XIV.] LIVING MATTER AND ITS EFFECTS. 231 



that the material was probably deposited at the bottom of 

 lakes or in marshes. 



When a living diatom is examined, it is seen that the 

 siliceous case incloses a particle of protoplasm. A diatom 

 is, in fact, a simple vegetable cell, but its pecuharity lies 

 in the fact that the cell is able to separate, or secrete, from 

 the surrounding water, that chemical combination which is 

 called " silica," and which exists, in minute proportion, dis- 

 solved in most natural waters. The silica thus appropriated 

 by the diatom forms a solid case, which incloses the proto- 

 plasm, and often exhibits a beautifully-sculptured surface. 



Fig. 64. — Microscopic section of diatomaceous deposit, Moume Mountains. 

 Ireland. Magnified about i6o diameters. 



On the death of a diatom, the protoplasm decomposes 

 and disappears ; but the siliceous shield, although it is very 

 slowly dissolved by water, is not easily perishable, and it 

 therefore remains as a solid body at the bottom of the water. 

 It is true the diatoms are very minute, but they compensate 

 for this by their extraordinary abundance. In some 

 estuaries, they are so abundant that the accumulation of their 



