Chapter VIII 



THE ENVIRONMENT 



WEATHERING OF ROCKS 



Apart from the actual finds of fossilized remains, geology can also 

 evaluate the conditions which reigned at a certain time on the sur- 

 face of the earth. That is, in those cases where enough data are 

 preserved upon which to base some concept, geology can make out 

 whether there possibly has been, in the early past, a reducing atmos- 

 phere: an atmosphere totally different from our present oxidizing one, 

 such as is postulated by biologists and astronomers. 



As stated before, this is possible because such a different atmos- 

 phere would not only affect life, but also chemical processes on the 

 surface of the earth. Such a reducing atmosphere would make its 

 influence felt in all contemporary exogenic processes. 



The most important of the exogenic processes for our problem, 

 is the sequence weathering — erosion — transportation — sedimentation. 

 Dependent upon local characteristics of erosion — transportation — 

 sedimentation, such as speed of transportation, sedimentation on low- 

 lands or in oceans, weathering may take place during the latter parts 

 of this sequence too. For a determination of the character of the 

 early atmosphere the type of weathering in those days is of para- 

 mount importance. The influence of the other exogenic processes 

 serves more or less as a background for its setting. 



To look further into the effects of weathering, it is necessary first 

 to make a short digression, and review the composition of the rocks 

 of the earth's crust, because this now enters into the picture. The 

 most common elements of the crust are silica, Si, aluminium, Al 

 and oxygen, O. Most minerals of the crust are comjx)unds of these 

 elements, either silicium oxide or quartz, Si02, or silicates: com- 

 pounds of Si, O and Al. Into the silicate compounds are incorporated 

 the base metals Ca and Mg, the alkalis K and Na, metals, such as 



