224 Geological Society, 



again, a piece a quarter of an inch in diameter, presenting the rough- 

 est aspect, be examined under a power of 120 linear, illuminated by a 

 Lieberkuhn, the surface, under favourable circumstances, will pre- 

 sent a complex mass of small, contorted tubuli, occasionally fur- 

 nished at the apex with a minute perforation. 



The structure and other characters of the tabular flints are stated 

 to accord perfectly with those of the nodular masses, except that the 

 under surface has a still more marked spongeous aspect, and that spi- 

 cula and Foraminifera are more abundant. The absence of any ap- 

 parent base or point of attachment in the great mass of nodular chalk 

 flints, the author says (considering them undoubtedly of spongeous 

 origin), may be accounted for by supposing that the gemmule was 

 originally attached to some minute fragment of a shell or other sub- 

 stance, and that its further development took place while recumbent 

 on the mud or silt. 



The perpendicular and oblique veins of flint between Brighton 

 and Rottingdean, are reported to present exactly the same internal 

 characters as the tabular and nodular flints, and to agree externally 

 with the former. The occasional existence of a fissure filled with 

 chalk, in the centre of the vertical layers, Mr. Bowerbank conceives, 

 may indicate that the sponge had grown from the two sides of the 

 crevices, but had not in all places been able to unite. The sides of 

 these flint veins are not studded with Foraminifera in a manner simi- 

 lar to that of the tabular horizontal layers. 



Mr. Bowerbank next examined the flint with which Echinites and 

 shells of the chalk are often entirely or partially filled and enveloped, 

 and he states, that the results were the same, both with reference to 

 the exterior and the interior of the flint. In those cases in which 

 the Echinite is only partially filled, he infers that the portion so occu- 

 pied was originally a sponge, because its surface is uneven ; for had 

 the flint been deposited in an empty shell or Echinite, it would pre- 

 sent an uniformly flat surface. Again, he states, that the projecting 

 of the flint through the two openings of the Echinite, with an ex- 

 tension to a greater or less distance, is owing to the sponge having 

 grown outwards through these orifices ; and the envelopment of an 

 organic body by a tabular mass of flint, he explains by reference to 

 the habit of recent sponges to invest testacea or other marine bodies. 

 In some cases, he has found minute but deep depressions on the 

 surface of flints filling Galerites, and immediately opposite to the 

 ambulacral pores ; and he ascribes the origin of the depressions to 

 streams of water drawn in through the orifices to supply the wants 

 of the living sponge. 



Mr. Bowerbank was afterwards induced to extend his examination 

 to the flints which invest the zoophytic bodies of the Wiltshire chalk. 

 By carefully cleaning the interior of some of these flints, he discover- 

 ed spicula projecting from all parts, however different the character 

 of the inclosed body ; and the spicula appeared to have no reference 

 to it, none of them being found on its surface. Under the micro- 

 scope, the investing flint presented in every respect the same appear- 

 ance as that exhibited on the lower surface of the tabular flints, ha- 



