GEOLOGICAL. 



189 



superficial measurement, that is, their entirety, as now 

 presented to your eye, is one hundred and sixty thou- 

 sand times larger than they really are. I mention this 

 " superficial measurement " here as I have done elsewhere 

 in the book, that you may try to realize what is meant 

 by "the invisible world," for without the microscope 

 they could never have been discovered. 



These four "frustules," as they are called, are all 

 fossil, and appear to be flat discs, but they are slightly 

 convex, and consist of two valves, united by their outer 



Fossil Diatom acese. 



edges ; the top middle figure represents the two valves 

 thus united, forming a sort of case in which all the vital 

 processes of their little lives are performed. 



Having already shown you the exquisite siliceous 

 skeletons of such plants as the deutzia, I need do no more 

 than remind you of the marvellous way in which this 

 element is diffused throughout the vegetable kingdom, 

 and that just as our own bony skeletons are formed of 

 lime, the base of which is chalk, so the whole vegetable 

 world is indebted to flint for the framework of that 



