618 PKOF. W. J. SOLLAS ON THE ESTTJAEIES OF 



tain addition of these constituents is now being made from some 

 other source, in order to maintain the proportion unchanged. 



In the next place, it is well known that siliceous sponge-spicules 

 when left to the action of sea-water * slowly dissolve, the first signs 

 of solution to make their appearance being usually an enlargement 

 of the excessively fine canals which perforate them axially. The 

 calcareous spicules of the Calcispongise are, as one might expect, but 

 to a far greater extent than one would expect, more soluble than 

 siliceous ones, and thus their preservation is exceedingly rare. 



The spicules which occur in the alluvium of the Severn, par- 

 ticularly the broken ones, generally give marked evidence of this 

 solution ; the canals of whole spicules are frequently, and of frag- 

 ments always, obviously enlarged. An interesting difference in 

 this respect is to be noticed between the spicules of the super- 

 ficial alluvium at Avonmouth and that deeper seated, say at about 

 12 feet from the surface, as in the cutting for the Severn tunnel ; 

 those from the latter, having been exposed to solvent action for a 

 much longer time than the former, are traversed by canals from 

 •1- to | of their diameter, sometimes more, so as to present us with mere 

 shells or husks, though sometimes less ; but the canals of those 

 found near the surface are seldom as much as l of the diameter of 

 the spicule, and often less. As, however, it is the superficial 

 alluvium which is now in course of erosion by streams and waves, 

 we may in considering the source of the spicules found in the tidal 

 waters confine our attention exclusively to it f . With respect to 

 the spicules now found in the tidal waters, one may as well state 

 at once that they also show marked signs of solution, not quite 

 to the same extent perhaps as the fragments found in the alluvium, 

 although the difference is very slight. Entire uninjured spicules, un- 

 touched by solution, are, on the whole, slightly commoner in the 

 former. 



Admitting that there is very little difference in the average size 

 of the canals of the spicules found in the superficial alluvium and 

 those floating about in the estuary, one might still argue that had 

 the latter been derived from the former, they should, making allow- 

 ance for their additional soaking in the water, present us with 

 larger canals. But this is certainly not the case, the difference, if 

 any, being the other way : and thus one might conclude that the 

 floating spicules have not been washed out of the alluvium. Con- 

 sidering, however, the important conclusions which would folloAV 

 such an admission, I shall not lay any stress on evidence of such a 

 superfine nature, but prefer to adduce two sets of facts, which have 

 the appearance of bringing the matter to a demonstration. The 



* Salt water is not necessary to this solution, as is shown by the fact that 

 the spicules of Spongilla found in river-ruud frequently exhibit greatly en- 

 larged canals. 



t If, however, tracts of alluvium form part of the bed of the estuary, the 

 tide must by this time have denuded down to the lower deposits, and the 

 spicules derived from them would be those which have undergone extreme 

 solution ; and as the spicules of the recent mud do not possess greatly enlarged 

 canals, this source is excluded. 



