Becker — Fractional Crystallization of Rocks. 261 



denies the applicability of the Ludwig-Soret law. In this he 

 seems to me correct, but I fail to see that he gives adequate 

 reasons for the rejection. He resorts to the separation of 

 magmas into immiscible fractions for a working hypothesis, 

 but without showing how the necessary variations of tempera- 

 ture are to be accounted for. Mr. Alfred Harker* also regards 

 the Ludwig-Soret law as inapplicable to magmatic segregation, 

 which he seeks to explain by the molecular flow attendant 

 upon crystallization. The maximum rate of molecular flow is 

 thus provided for, but I have shown that even under these 

 most favorable circumstances the time required for the separa- 

 tion of considerable masses of material from one another 

 would be practically infinite in any solutions of known prop- 

 erties. Mr. Michel-Levy again, whose researches in physics 

 give his opinions on the segregation of magmas the greatest 

 weight, has reviewed the hypotheses of Messrs. Brogger and 

 Iddings. He points out the enormous time required for the 

 process and, as others have done, the impeding influence of 

 viscosity. The results of experiment, he thinks, are more favor- 

 able to the old theory of superposition of magmas in the order 

 of decreasing density. He finds many objections both to the 

 hypotheses and to the evidence in their favor, and the only 

 point which he regards as certain is that there are some con- 

 sanguineous rocks. These, he thinks, probably came from a 

 reservoir in which the initial magma has undergone only such 

 modifications as were consistent with the preservation of its 

 distinct individuality. f It seems needless to enlarge further 

 on the unsatisfactory condition of the theory of differentiation. 

 On the othe'r hand, the simple principle of fractional crystal- 

 lization, which is the very opposite of magmatic differentiation, 

 is in most respects thoroughly well understood, it is known to 

 be practicable by hundreds of thousands of experiments, many 

 of them on a fairly large scale, and its action is so rapid as to 

 bring about in days diversities of composition which it would 

 take centuries to bring about by processes depending on molec- 

 ular flow. In dikes and laccolites of mobile lavas fractional 

 crystallization seems inevitable, while the convection attend- 

 ing it is inconsistent with segregation by molecular flow. 

 Surely it is worth the while of lithologists to consider in how 

 far differences in such rocks as are beyond a doubt genetically 

 connected can be accounted for by a process which is almost 

 inseparable from consolidation. 



Washington, D. C, June, 1897. 



* Quart. Jour. Geol. Soc. London, vol. 1, 1894, p. 311. 



fBull. Soc. Geol. de France (3), vol. xxiv, 1896, p. 123. I should have been 

 glad to reinforce some of the reasoning in a paper printed in this Journal, vol. hi, 

 p. 21, by reference to Mr. Michel-Levy's paper cited above ; but it did not come 

 under my eyes in time. 



