241- Prof. Ansted on the Zouloyical condition of Chalk Flints, H^c. 



the microscope, deep circular excavations being traceable, and 

 small fragments of shells and other extraneous matters being 

 partly imbedded in and partly adhering to the surface. 



Mr. Bowerbank having thus examined and described the na- 

 ture of the flint which forms tabular masses interstratified with 

 the chalk, extended his observations to the internal casts of sea- 

 m'chins and other shells of Radiata common in the chalk, some of 

 which are wholly and others only partially siliceous. In these 

 cases, on clearing away the chalk with care, it appeared that the 

 flint did not in any case present an even siu-face, such as would 

 have resulted from a fluid depositing the siliceous matter, but, on 

 the contrary, that the surface was undulated, projecting above 

 the surrounding parts, and offering the same characters as those 

 observed in flint nodules. It therefore appeared that these re- 

 mains must be referred to the same origin as the tabular flint. 



Mr. Bowerbank, from such observations as these, has arrived 

 at the conclusion, that all flint, in the common acceptation of the 

 term, has been formed upon spongeous bodies as nuclei. Anxious 

 to verify so far as I could these conclusions and convince myself 

 of their correctness, I have now to record an observation of my 

 own of a similar kind. 



In this experiment, which I look upon as to a certain extent an 

 experimentum crucis, I selected from the collection in the Wood- 

 wardian Museum* a small specimen of the purest black flint, in 

 which a common Cidaris of the chalk was imbedded, part of the 

 shell projecting beyond the surface of the flint and that part still 

 consisting of carbonate of lime. The general appearance of the 

 specimen is strongly in favour of the idea of a pulpy tenacious 

 fluid having received on its surface the sinking shell, whose weight 

 was not sufficient to cause it to sink entirely beneath the surface. 



Examining first of all fragments of the flint not contained 

 within the shell, they presented the appearance described by INIr. 

 Bowerbank and figured in the Geol. Trans. ; they did not indeed 

 exhibit distinct marks of reticulated tissue, but there could be no 

 doubt whatever of their organic origin. I then selected several 

 small portions chipped off from within the fossil, and one of these, 

 although exceedingly minute, was fortunately sufficiently uniform 

 in its texture to admit of employing a very high power. I was thus 

 enabled clearly to distinguish the peculiar reticulated tissue, the 

 existence of which puts beyond question the nature of the organized 

 being upon which this portion of the silica had formed. The di- 

 stinct texture was not made out without a power of 1200 linear, 

 and the appearance was still more striking with a power of 2500. 

 In this case then, the substance which has partially filled a shell 

 not broken and resting on the surface of a flint, is itself of the 

 * The Geological Museum in the University of Cambridge. 



