BY H. I. JENSEN. 519 



the origin of our planet be accepted, according to which the 

 earth grew by gradual accretions of meteoric material from 

 without. In this case, unless we suppose that the earth received 

 widely different kinds of material at different periods of its 

 growth, we have a particularly good ground to expect the same 

 plutonic and volcanic sequence in all areas and at all times, or 

 to ask Dr. Chamberlin to explain more fully by what curious 

 processes of differentiation the enormous differences in igneous 

 succession have been brought about. A little thought given to 

 this question will, I think, convince any petrologist that the 

 variability of the Plutonic sequence is a powerful objection to 

 the planetesimal hypothesis. 



(3) The hypothesis that alkaline rocks may be derived from 

 the assimilation or fusion of alkaline sedimentary beds at great 

 depths, derives some support from the fact that foyaitic rocks 

 usually contain chlorine and sulphates in proportions much greater 

 than other magmas. 



(4) One phase (and usually the first) of activity in an alkaline 

 volcanic epoch is the eruption of plugs or mamelons of highly 

 viscous, siliceous lava, such as would be formed by the refusion 

 of alkaline sedimentary beds, or the mixing of such a molten 

 mass with quartzitic rocks. In this connection it is signifi- 

 cant to state that the fused alkalies, especially in the presence of 

 water, exert great solvent power on quartz. The proportion of 

 iron met with in these rocks is also very nearly that which would 

 occur in a chemical precipitate from a supersaturated ocean, or 

 drying up basin (cp. The Old Red Sandstones). 



The Planetesimal Hyyothesis. — In his able Presidential Address 

 to this Society* on geophysics. Mr. Thomas Steel, F.L.S., advanced 

 some strong astrophysical objections to the planetesimalhypothesis. 



If some of the mathematical opponents of the hypothesis were 

 set to work to find objections to it with the same assiduity that 

 some critics have displayed in finding objections to that of 

 Laplace, I dare say it would be found just as vulnerable. 



* These Proceedings, Vol. xxxii. p.l, 1907. 



