444 Dr. Q. J. Rinde — Organic Origin of Chert. 



are built up by these organisms, the portions preserved do not suffice 

 to determine satisfactorily a single species. The sponges, like those 

 in the Cretaceous rocks, have lived for successive generations on the 

 areas of these Chert-beds, and after the death of the animal and 

 the decay of the soft structures, the microscopic spicules of their 

 skeletons have gently fallen and intermingled together on the sea- 

 floor. When the sponges have existed continuously and nearly 

 exclusively in the same area for long periods, massive beds of Chert 

 have been produced by the accumulation and partial solution of their 

 spicules ; where, however, the sponges have grown in association 

 with Crinoids and Polyzoa, then the Chert is in nodular aggregates, 

 or the interspaces between the Crinoidal and other calcareous 

 structures have been filled up by the silica resulting from the partial 

 solution of the sponge-remains producing Cherty limestones. 



Under certain conditions, however, when the calcareous portions 

 of the rock much exceed the sponge-remains, these latter are 

 frequently partially or entirely dissolved and replaced by calcite. 

 This substitution has taken place in some of the dark limestones of 

 the Calp series near Dublin, in which by the aid of a simple lens 

 numerous spicules can be detected, but on treating the rock with 

 acid, the spicules are dissolved away equally with the limestone 

 matrix, leaving only a few small hollow fragments in the residue. 

 The spicules in this limestone are considerably larger than those in 

 the Chert-beds, and appear to belong mostly to Tetractinellid sponges. 



It is not my intention to enter now into a detailed comparison of 

 the Irish Carboniferous Chert with that in the Yoredale series 

 of Yorkshire and North Wales ; it will suffice to state that in all 

 essential features the rocks from these different localities are very 

 similar, so that in many cases it would be impossible to distinguish 

 hand-specimens of the Irish from the Welsh Chert. The organic 

 nature of the English and Welsh Carboniferous Chert, as produced 

 from sponge-remains, is far more distinctly shown than in the case 

 of the Irish beds, for the spicules are much better preserved, and 

 the beds have been less altered by fossilization. Sponge-life also 

 has been more continuous in the Yorkshire and North Wales areas, 

 or, at all events, these organisms have been less intermingled with 

 ■Crinoids and other animals possessing skeletons of carbonate of 

 lime, so that their remains form continuous series of Chert-beds. 

 In some of the Yorkshire areas there are beds of Chert 18 feet 

 (5"4:m.) in thickness without a break, and in North Wales there is a 

 oontinuous series 350 feet (105m,) in thickness, without the inter- 

 vention of Limestones. I hope shortly to give a detailed notice of 

 these remarkable and hitherto unequalled sponge-beds, which, alike 

 with those in the Irish Carboniferous rocks, conclusively show that 

 in the interval between the Carboniferous Limestone proper and the 

 Millstone Grit, siliceous sponges existed in greater force and played 

 a more important part as rock-builders, than at any subsequent 

 geological period. 



The organic nature of these Carboniferous Chert and siliceous 

 rocks, though strenuously denied by some geologists, has been 



