DIASTROPHISM AND THE FORMATIVE PROCESSES 693 



necessity of the limited amount of planetesimal matter, the large 

 amount of space through which it was distributed, and the mutual 

 relations of the planetesimal orbits, as already brought out. There 

 were besides obstacles to growth beyond quite small sizes. Even 

 if these obstacles be supposed to have been ineffectual, time for 

 growth from the minute sizes to five-mile bolides must have inter- 

 vened before the latter could function as crater-formers. They 

 could thus have come into function only at a late stage. But 

 accretion could not have been suspended in the meantime. They 

 could therefore have come into function only as a partial source of 

 lunar accretion. Growth from the smaller planetesimals must have 

 gone forward during all the intervening period. Accretion simply 

 by such giant planetesimals is thus incompatible with the funda- 

 mental conditions postulated by the basal hypothesis on which it 

 rests. 



The hypothesis is not much more promising if planetoids are 

 substituted for the supposed giant planetesimals, for, by the 

 mechanics of the case, the planetoids were given courses less favor- 

 able to aggregation than were the planetesimals, and hence greater 

 intervals between their infalls must in consistency be assumed. 

 This is in harmony with the observed fact that at least eight hun- 

 dred planetoids are still following their own individual paths in a 

 relatively limited tract and yet no collision or even dangerous 

 approach to one another has been noted during the whole period 

 of astronomical observation. In addition to this, we have found 

 reasons for doubting whether planetoids could organize as nuclei 

 of the planetary type under the differential stresses of the solar 

 attractions that prevail in the region of the earth and in regions 

 still nearer the sun. 



The hypothesis that the pits of the moon were formed by the 

 impacts of great meteorites offers no presumption that one infall 

 would be followed by another in the same spot within any short 

 period. As a cause of general melting, this is even more unpromis- 

 ing than the preceding. 



The discussion thus far has proceeded on the assumption that 

 the pits of the moon are the scars left by the impacts of great 

 bolides of one sort or another. Before turning to the next topic, 



