92 MICROSCOPICAL OBJECTS FOUND 



That as regards the siliceous elements of 

 calcareous strata, few recent deposits exist in 

 which there are not some siliceous cases of the 

 organisms, regarded by Ehrenberg as siliceous 

 Infusoria, but that in the majority of instances, 

 grains of sand constitute the only visible form in 

 which silica exists in any abundance. In this 

 state it is found more or less in all. 



It is obvious, then, that if any calcareous 

 deposits are now forming, the results of chemical 

 decomposition, they are not in accordance with 

 the ordinary plan followed by nature in the accu- 

 mulation of calcareous strata either at the present, 

 or during the Tertiary and Cretaceous periods. 

 At the same time, we must not lose sight of the 

 fact, that calcareous deposits may be formed under 

 water, the results of chemical action alone. The 

 Travertinsof Italy afford sufficient evidence of this; 

 and if similar calcareous springs existed extensively 

 under the waters of any confined ocean, there is 

 no apparent reason why they should not lead to 

 the production of calcareous deposits, such as are 

 found at San Vignome and at San Filippo. The 

 only approach to an appearance of the kind which 

 has come under my notice, in all the specimens of 

 recent sediment which I have examined, was in 



