604 CLARKE: EARLY FORMS OF LIFE 



of life must have affected the course of evolution, although with 

 exceeding slowness. Every variation in the composition of the 

 fluviatile or oceanic salts modified the conditions under which 

 life developed. As living organisms multiplied, they in turn 

 altered the composition of the waters, by just so much as they 

 withdrew lime, or magnesia, or phosphoric oxide, or silica from 

 solutions and used them in building up the sedimentary cherts, 

 phosphorites, and limestones of today. 



To trace these changes in detail would be difficult, if not 

 impossible, but some of the ancient conditions can reasonably be 

 inferred. The chemical reactions involved in the discussion are 

 the same now as at the beginning, although the results produced 

 by them have varied from time to time and place to place. The 

 chemical elements are not uniformly distributed; at one point 

 there is more silica, at another more lime; and organisms with 

 siliceous, calcareous, or phosphatic shells or skeletons developed 

 in accordance with their surroundings. Where the primeval 

 waters were relatively rich in silica, siliceous organisms were most 

 readily evolved; where lime predominated, the development of 

 calcareous organisms was favored. So much seems to be clear. 



It has already been stated that the land surface of the earth 

 was at first composed of rocks such as form only one fourth of it 

 today; that is, of igneous, plutonic, or crystalline rocks with no 

 sedimentaries of organic origin. At present persilicic rocks of 

 granitic or granitoid type are the most abundant of these, and 

 they are relatively poor in lime. In this respect it is highly prob- 

 able that the earliest rocks followed the same rule. 



Many analyses of river waters have been made, some of them 

 with reference to their geological relations. Waters emerging 

 from areas of sedimentary rocks, or from basaltic regions, are 

 quite unlike those which issue from granite and its congeners. 

 The meteoric waters act differently upon different kinds of rock, 

 and take up dissimilar loads of soluble substances. Waters from 

 limestone or from certain ferromagnesian rocks are relatively 

 rich in lime; those from dolomite contain a larger proportion of 

 magnesia, and so on; the waters varying in composition as the 

 rocks themselves vary. Each water at its point of origin has its 

 own chemical characteristics, which are fixed by its Jithologic 



