from Oodstone Firestone. 278 



interspersed with irregularly shaped siliceous particles thickly 

 covered with the discs or spherules already mentioned. Here 

 and there may be seen the internal casts of foraminifera and 

 sometimes the spicule of a sponge that has not been entirely 

 broken up, but they are not so evident as in some selected 

 specimens. We find a very similar structure in chert, which 

 occurs abundantly at Tilburstow Hill. Chert is easily splintered 

 into fragments sufficiently thin to show its structure when 

 mounted in Canada balsam. Such splinters often exhibit the 

 internal casts of sponge spicules very perfectly, but the outer 

 siliceous skeleton of the spicule itself has either been entirely 

 dissolved away, or become merged in the matrix of the chert so 

 as to be indistinguishable. Parts of the chert show numerous 

 circular markings, and it seems to me that these were originally 

 spherules such as we find in the firestone, which have continued 

 to grow by accretion of silica until they have overlapped each 

 other, and a solid stone is the result. So far as I can see this 

 appears to be the only difference between chert and firestone. 

 We know that hydratid sihca is soluble in alkaline carbonates, 

 which are only partially decomposed by the silica dissolved, and 

 it is owing to this fact that almost all spring, river, and sea water 

 contains silica in solution in minute quantities. When aided by 

 heat, as in the geysers of Iceland, large quantities are dissolved 

 and deposited as a petrifaction on surrounding objects as the 

 hquid cools (Miller). In this way probably the harder rocks 

 have been formed. 



We will now turn our attention to the green bodies before 

 mentioned, and try to trace their origin. These consist of a 

 mineral composed of silica and iron known as glauconite. At 

 Oamaru, New Zealand, there are large deposits of a chalky- 

 looking substance, which is composed of sponge spicules, 

 diatoms, Eadiolaria, and Foraminifera. The cavities of the 

 sponge spicules in some of the specimens that I have examined 

 contain vermiform bodies composed of glauconite, and which 

 appear to have been derived from the solution of the spicule 

 walls and subsequent deposition in another form, for we 

 frequently find these bodies in spicules almost closed at both 

 ends, with the exception of the small canal aperture ; and, as 

 their specific gravity is greater than the original spicule, it is 

 only to be expected that they will occupy less space. Hence the 

 cavity is never perfectly filled. The Foraminifera, after treatment 

 with acid, frequently leave perfect siliceous casts of the interior of 

 their shells. 



The question arises, are these glauconite casts formed previously 

 or subsequently to the upheaval of these beds ? The evidence 

 that I can offer is not conclusive. I have examined sponge 

 spicules dredged from great depths (3000 fathoms), and have not 



