboniferous chert than has hitherto been sus- 

 pected." " But," he continued, " I am not prepared to go to the 

 full length of Dr. Hinde's demands, as I understand them, nor to 

 abandon as untenable the proposition that much of the silica of 

 Carboniferous chert has been derived by a transmutation process 

 from the waters of the ancient seas." My observations of the in- 

 soluble residues do not support the idea that the siliceous con- 

 stituents were derived directly from the sea-water, that is to say, 

 from inorganic silica, but from siliceous organisms, with the excep- 

 tion, of course, of the detrital quartz. What, then, were these 

 organisms which contributed the silica? That sponge-life was 

 abundant in the Carboniferous sea there can be no doubt ; but it 

 is difficult to estimate to what extent, on account of the disintegration 

 and changes which these remains appear to have undergone. Dr. 

 Hinde states || that the silica of siliceous sponges may be either 

 " (a) amorphous or in the colloid state ; (b) chalcedonic or crypto- 

 crystalline, or (c) crystalline." This description certainly applies 

 to the siliceous remains which I have obtained from the limestone 

 at Clifton ; but I am not prepared to say that other organisms 

 besides sponges have not contributed. I think, however, we may 

 conclude that the amorphous, chalcedonic, and crystalline siliceous 

 constituents in the residues are of organic and not inorganic origin. 

 My thin slides of the limestone, together with the residues, afford 

 evidence which points to the conclusion that the greater portion of 

 the Carboniferous Limestone at Clifton was deposited in the form 

 of material not unlike that of the Chalk and the calcareous mud 

 now being deposited, in which siliceous organisms occur. Professor 

 Huxley once spoke •[[ of the Atlantic mud as "modern chalk;" I 

 regard the Middle series of the Carboniferous Limestone at Clifton 

 as Palaeozoic Chalk, though I am not sure that it was a deep-sea 

 deposit. 



* Phil. Trans, part 2, 1885, p. 433. t Proc. Royal Soc. vol. xlii. p. 304. 



t Geol. Mag. toI. iv. N. S. dec. 3, pp. 435-446. 



§ Geol. Mag. vol. iv. N. S. dec. 3, pp. 524-526. 



II Pal. Soc. vol. xl. p. 55. % ' Saturdaj E-eyiew,' 1858. 



