Prof. W. Thomson on the Depths of the Sea. 119 



ceous organisms of the most varied and beautiful forms — shields 

 of diatoms, spicules of sponges, and the wonderful netted skele- 

 tons of the Polycystina. The soft ealcareous mud is the home 

 of multitudes of exquisitely formed glassy and other siliceous 

 sponges ; the chalk, on the other hand, may be said to contain 

 no disseminated silica whatever. When chalk is dissolved in 

 acid, a few grains or crystalline fragments of silica remain ; 

 but these are apparently all of inorganic origin — fragments of 

 mineral matter. Instead, however, of disseminated siliceous 

 organisms, we have, in the chalk, bands and lines of flints — 

 lumps of amorphous silica, which seem to have filled up and 

 taken the shape of any cavities already existing in the beds. 

 Many of these flints are apparently quite shapeless ; but many 

 of them (such as the so-called " paramoudras " of the Antrim 

 chalk) have more or less distinctly the form of large cup-like 

 sponges. Often the shell of a sea-urchin forms the mould of 

 a flint, which fills it entirely, reproducing in relief on its ex- 

 ternal surface every suture and perforation of the inner surface 

 of the shell. The conclusion seems to be irresistible, that in 

 some way which we do not as yet thoroughly understand, but 

 to which some late observations of the Master of the Mint 

 seem to promise a clue, the organic silica, if I may use the 

 expression, is dissolved out of the calcareous matrix ; the so- 

 lution percolates into and through the cavities, the water being 

 gradually drained from the silica, which is in the colloid state, 

 by the walls of the cavities acting as porous media, till, on the 

 water being nearly or entirely removed, the silica " sets " into 

 flint. In the white chalk of England there is an exceedingly 

 beautiful group of fossils, called Ventriculites, which have 

 greatly puzzled paleontologists. They have usually the form 

 of graceful vases, tubes, or funnels, variously ridged or grooved 

 or otherwise ornamented on the surface, frequently expanded 

 above into a cup-like lip, and continued below into a bundle 

 of fibrous roots. The minute structure of these bodies shows 

 an extremely delicate tracery of fine tubes, sometimes empty, 

 sometimes filled with loose calcareous matter dyed with per- 

 oxide of iron. We have been in the habit of regarding the 

 Ventriculites as an extinct group, specially characteristic of the 

 chalk ; but, after examining several species, and studying 

 carefully Mr. Toulmin Smith's excellent observations on their 

 structure, I now thoroughly believe that they were siliceous 

 sponges, nearly allied to, if not identical with, the recent 

 order Porifera vitrea, and that the silica of their spicules 

 was removed, and went to add to the jelly-like material of the 

 flints, leaving the moulds only in the chalk. Ventriculites are 

 not extremely common in the white chalk, nor are they very 



