3 2 ° 



NATURE 



September j, ign 



igneous rocks in time, in space, and in environment, in- 

 cluding their relation to tectonic features ; the mutual 

 ition ol different rock-types and any indications of law 

 in the orde: of their intrusion or extrusion; and, in short, 

 all observable relations which may be presumed to h 

 genetic significance, fhe digestion of this mass of data 

 ha alread} led to certain generalisations, some of which 

 i i epted by almost all penologists, while others must be 

 regarded as still on their trial. 



Of ill- formei kind is the conception of petrographical 

 provinces, which was put forward by Prof. Judd twenty-live 

 years ago, and has exercised a profound influence on the 

 trend of penological speculation. It is now well established 

 that we can recognise more or less clearly defined tracts, 

 within which the igneous rocks, belonging to a given period 

 of igneous activity, present a certain community of petro- 

 graphieal characters, traceable through all their diversity 

 or at least obscured only in some of the more extreme 

 members of the assemblage. Further, that a province 

 possessing an individuality of this kind may differ widely in 

 this respect from a neighbouring province of like date; 

 while, on the other hand, a striking similarity may exist 

 between provinces widely separated in situation or in age. 

 It is natural to attribute community of chemical and 

 mineralogical characters among associated rocks to com- 

 munity of origin. The simplest hypothesis is that which 

 supposes all the igneous rocks of a given province to be 

 derived by processes of differentiation from a single parent- 

 magma. This may be conceived, for the sake of simplicity, 

 as initially homogeneous, though doubtless some of the 

 causes which contribute to promote heterogeneity were 

 operative from the earliest stage. Granted this hypothesis, 

 it follows that the points of resemblance among the rocks of 

 a province will indicate the nature of the common parent- 

 magma, while the points of diversity will throw light on the 

 causes of differentiation. The observed sequence in time of 

 the various associated rock-types will also have an evident 

 significance, especially if, as there are good reasons for be- 

 lieving, differentiation in igneous rock-magmas is largely 

 bound up with progressive crystallisation. Those peno- 

 logists, on the other hand, who attach importance to the 

 absorption or " assimilation " of solid rock-matter by molten 

 magmas, are bound to consider both the nature of the 

 chemical variation and the local distribution of the different 

 types with constant reference to the composition of the 

 country-rocks. The balance of opinion, and I think of argu- 

 ment, would assign the variation, at least in the main, to 

 differentiation ; and there are well-known principles, chemical 

 and mechanical, which theoretically must operate to pro- 

 duce a diversity of ultimate products from a magma origin- 

 ally uniform. How far these principles are in practice 

 adequate to the demands which have been made on them, 

 is a question not to be finally resolved without quantitative 

 knowledge which is still a desideratum. Experiment may in 

 time come to our aid. My design to-day is rather to offer 

 some remarks upon a distinct, though allied, problem — viz., 

 that presented by the petrographical provinces themselves. 



The geographical distribution of different kinds of igneous 

 rocks long ago engaged the attention of Humboldt, Boue\ 

 and oth ts, and the subject has always possessed 

 ■ tin interest in view of the association of most metal- 

 liferous deposits with igneous rocks. It has, however, 

 acquired a new importance 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 provim d the observed facts relative 



to their distribution. One theor; illy by 



Dr. G. F. Becker, invokes primaeval differences in com- 

 position between different parts of the globe, which have 

 persisted throughout logical time. It involves the hypo- 

 thesis thai igneous rock-magmas result from the refusion of 

 pre-existing rocks within a limited area. Indeed Becker 

 discards altogether the doctrine of differentiation, and con- 

 ceives th varied assemblage of rocks in a given province as 

 produced by admixture from .1 certain number of primitive 



lys, should be recognisable l>\ their wide 

 distribution and constant character. 1 1 is clear, however, 

 that, on the hypothesis of admixture, the primitive types 

 must be those of extreme composition. These are, in fact, 



uid the most variable, pointing not to 

 admixture but to differentiation as the cause of the diversity. 



A theory which attril thi special characteristics of petro- 



graphical provinces to permanent heterogeneity in the com- 

 position oi the globe is difficult to reconcile with the small 

 extenl and sharp definition of some strongly cl 

 provinces, such as thai of Assynt or of the Bolv 

 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 Carl iferous, but, in respect of mineralogical 



and , li , al ci mposition, 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 por- 

 phyrites and other dyke-rocks. In the Carboniferous, on the 

 other hand, we find porphyritic basalts, mugearites, and 

 trachytes (including phonolitic types), with picrites, tes- 

 chenites, monchiquites, orthophyres, and other allied rocks. 

 It would be possible to cite many other cases illustrating the 

 same point. 



1'ke Alkaline and Calcic Branches. 



The two Scottish suites of Upper Palaeozoic 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, using 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 penologists ; but the characteristics of 

 the two branches, which are too well known to need re- 

 capitulation here, are more clearly definable in mineral- 

 ogical than in chemical language. This twofold division 

 of igneous rocks is, of course, in no wise a final or exhaus- 

 tive 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 clear 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 re- 

 lationship among them can scarcely be considered as proved. 

 It is perhaps permissible to suggest that, e.g. , 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 i: solfataric origin of the albite being in 



this connection immaterial. 



A given petrographical province is either of calcic or of 

 nil. aline faeies, 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 

 \ l tern Pembrokeshire, recognises the alkaline affinities 

 of most of tb us some of the more basic types 



to the opposite branch. In a verj varied assemblag 

 not infrequently men with a ft types which. 



occurring in a calcic province, recall the characters of 

 alkaline rocks, or conversely. Such anomalies have been 

 pointed out by Daly, Whitman Cross, and others. They are 

 found among the later derived types, referable to prolonged 

 "i repeated differentiation, and they are' to be expected 

 especially where the initial magma was not very strongly 

 characterised as either calcic or alkaline. 



Having regard to the known exposures of igneous rocks 

 over lb- land-surface of the globe, it seems that 



there is a very decided preponderance of the calcic over 

 the alkaline branch. This, as we shall see, is probably a 

 fact of real significance, but it is nevertheless noticeable 

 that increasing knowledge tends partly to redress the balance. 

 In our own country, in addition to the Scottish Carboniferous 

 rooks and those probably of Ordovician age in Pembroke- 

 shirr, we have tli' remarkable Lower Palaeozoic intrusions 

 > nt. in Sutherland, of strongly alkaline character, as 



NO. 2184, VOL. 8/] 



