Flints of the Upper or White Chalk. 189 



in water was observed to be incompatible with the coexistence 

 of soluble silicic acid, till the proportion of the latter was 

 reduced to nearly 1 in 10,000 " {he. cit. anteh, pp. 336, 337). 



Of course, between the most highly developed cherty form 

 of flint and that in which is an admixture of foreign particles, 

 of whatever kind these may be, there is an almost infinite 

 gradation, depending, as I have in a former page pointed out, 

 on the replacement, now taking effect for the frst time, of 

 carbonate of lime by silica. In order to understand by what 

 very simple means this result is brought about, I will endea- 

 vour to illustrate it by the diagrams in the Plate accompanying 

 this paper, representing four of the most common forms in 

 which the typical nodular flints are met with in the Chalk, all 

 peculiarities as regards external form being of course dis- 

 pensed with as irrelevant to the present inquiry. For this 

 reason each of the four nodules is supposed to have been more 

 or less spherical (a condition, by the way, in which they are 

 not unfrequently met with), and to have been split in half, so 

 as to exhibit the flat and broken surface of one of the hemi- 

 spheres. 



In figure 1 we have a solid mass of typical black flint, sur- 

 rounded exteriorly by a whitish crust or layer, the thickness of 

 which is immaterial, inasmuch as it depends almost entirely on 

 the degree of comminution and purity of the deposit in the spot 

 at whicli it was formed. The portion which is now a mass 

 of the typical black flint (marked B in all the figures) con- 

 sisted originally of an accumulation of effete sponge-spicules 

 and network, which, immediately after the death of the parent 

 sponge to which they belonged, became loosely distributed 

 within the substance of the also effete investing protoplasmic 

 nidus *. Here they would be retained, more or less free 



* " Effete " is not meant to denote a state of decomposition in the common 

 acceptation of the word, inasmuch as every known fact tends to show that 

 no such process takes place at profound depths in the ocean. Disintegra- 

 tion (i. e. tumbling to pieces) may, and no doubt does, take place, either 

 in obedience to mechanical, chemical, or molecular forces, under the ope- 

 ration of which dead organic matter is enabled to enter into new comoi- 

 nations. This distinction is more important than it at first sight appears to 

 be, since there is reason to believe that in such an elementary substance as 

 sponge-protoplasm, and likewise in the examples known to every algologist, 

 in which the development of a protoplasmic nidus or '' thallus " is often so 

 enormous in comparison with the dimensions and apparent capabilities of 

 secretion of the organisms producing it, that it is extremely difficult to 

 understand by what subtle or simple function (if it be indeed simple) such 

 a massive adjunct can be produced and maintained for lengthened periods. 

 The singularly rapid disruption of this adjunct, following upon the death 

 of the organism of which it formed a part, furnishes one of the most in- 

 structive and significant commentaries we could desire upon the complete 



Ann. & Mag. N. Hist. Ser. 5. Vol. vii. 14 



