190 Dr. Wallich on the Origin of the 



from contact with sea-water, owing to the insolubility and 

 coherence of the protoplasm, and would in due time yield to 

 the powerful solvent influence of this substance (an influence 

 which is at its maximum when exerted between substances 

 that are colloidal), and eventually, by parting with their 

 hydration in obedience to the law which governs these bodies, 

 as their state of pectization became more perfect, would become 

 consolidated into black flint. 



Wherein, then, does the external whitish layer or crust of 

 the nodule, and which also forms a crust or coating over the 

 surface of the tabular layers of flint, differ in its mode of 

 formation from that I have just been describing — namely, of 

 the central mass of black flint ? The explanation, I contend, 

 is both simple and conclusive. At this stage of the forma- 

 tion of the nodular mass (not in point of time, but of material) 

 replacement of the carbonate of lime by silica, rarely/, if ever, 

 a complete j^'i'ocess *, comes into operation. An examination 

 of sections of nodules embracing both a portion of the true flint, 

 and of its outer investing crust of cher-t will show that the transi- 

 tion from pure flint to well-pronounced chert is a gradual one, 

 so gradual, indeed, that it would often be difficult, were not a 

 difterence of colour apparent, to recognize it, except by the im- 

 pairment of the vitreous character, conchoidal fracture, and 

 translucence which distinguish the flint from tlie crust of chert. 

 Seen, however, as a section under the microscope, it is always 

 visible. On the outer aspect of the cherty crust, on the other 

 hand, there is no gradation observable either as regards colour 

 or texture ; for, although in nearly every nodular mass a 

 certain quantity of unmetamorphosed chalk is closely adherent 

 to it externally, a very little trouble will show that not a 

 trace of siliceous percolation has extended beyond the peri- 



dependence of organic matter for its continuance, as living matter, upon 

 the unknown quantity we call life. A mere breath destroys the hnk 

 that binds together the animate and inanimate ; and, as if eager to regain 

 the sway they once enjoyed, when life " was not," the material forces 

 of nature set about their normal task of disintegrating and recon- 

 structing the elements which had for a time so successfully set them at 

 defiance. If we extend this conception (and there is no reasonable 

 ground for saying we have no warrant for so doing) to the stupendous 

 development of sponge-life at the bed of the ocean, we shall not expe- 

 rience difficulty in comprehending how the silica and the protoplasm of 

 the sponge, which respected each other's rights so long as the vital force 

 presided over them, should, under the now unrestrained action of their 

 powerful combining tendencies, interact upon each other in the way 

 that has here been indicated. 



* Mr. Soilas (at p. 449 of his paper) speaks of the white outer crust of 

 the flints as " an incompletely salicified laym' of chalk.'' 



