TBANSACTIONS OF SECTION C. 689 



Belgium—' Bulletin de I'Acad. Royale de Belgique,' s. 2, t. 4G, pp. 471-499. In 

 1881 Professor Sollas pointed out that in sections of some of the very specimens 

 described and figured by Professor Hull in the paper referred to above ' sponge 

 spicules make up the larger part of the chert '— ' Ann. and Mag. Nat. Hist.' s. 5, 

 vol. vii. p. 141. In 1885 I suggested that the Irish chert was probably derived 

 from sponge remains, the same as the Cretaceous chert in the south of England— 

 ' Phil. Trains.' Part II. 1885, p. 43.3. During the present year Messrs. Hull and 

 Hardman brought papers before the Royal Society reiterating their former views 

 as to the inorganic origin of the chert, and stating that there was absolutely no 

 evidence for the suggestion I had made as to the derivation of the silica of the 

 chert from sponge remains, and that I had mistaken fragments of criuoids for 

 sponge spicules—' Proc, Royal Soc' vol. xlii. pp. 304 et seq. 



In order if possible to determine this question, I went to Ireland last July 

 and examined the carboniferous chert in the localities whence Professor Hull ob- 

 tained the specimens on which he founded his conclusions as to its inorganicorigm, 

 visiting various places in Queen's County and Kilkenny to the south, and in Fer- 

 managh and Sligo to the north-west of Ireland, and in the vicinity of Dublin. The 

 chert in these diiferent localities is mainly of the same character— a dark, hard, 

 compact, siliceous rock, frequently without being afiected by acid, though in the 

 cherty limestones, calcite, in the form of organic structures, is intermingled m 

 various proportions with the silica. The chert occurs either iu layers of nodules 

 imbedded in limestone, not dissimilar to the flints in the Upper Chalk, or in dis- 

 tinct beds from one to five inches in thiclaiess, which may be either independent 

 of the beds of limestone, or form central masses with limestone above and below 

 them. These chert beds occur throughout the Upper Limestone Series of the Irish 

 Carboniferous, which has a thickness of from 600 to 800 feet ; in places they con- 

 stitute from one-tenth to one-fifth of the total mass of the rock : Professor Hull, 

 however, estimates that they form almost a half or a third of the entire thickness. 

 Accepting the lower estimate, the total thickness of the cliert iu the series would 

 be from foO to 150 feet. . t • • j 



In microscopic sections of specimens from every locality I visited sponge- 

 spicules are present ; they are more abundant in the beds of chert in which there is 

 no apparent calcite, and the rock in many instances is filled with them. Further, 

 in frequent instances the compact, dark chert-beds weather so as to form both on 

 their upper and under surfaces a porous, siliceous, granular crust of a grey tint and 

 harsh to the feel. This crust under favourable conditions can be seen to be com- 

 posed of innumerable minute sponge-spicules, intermingled and, as it were, felted 

 together. There is therefore direct and undisputable evidence that the silica in 

 the chert is due to the accumulation and partial solution of these sponge-remains, 

 and that it has not been derived as a direct chemical deposit from sea-water. 



Through the courtesy of Professor Hull I examined under the microscope the 

 rock-sections which he described and figured in his original paper, and though 

 they had not been specially selected, there were sponge-spicules present in all of 

 them ; and I can fully confirm the published statement of Professor Sollas that in 

 some, spoDge-spicules make up the larger part of the chert. These sections make it 

 evident that Professor Hull did not recognise the forms to be spicules, but that he 

 regarded them as sections of crinoid structures. 



During the last year I have studied the Carboniferous chert in the Yoredale 

 Series of Yorkshire and North Wales, and I am now preparing a description of 

 this rock. In all essential features it resembles the Irish chert, but the evidence of 

 its derivation from sponge-spicules is far clearer, since the rock from these places 

 has been less altered by fossilisation, and in many sections the chert is distinctly 

 an agglomeration of spicides, whose forms are nearly as perfect as those of existing 

 sponges. The beds of chert in Yorkshire are more continuous than those in Ire- 

 land ; in some instances they form an uninterrupted series eighteen feet in thick- 

 ness ; this, however, is far exceeded by the beds in North Wales, where in borings 

 they are proved to reach 350 feet in thickness without the intervention of 

 limestones. 



The organic orio-in of the Carboniferous chert, so strenuously denied by the 

 1887. Y T 



