Ixxxiv PKOCEEDINGS OP THE GEOLOGICAL SOCIETY. [May 1901', 



glass, separates into distinct minerals which do not, as a rule, con- 

 solidate simultaneously. Bat the acceptance of this fact does not 

 involve the acceptance of the differentiation-theory of the origin of 

 petrographical species, for, as M. Michel Levy points out, the crystal- 

 lization of a magma under ordinary circumstances does not com- 

 mence until it has reached a pasty state. MM. Fouque & Levy 

 observed no tendency to differentiation, of the kind required to 

 produce petrographical species, in their celebrated synthetical expe- 

 riments. The centres of crystallization were uniformly distributed 

 throughout the masses, which were too viscous to allow of any 

 appreciable movement of the first-formed minerals. Nevertheless, 

 the facts observed by Darwin and others clearly prove that in large 

 masses of lava, even at the surface of the earth, movement of crystals 

 is possible in igneous magmas, and M. Michel Levy himself admits 

 that such movement may become an important factor under certain 

 circumstances. 



Mr. Harker has suggested another way in which crystallization 

 may operate, so as to produce variation in a mass of rock. He has 

 shown that the Carrock-Fell gabbro varies in composition from the 

 centre to the sides, and that, as so frequently happens in eruptive 

 masses, the latter are more basic than the former. He considers 



* that the differentiation took place by diffusion in a fluid magma, but not 

 as a process distinct from and quite anterior to crystallization. It was, 

 as I believe, effected in a quasi-saturated magma, concurrently with the 

 crystallization of the earlier-formed minerals ; . . . . the characteristic of all 

 [such occurrences] is that the several constituents are concentrated in 

 a definite order, which is identical with the order in 

 which they crystallize out from the magma.' 



All theories which depend on diffusion or molecular flow have 

 been criticized by Mr. Becker on the ground that the rate of diffusion 

 is too slow to produce the results attributed to it in any reasonable 

 time. He shows that, in the case of a column of water resting upon 

 a layer of copper-sulphate, the lapse of 1,000,000 years would bo 

 required to produce sensible discoloration at a height of 350 metres, 

 or semis aturation at a height of 84 metres ; and he considers that 

 the molecular flow of any compound in a silicate-magma would 

 probably be at least 50 times less rapid, so that a mass of lava 

 1 cubic kilometre in volume 



' would not have had time to segregate into distinctly different rocks by 

 molecular flow if it had been kept melted since the close of the Archaean 

 period.' 



