SOURCE OF SILICA 



161 



million, * while in springs it is no less than one hundred 

 and fifty-two parts in a million, t This is susceptible of a 

 simple explanation ; as soon as the water begins to flow 

 at the surface of the ground it becomes the home of those 

 simple plants, the diatoms, which extract the silica and 

 convert it to their own purposes, building up out of it a 

 pair of fragile valves of glass-like opal (Fig. 47) . Wherever 

 water is found, in pools or lakes or rivers or canals, these 

 living atomies are present, engaged 

 in making inroads on its store of 

 dissolved silica, and thus it suffers 

 a continual impoverishment through 

 the whole course of its journey to 

 the sea. 



The ultimate source of the silica is 

 the primitive substance of the earth's 

 crust, granite, or rocks such as 

 granite, and in particular one mineral 

 which these contain, i.e., the felspar. 

 It is to the decomposition of this 

 mineral that most of our sedimentary 

 rocks and most of the silica carried in 

 solution by water must be traced. 



Having reached this stage, our 

 labours as geologists are concluded. 

 To investigate the origin of the 

 molten planet is no function of ours, 

 and we may safely leave it in the hands of the astronomer, 

 whose business it is. 



Briefly to recapitulate our results, we perceive some 

 four or five millions of years ago a clear blue sea extend- 

 ing above the site of the British Isles ; over its floor a 

 sheet of chalky ooze was spread, formed by the birth, 



* This is the average of twenty-five analyses, 

 f This is the average of eight analyses of as many deep wells. 



12 



FIG. 47. Siliceous 

 Shell of a Common 

 Diatom, Pinnu- 

 laria, magnified 

 and somewhat dia- 

 gramatic. On the 

 left, as seen en 

 face ; on the right, 

 a lateral view. 

 After Prantl. 



