208 T. C. CHAMBERLIN GROUNDWORK OF EARTH S DIASTROPHISM 



In the case of the moon, the logic is even more cogent, for even in its 

 present cold full-grown state its gravity is insufficient to hold an appre- 

 ciable atmosphere. Certainly, then, in its earlier and hotter stages, if it 

 were once molten, all its volcanic gases should have escaped as fast as 

 they were set free, and it should have cooled into a solid, gasless mass. 

 The highly explosive vulcanism of the moon implied by the special fea- 

 tures of its present pitted surface and the long lines of debris that radiate 

 from some of its craters, irrespective of the reliefs of the surface, find 

 ready cause under the alternative view, because it supposes that gaseous 

 and gas-forming materials were entrapped during the planetesimal 

 growth. This entrapped material would become an ample source of ex- 

 plosive vulcanism at a later stage, when internal conditions, like those 

 previously sketched in connection with the earth, brought into play their 

 solvent and gas-freeing activities, while differential pressure forced the 

 gases and other mobile matter to the surface. The fragmental state of 

 the outer matter in the absence of the cementing effect of a hydrosphere 

 should give peculiar effect to the explosive action. 



Bearings of this View on igneous Petrology 



The prevailing view of magmas, when regarded as inheritances from 

 a once molten earth, usually makes them either residues of the original 

 molten mass or else re-solutions, e?i masse, of rock previously solidified. 

 In either case the solidifying process is usually interpreted as involving 

 a differentiation into a series of derivatives which separate more or less 

 successively from the original magma. Magmatic dijferentiation thus 

 becomes the essence of igneous petrology. Its problems become essen- 

 tially questions of descent from an assumed primitive, or quasi-primitive, 

 magma. In the field, the critical issue is to find all the derivatives re- 

 quired to make up, quantitatively and qualitatively, a consistent primi- 

 tive, or quasi-primitive, magma. The embarrassments are many and 

 formidable. 



On the other hand, if magmas consist merely of partial solutions of 

 heterogeneous mixtures, such as would naturally be formed by a multi- 

 tude of small bodies of different natures falling in at random, they would 

 quite certainly become highly diversified in the making. The primary 

 problem would then lie in the generation of the magmas; in the ascent 

 of magmas rather than their descent. While differentiation in the process 

 of solidification Avould still remain a factor, it would be a secondary mat- 

 ter, in the sense that it was necessarily conditioned by the previous gener- 

 ative process. The inquiry in the field, in this case, should start with the 

 evidence as to what petrological species and varieties are actually present, 



