PRESIDENTIAL ADDRESS. '6T6 



has always possessed a certain interest in view of the association of most metal- 

 liferous deposits with igneous rocks. It has, however, acquired a new import- 

 ance in recent years in connection with questions of petrogenesis which are 

 still under discussion. The problem is, in brief, to account for the existence of 

 petrographical provinces and for the observed facts relative to their distribution. 

 One theory, advocated especially by Dr. G. F. Becker, invokes primaeval differ- 

 ences in composition between different parts of the globe, which have persisted 

 throughout geological time. It involves the hypothesis that igneous rock-magmas 

 result from the refusion of pre-existing rocks within a limited area. Indeed 

 Becker discards altogether the doctrine of differentiation, and conceives the 

 varied assemblage of rocks in a given province as produced by admixture from a 

 certain number of primitive types. These, he says, should be recognisable by 

 their wide distribution and constant character. It is clear, however, that, on 

 the hypothesis of admixture, the primitive types must be those of extreme com- 

 position. These are, in fact, always the rarest and the most variable, pointing 

 not to admixture but to differentiation as the cause of the diversity. A theory 

 which attributes the special characteristics of petrographical provinces to per- 

 manent heterogeneity in the composition of the globe is difficult to reconcile with 

 the small extent and sharp definition of some strongly characterised provinces, 

 such as that of Assynt or of the Bohemian Mittelgebirge. A more fatal objection 

 is that petrographical provinces are not in fact permanent. A good illustration 

 is afforded by the midland valley of Scotland, an area our knowledge of which 

 has been much enlarged by the recent work of the Geological Survey. It was 

 the theatre of igneous activity in Lower Old Red Sandstone times and again in 

 the Carboniferous, but, in respect of mineralogical and chemical composition, the 

 two suites of rocks present a striking contrast. The Old Red Sandstone lavas 

 are mostly andesites, though ranging from basalts on the one hand to rhyolites 

 on the other, and the associated intrusions are mainly of diorite, quartz-diorite, 

 and granite, with porphyrites and other dyke-rocks. In the Carboniferous, on 

 the other hand, we find porphyritic basalts, mugearites, and trachytes (including 

 phonolitic types), with picrites, teschenites, monchiquites, orthophyres, and other 

 allied rocks. It would be possible to cite many other cases illustrating the same 

 point. 



The Alkaline and Calcic Branches. 



The two Scottish suites of Upper Palseozoic rocks just mentioned fall into 

 opposite categories with reference to what is now becoming recognised as the 

 most fundamental distinction to be made among igneous rocks. The earlier set 

 is typical of the andesitic division and the later of the tephritic ; or, rising other 

 equivalent names, the one belongs to the calcic (or 'alkali-calcic') branch and the 

 other to the alkaline. I will adopt the latter terminology as being generally 

 familiar to petrologists ; but the characteristics of the two branches, which are 

 too well known to need recapitulation here, are more clearly definable in minera- 

 logical than in chemical language. This two-fold division of igneous rocks is, of 

 course, in no wise a final or exhaustive treatment of the subject; but as a first 

 step towards a natural or genetic classification it seems to be established beyond 

 question. No third branch in any degree comparable with the two and distinct 

 from them has been proposed. The charnockites and their allies represent but 

 a single rock-series, and Rosenbusch has not made clesr his reasons for separating 

 them from the calcic rocks. The 'spilitic' suite of Dewey and Flett is made to 

 embrace a somewhat miscellaneous collection of types, and any close genetic 

 relationship among them can scarcely be considered as proved. It is perhaps 

 permissible to suggest that, e.o., the quartz-diabases are, here as in Scotland, 

 quite distinct in their affinities from the types rich in soda. These latter, con- 

 stituting the bulk of the proposed suite, would seem to belong quite naturally to 

 the alkaline branch, the question of the magmatic or solfataric origin of the 

 Eblbite being in this connection immaterial. 



A given petrographical province is either of calcic or of alkaline facies, 

 typical members of the two branches not being found together. The apparent 

 exceptions are, I think, not such as to modify very seriously the general rule. 

 Mr. Thomas, in describing an interesting suite of rocks from Western Pembroke- 

 shire, recognises the alkaline affinities of most of them, but assigns some of the 

 more basic types to the opposite branch. In a very varied assemblage we not 



