372 TRANSACTIONS OF SECTION C. 



wollastonite in -a roCK must have crystallised below 1190°, pryites below 450", 

 and so for other cases. We may confidently hope that, with the aid of such data, 

 we shall soon be enabled by simple inspection, to lay down in degrees the 

 temperature-range of crystallisation of a given igneous rock. 



There are now several laboratories where high-temperature research, of the 

 rigorous order indicated, is being carried out; but the work is peculiarly arduous, 

 and results come slowly. Some branches of the inquiry, notably those involving 

 high pressures, and again the investigation of systems into which volatile com- 

 ponents enter, are as yet virtually untouched. For these reasons it would be 

 premature to hazard at this stage any more detailed forecast of the services to 

 be rendered to petrology by synthetic experiment. 1 will accordingly leave this 

 attractive subject, and pass on from the laboratory to the field. 



Geographical Distribution of Igneous Bocks. 



Here the existing situation is very different. Instead of following out definite 

 lines already laid down, we are concerned in reducing to order a great mass of 

 discrete facts drawn from many sources. The facts which enter into considera- 

 tion are those touching the distribution of various igneous rocks in time, in 

 space, and in environment, including their relation to tectonic features ; the 

 mutual association of different rock-types and any indications of law in the 

 order of their intrusion or extrusion ; and, in short, all observable relations which 

 may be presumed to have a genetic significance. The digestion of this mass of 

 data has already led to certain generalisations, some of which are accepted by 

 almost all petrologists, while others must be regarded as still on their trial. 



Of the former kind is the conception of petrographical provinces, which was 

 put forward by Professor Judd twenty-five years ago, and has exercised a pro- 

 found influence on the trend of petrological speculation. It is now well estab- 

 lished 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 petrographical 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 thi6 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 community 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 homo- 

 geneous, though doubtless some of the causes which contribute to promote hetero- 

 geneity 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 believing, differentiation in igneous rock-magmas is 

 largely bound up with progressive crystallisation. Those petrologists, 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 

 argument, 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 produce a diversity of ultimate products from a magma origi- 

 nally 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 other geologists, and the subject 



