192 Mr. BowERBANK on the Siliceous Bodies of 



Dr. Grant, in his ' Outlines of Comparative Anatomy,' in treating of the Pori- 

 phera, concludes his observations by saying, " The abundance of siliceous needles 

 in the skeletons of the lowest Poriphera assists in their conversion into flint, when 

 their remains have been exposed for ages in chalk or other strata traversed by 

 silicifying percolations" (p. 9). I am strongly inclined to believe, that the siUceous 

 spicula, in the cases under consideration, have not influenced the deposition of the 

 flint in so great a degree as we may at the first view be inclined to believe, for 

 there are circumstances attending this accumulation of the siliceous matter which 

 are exceedingly curious in their nature. We find that the deposit of the silex is in 

 no case limited or determined by the immediate presence of the spicula, nor do we 

 observe, when thin sections of these bodies are viewed with the microscope, that 

 the spicula have operated as nuclei from which the silex has radiated, in the form 

 of needles or crystals driving the surrounding bodies before them, in the manner 

 that we so frequently observe in silicified wood ; while, on the contrary, we fre- 

 quently notice in the chalk-flint spongites, that portions of the large ex-current 

 canals are not flUed with silex, and the spicula are seen projecting into them 

 without even the slightest incrustation of siliceous matter upon their surface. 

 On the contrary, the siliceous is in all these cases bounded by the extent of the 

 animal matter of the sponge. Whenever a single tube, or a thin layer of tubes, 

 has been projected from the mass, although it may have been imbedded in the 

 chalk, there does the siliceous matter appear to follow, as if under the powerful 

 influence of some species of elective attraction, with the laws of which we are at 

 present unacquainted, but which I trust will hereafter become a subject of investi- 

 gation to some of our able chemists ; and this attraction of the silex to the animal 

 remains is so complete, that the chalk adhering to the surface of the flint is as free 

 from siliceous particles as that which is far removed from the mass ; and a hundred 



pearances which we observe in the corresponding parts of the siliceous portions of the great chert-bed. The 

 clear cherty nucleus is enveloped by a thick coating of spongeous structure, which has been rendered 

 opaque by a partial infiltration of the surrounding noaterial. It abounds in large and small canals, similar 

 in appearance to the in-current and ex-current canals of the recent sponge tribe, and spicula are frequently 

 found imbedded in it. From all these circumstances, therefore, we may reasonably conclude, that these 

 nodules are only detached specimens of the same species of sponge, which we find, under circumstances 

 more favourable for their increase, uniting into vast masses, and forming the great chert-bed of the Port- 

 land oolite. 



At the Whetstone excavations of Broad Hemberry and Punchey-down, I found the chert nodules 

 numerously dispersed amid the greensand beds, and presenting very much the same external appearance 

 as those of the Fovant and Shaftsbury greensands. Upon breaking a considerable number of them, a 

 clouded, white appearance, very similar to that observed in the Portland chert, was frequently to be 

 seen ; and upon examining this by the aid of a leiberkuhn and a power of two hundred linear, I found 

 the spongeous texture abundantly dispersed through the specimens, and differing but slightly from the 

 tissue described as occurring in the greensand cherts from Lyme Regis. — March 1841. 



