120 THE DAWN OF LIFE. 



" It appears probable from these observations that glauconite 

 is formed by chemical reactions in the ooze at the bottom of 

 the sea, where dissolved silica comes in contact with iron 

 oxide rendered soluble by organic matter; the resulting 

 silicate deposits itself in the cavities of shells and other 

 vacant spaces. A process analagous to this in its results, has 

 filled the chambers and canals of the Laurentian Foraminifera 

 with other silicates ; from the comparative rarity of mechani- 

 cal impurities in these silicates, however, it would appear that 

 they were deposited in clear water. Alumina and oxide of 

 iron enter into the composition of loganite as well as of glau- 

 conite ; but in the other replacing minerals, pyroxene and 

 serpentine, we have only silicates of lime and magnesia, which 

 were probably formed by the direct action of alkaline silicates, 

 either dissolved in surface-waters, or in those of submarine 

 springs, upon the calcareous and magnesian salts of the sea- 

 water." 



[As stated in the text, the canals of Eozoon are sometimes 

 filled with dolomite, or in part with serpentine and in part 

 with dolomite.] 



(B.) SILURIAN LIMESTONES HOLDING FOSSILS INFILTRATED WITH 

 HYDROUS SILICATE. 



Since my attention has been directed to this subject, many 

 illustrations have come under my notice of Silurian limestones 

 in which the pores of fossils are infiltrated with hydrous 

 silicates akin to glauconite and serpentine. A limestone of 

 this kind, collected by Mr. Robb, at Pole Hill, in New Brunswick, 

 afforded not only beautiful specimens of portions of Crinoids 

 preserved in this way, but a sufficient quantity of the material 

 was collected for an exact analysis, a note on which was pub- 

 lished in the Proceedings of the Eoyal Irish Academy, 1871. 



The limestone of Pole Hill is composed almost wholly of 

 organic fragments, cemented by crystalline carbonate of lime, 

 and traversed by slender veins of the same mineral. Among 

 the fragments may be recognised under the microscope por- 

 tions of Trilobites, and of brachiopod and gasteropod shells, 

 and numerous joints and plates of Crinoids. The latter are 



