116 



MISCELLANEOUS GEOLOGICAL CONSIDERATIONS 



deductions of comparative anatomy can lead to false conclusions 

 because the historical development had been different. This warns 

 us against accepting similar lines of thought in comparative bio- 

 chemistry. 



THE IMPORTANCE OF CLAYS 



Clay is one of the most abundant rock types at present on the sur- 

 face of the earth, and from what we know of the oldest sediments 

 laid down in early pre-Cambrian times, it will have been about as 

 abundant then. Clays are very fine-grained rocks. They are formed 

 by a group of related minerals, the clay minerals, which are all of 

 a similar build, being formed by silicates of aluminium, with much 

 water and various other cations, and with a decidedly flaky structure. 

 They resemble muscovite, both in chemical composition and in the 

 structure of their crystal lattice, but the individual crystals are of 

 much smaller size. Their crystal lattice consists of well-defined, thin, 

 parallel layers of strongly bounded ions, separated by voids in which 

 only a few hydroxyl groups or cations are found (Fig. 33). The 

 distance between the individual parallel layers of the lattice can vary 

 strongly, depending on the number of ions incorporated in the voids. 



o 



o 



OH 







Mg.Al^ 

 St .Al"" 



nH^O 



1+ 



Fig. 33. Schematic section of the crystal lattice of montmorillonite, one 

 of the clay minerals. Each layer of silicate is separated by a loosely 

 bound number of water molecules, through which the clay swells when 

 wetted and shrinks when drying. Maximum distance between silicate 

 layers is about 14 A (from Bijvoet et al., 1948). 



