Carboniferous Sponges from Ayrshire. 431 



along with Polyzoa and Ostracoda ; while another group is 

 almost exclusively made up of the remains of large Orinoids. 

 Cephalopods and Gasteropods are rare in this series of lime- 

 stones ; and the teeth of Placoid fishes are only occasionally 

 met with. The Sponge-remains are not confined to a single 

 horizon, though they appear to be more common in the shelly 

 limestone. Neither are they restricted to the purer limestones, 

 being found in the calcareous shales as well. The Central 

 Scottish limestones were not deposited in so deep a sea as 

 that in which the English limestones were laid down ; and in 

 the west of Scotland there is evidence of repeated oscillations, 

 whose occurrence goes against the supposition that the water 

 was deep. The shales which separate the sponge-bearing 

 limestones from the inferior division of the Lower Limestone 

 series are argillaceous and bituminous, with beds of foul coal 

 and of volcanic ash ; while above the upper division of the 

 same series (that is, above the sponge-bearing beds), as at 

 Trearne, Stigmaria and other coal-plants with carbonaceous 

 shales tell of recurrent land and shallow water. From these 

 considerations and from comparison of the list of fossils given 

 by Mr. Craig in the paper above referred to, we do not think 

 the Hyalonema lived in such deep water as do some, at least, 

 of its modern representatives — not all, if H. cehuense^ Higgins, 

 was obtained by diving ; for, as Mr. Higgins says, the depth 

 could not, then, have exceeded 60 feet. 



It is in the joints and fissures of tlie Cunningham-Baidland 

 limestone that the sponges and other fossils have been found 

 in most perfect condition amongst the weathered ddbris of the 

 rock. The limestone in this quarry and generally throughout 

 the district is greyish white ; but the rotten limestone of the 

 fissures is dark reddish brown, the colour being found, on 

 analysis, to be due to the presence of iron associated with 

 three or four per cent, of manganese. Distributed throughout 

 the limestone in this and other quarries where it is worked 

 are thin bands and nodular masses of greyish flint which 

 enclose the organisms of the deposit, and which seem to have 

 been formed by chemical segregation from silica in solution. 

 Mr. R. 0. Wood has suggested as the source of part, at least, 

 of this silica, the volcanic rocks and ashes which are so plen- 

 tiful in the superjacent strata ; and the existence of thermal 

 springs may also have had a share, as was suggested by Prof. 

 E-amsay in the case of many of our altered Scottish limestones. 

 To the possible presence of thermal springs Mr. Young is 

 inclined to attribute the abundance of corals met with on this 

 horizon throughout the western district. It is worthy of note 

 that some of the apparently good limestones are unfit for the 



