370 Fra7ik Carney 



forming larger particles. The orbits of single and combined 

 planetesimals cross or pass near the orbits of nuclei, whereupon 

 the smaller bodies unite with the larger. By this accretion proc- 

 ess bodies are formed which dominate a certain amount of space 

 from their centers. These growing masses through further accre- 

 tions became initial planets. Some of these may unite with 

 others, but eventually the members of our present solar system 

 were formed. 



But we are concerned with neither hypothesis any further than 

 the aid it offers in understanding the origin of rocks ; both accord 

 in the principle that sedimentary rocks contain the weathered 

 products of older rocks. 



Age of weathering processes. The Nebular hypothesis, however, 

 gives us only a limited zone of secondary rocks, while the Planet- 

 esimal teaches that secondary rocks were in process of formation 

 from a time when the earth was but a fraction of its present 

 size. In the various processes of weathering, water and certain 

 chemicals are the essential factors. But there was a time, it is 

 thought, when the earth was not surrounded by an atmosphere, 

 and its surface bore no hydrosphere. In the absence, then, of 

 these two spheres, it is probable that the original rock suffered 

 very little change, certainly not all the changes which we know of 

 as weathering. An atmosphere is matter in the gaseous form. 

 Only when a planet becomes large enough to exert sufficient 

 gravity to hold gaseous matter near its surface, will it have an 

 atmosphere; for this reason, the early atmosphere of a growing 

 planet consists of the heavier gases. 



The moon has no atmosphere; hence we believe that the gravi- 

 tative power of the moon is not sufficient to control hydrogen, 

 oxygen and other gases. We are also told that Mars, which is 

 about one-tenth the size of our earth, has a very slight atmosphere. 

 From this fact we infer that Mars is large enough to control some 

 gases. If the analogy of these two members of our solar system 

 may be used in interpreting the theoretical history of the earth, 

 it must be concluded that the earth has been surrounded with 

 at least a slight atmosphere from the time it was about one-tenth 

 its present size. The surface of the moon appears not to have 

 a hydrosphere. Whether Mars has any water on its surface, is 

 a mooted point. Theoretically, however, we would infer that 

 a growing planet should have a gaseous envelope before the de- 



