W. J. SOLLAS ON PHAKETROSPONGIA STKAHANI. 253 



washing with distilled water and drying, examined both as an 

 opaqne and as a transparent object by powers of 75 and 140 diameters. 

 It was then found that the skeletal network had dissolved freely 

 and uniformly ; and the undissolved remainder, still exhibiting a 

 spicular structure, presented a smooth even surface sunk considerably 

 below the level of the surrounding interstitial matter ; here then 

 spicules and fibre alike consisted solely of carbonate of lime. The in- 

 terstitial matter, however, presented an interesting difference : part, 

 being calcareous, had dissolved ; but another part of a different com- 

 position had resisted solution, and remained behind as an insoluble 

 network, imbedding green grains, silicified Foraminiferal shells, 

 casts, and other foreign particles in a siliceous cement. Thus the 

 original calcareous composition of the infilling material has been in 

 this case, to a great extent, exchanged for a siliceous one ; but pre- 

 cisely the same alteration has occurred to the interstitial matter of 

 every specimen of Pharetrospongia which I have examined ; and 

 since siliceous infiltration is a very rare phenomenon among the 

 fossils of the Cambridge Grecnsand, while in Pharetrospongia it is a 

 constant occurrence ; and since, again, we have already shown that 

 the fibres of Pharetrospongia once possessed a siliceous composition, 

 which they have since exchanged for a calcareous one, it seems not 

 unfair to conclude that the silica under consideration found its way 

 into its present position from the fibres of the sponge itself — that, 

 in fact, there has been an interchange of material between the in- 

 terstitial substance and that of the sponge-fibre, an outflow of dis- 

 solved silica from the one being compensated by an influx of calcic 

 carbonate from the other. 



It is scarcely possible within the limits of an appendix to discuss 

 this change fully; and, reserving that for another place, I must 

 now proceed to describe the more important results which have been 

 obtained chiefly, though not exclusively, from an examination of 

 specimens in which a phosphatic infiltration has taken place in ad- 

 dition to a chalk-marl infilling. On etching the surface of one of 

 these specimens with acid the same results follow as with the calca- 

 reous examples, with one exception, however ; for in these specimens 

 the skeletal fibre, though almost wholly converted into carbonate of 

 lime, is not altogether so, but leaves behind on solution a number 

 of beautifully preserved spicules having a siliceous composition 

 beyond a doubt, and present in such abundance as to furnish plenty 

 of examples perfect enough for accurate measurement ; at the same 

 time they are exclusively restricted to the borders of the skeletal 

 fibre, and do not occur in its central portions, since they are not 

 only absent from the latter position, as seen on the eroded surface 

 of the fossil, but are also not to be detected in the sediment which, 

 after solution, is collected on washing with distilled water. Connected 

 with this fact is the observation that in some cases the dissolved silica 

 of the spicules has been deposited about the exterior of the spicular 

 fibre to such an extent as to form a thin siliceous envelope to it. 



