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SCIENCE 



[N. S. Vol. XXXIV. No. 873 



terms Atlantic and Pacific as names, or at 

 least synon3'ms, for the two brauehes of 

 igneous rocks. Others, perhaps with some 

 justice, deprecate the use of the same terms 

 in a petrographical as well as a tectonic 

 sense, so long as the implied relationship is 

 still a matter of discussion. 



I would point out in passing that the 

 association of the alkaline rocks with areas 

 of subsidence helps to explain the rela- 

 tively small part which they play in the 

 visible portion of the earth's surface. We 

 may not unreasonably conjecture, for in- 

 stance, that the volcanic islands scattered 

 sparingly over the face of the Atlantic 

 Ocean, from the Azores to Tristan 

 d'Acunha, are merely fragments of a very 

 extensive tract of alkaline rocks now sub- 

 merged. 



The generalization associated with the 

 name of Becke, in so far as it may ulti- 

 mately commend itself to general accept- 

 ance, must have an important bearing on 

 the problem of the origin of petrographical 

 differences. The time is not ripe for any 

 dogmatic pronouncement, but I will ven- 

 ture to indicate briefly the general trend 

 of the inferences to be drawn. It seems 

 clear that only a trivial effect at most can 

 be allowed to original and permanent 

 heterogeneity of the earth's crust, or to 

 such accidents as the absorption by an 

 igneous magma of a limited amount of the 

 country-rock. The division between alka- 

 line and calcic regions, and the separation 

 of distinct provinces within such regions, 

 point rather to the same general cause 

 which, at a later stage, produced the diver- 

 sity of rock-types within a single province, 

 that is to magmatic differentiation. Here, 

 however, the differentiation postulated 

 must be on a very wide scale, and must 

 take effect in the horizontal direction. Its 

 close connection with crust-movements 

 clearly indicates' differential stress as an 



essential element in the process. The ac- 

 tual mechanism can be at present only a 

 matter of speculation, but I think the clue 

 will be found in such observations as those 

 of Mr. Barrow on the pegmatites of the 

 Scottish tlighlands. Conceive an exten- 

 sive tract to be underlain by a zone 

 which is neither solid nor liquid, but com- 

 posed of crystals with an interstitial fluid 

 magma. If this be subjected to different 

 pressures in different parts of its horizontal 

 extent, its uniformity will necessarily be 

 disturbed, the fluid portion being squeezed 

 out at places of higher pressure and driven 

 to places of lower pressure. The precise 

 nature of the differentiation thus set up 

 will depend on the relative compositions of 

 the crystalline and fluid portions, and the 

 subject could not be very profitably dis- 

 cussed without fuller knowledge concern- 

 ing the order of crystallization in rock- 

 magmas. Whether or not the explanation 

 be ultimately found in this direction, the 

 relation between the two tectonic types and 

 the two branches of igneous rocks must, I 

 think, find a place in the final solution of 

 the problem. 



I intimated at the outset that my re- 

 marks would not be confined to matters al- 

 ready settled and indisputable. It will be 

 easily understood that some statements 

 which I have made, for the sake of clear- 

 ness, without qualification are subject to ex- 

 ceptions, and exceptions have, indeed, been 

 urged by critics whose opinions are en- 

 titled to respect. The most uncompromis- 

 ing of these critics. Dr. Whitman Cross, 

 has laid it down that: "Only generaliza- 

 tions without known exceptions in experi- 

 ence can be applied to the construction of 

 a system that may be called natural." I 

 hold, on the contrary, that such a science 

 as geology can be advanced only by the in- 

 ductive method, which implies provisional 

 hypotheses and successive approximations 



