GEOLOGICAL DISTRIBUTION. 103 



The chert beds are well developed in the Upper-Limestone series of the 

 County of Sligo ; more especially at the hill of Keishcorran near Ballymote, on the 

 higher slopes of Knock-na-Rea, near the town of Sligo, and in the ridge of Ben 

 Bulben to the north of Sligo Bay. The separate bands of chert vary from one to 

 five inches in thickness, with intervening layers of blue limestone. The chert 

 bands are frequent, and I should judge that in different places they form from one- 

 tenth to one-fifth of the total mass of the rock. Beds of siliceous clay — probably 

 resulting from decayed chert — and like the material in the Ayrshire deposits, filled 

 with loose spicules, have been met with near the summit of Ben Bulben, and their 

 contents described by Mr. H. J. Carter, F.R.S. 1 



It is evident that to produce the enormous accumulation of spicules sufficient 

 to build up beds of rock like those referred to in Yorkshire, North Wales, and 

 Ireland, reaching in one place a maximum thickness of 350 feet (105 m.), 2 Sponge 

 life must have been extremely abundant and persistent in the Carboniferous epoch, 

 more so, perhaps, than at any subsequent period. It is true that the number of 

 species yet recognised from these thick deposits of Sponge remains is comparatively 

 limited, but it is hardly safe to conclude from this fact that there was but little 

 variety of form in the group at this period, for, owing to the general and complete 

 manner in which the Sponge-skeletons have been reduced into their component 

 elements, and their unfavorable condition of preservation, all other generic and 

 specific characters, beyond those of the form and proportions of the individual 

 spicules, have been obliterated. The fact that not a single example of an entire 

 Sponge has been up to the present discovered in any of the Sponge-beds of York- 

 shire, North Wales, and Ireland is a striking proof of the complete manner in 

 which their skeletons have been broken up. In this respect fossil Sponges have 

 undergone the same reducing process as the Crinoids, of whose remains the 

 massive beds of limestones in the Yoredale series mainly consist. Whilst in the 

 limestones we rarely meet with more than the disarticulated joints and plates of the 

 stems and calyces of Crinoids, in the chert and siliceous rocks intercalated with 



1 ' Ann. and Mag. Nat. Hist.,' ser. 5, vol. vi, 1880, p. 209. 



2 Professor Sollas has lately stated that the extraordinary profusion of Sponge-spicules in modern 

 marine deposits and in the ancient stratified rocks is due to the fact that the living Sponge is 

 constantly producing and disengaging spicules, and that during the process of " growth the spicule 

 slowly passes from the interior to the exterior of the sponge, and is finally, in at least some Sponges 

 (Oeodia, Stelletta), cast out as an effete product" (" Sponges," ' Encyclopaedia Britannica,' ninth edition, 

 vol. xxii, p. 420). Hitherto these deposits of Sponge remains have always been regarded as arising 

 from the disintegration of their skeletons after the death of the Sponge, and this still seems to me the 

 more probable explanation. Prof. Sollas' statement is so marvellous that it will require strong 

 confirmatory evidence before it can be accepted. At present none is given, and the general experience 

 of other observers points in an opposite direction, viz. that in the growth of the Sponge the skeletal- 

 spicules gradually tend to become firmer and more deeply embedded in the living tissues of the 

 organism. 



