PRESIDENTIAL ADDRESS. 375 



loading tectonic features of the globe. It is very noticeable that petrographical 

 provinces, and in particular provinces belonging to opposite branches, are often 

 divided by important orographic lines. This is illustrated by the C'ordilleran 

 chain in both North and South America, and again by some of the principal 

 arcs of the Alpine system in Europe. If now we examine the actual distribution 

 more closely, in the light of Suess's analysis of the continents and oceanic basins, 

 we perceive another relation still more significant. It is that, as regards the 

 younger igneous rocks, the main alkaline and calcic regions correspond to the 

 areas characterised by the Atlantic and Pacific types of coast-line respectively, 

 I briefly drew attention to this correspondence in 1S96, and a few years later 

 Professor Beckc, of Vienna, arrived independently at the same generalisation. 

 Recalling the two classes of crust-movements discriminated by Suess, he says it 

 appears that the alkaline rocks are typically associated with subsidence due to 

 radial contraction of the globe, and the calcic rocks with folding due to lateral 

 compression. The greater part of Becke's memoir is devoted to a comparison of 

 the two branches in respect of chemical composition ; but here, I think, he has 

 been misled by taking as representative of the whole alkaline 'Sippe' or tribe 

 the rocks of one small and peculiar province, that of the Bohemian Mittelgebirge. 

 Some petrologists have followed Becke in adopting the terms Atlantic and 

 Pacific as names, or at least synonyms, for the two branches 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 relatively small part which they play in 

 the visible portion of the earth's surface. We may not unreasonably conjecture, 

 for instance, 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 submerged. 



The generalisation associated with the name of Becke, in so far as it may 

 ultimately commend itself to general acceptance, 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 venture 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 alkaline 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 diversity 

 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 actual 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 Highlands. Conceive an extensive tract to be 

 underlain by a zone which is neither solid nor liquid, but composed 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 dis- 

 turbed, 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 discussed without fuller 

 knowledge concerning the order of crystallisation 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 remarks would not be confined to matters 

 already settled and indisputable. It will be easily understood that some state- 

 ments which I have made, for the sake of clearness, without qualification are 

 subject to exceptions, and exceptions have, indeed, been urged by critics whose 

 opinions are entitled to respect. The most uncompromising of these critics, Dr. 

 Whitman Cross, has laid it down that : ' Only generalisations without knowi. 



