98 BRITISH PALEOZOIC SPONGES. 



stone Bay a single specimen of Beceptaculites was discovered by the late Mr. 

 Champernowne. 



Carboniferous System. — In the earlier rocks of the Palaeozoic group, fossil 

 Sponges are, as a rule, of rare occurrence, and they are altogether insufficient to 

 impress a distinctive character on the rocks in which they are found, but in certain 

 portions of the Carboniferous epoch they existed in such great numbers, and for 

 such prolonged periods, that by the gradual accumulation of their broken-up 

 skeletons beds of chert and siliceous rock of considerable thickness have been 

 formed. Owing to the microscopic dimensions of the elementary spicules of which 

 these rocks are mainly composed, and to the fact that subsequent changes have 

 greatly altered and partially obliterated their original characters, the organic origin 

 of these rocks, though oftentimes suspected, has not hitherto been satisfactorily 

 ascertained. But from a microscopical examination of specimens which I have 

 myself obtained from the chert and siliceous rocks of the Yoredale series of York- 

 shire and North Wales, and from the corresponding horizon in Ireland, I have 

 discovered that these rocks are composed of the debris of siliceous Sponges, and 

 that they are veritable Sponge-beds. 



In some cases these Sponge-beds occur as irregular, nodular masses, or accre- 

 tions of grey or dark chert, in the midst of beds of limestone, resembling in 

 appearance flints in chalk ; in others, the chert forms distinct beds, often variously 

 banded, from one or two inches to eight feet in thickness ( - 025 — 2*43 m.). These 

 chert beds appear to be free from limestone. Frequently, however, the Sponge- 

 beds assume the form of grey or dark, compact, fine-grained siliceous rocks, 

 distinctly bedded, and often having a mottled or banded structure. Occasionally 

 a small proportion of lime is present in them. 



Only exceptionally are the spicular contents of these rocks visible to the unaided 

 sight, in the majority of cases they can only be seen in thin sections under the micro- 

 scope or with the aid of a platyscopic or other good lens. Thin sections of the least 

 altered specimens show a confused mass of spicules of various forms and sizes, thickly 

 and indiscriminately mingled together, but usually with a generally horizontal 

 arrangement in the plane of the rock-bedding. These spicules appear to be perfect 

 in form, and their axial canals in many cases have been preserved. In general, 

 however, they have been altered in various degrees by the effects of fossilization, 

 so that it is possible in a series of sections, or even in different parts of the same 

 section, to trace the gradual changes from rocks filled with spicules nearly in their 

 original condition, to those in which they have become merged into a matrix of 

 translucent silica, leaving but few traces of their former presence. 



It is difficult to ascertain the characters of the spicules merely from sections 

 of the rocks, but in a few cases the cherty Sponge-beds have decayed in such a 

 manner that their component spicules, or at least the larger forms, can be obtained 



