Vol. 50.] BASIC ERUPTIVE ROCKS OF GRAN. 35 



VII. Conclusions. 



In the preceding pages I have tried to set forth, in a hrief resume, 

 a series of examples illustrating the fact that the compositions of 

 eruptive rocks are sensible functions of the composition of the 

 mother-magma, and of the manner in which this latter has been 

 differentiated during cooling and diminution of pressure. 



A magma of a composition closely allied to that above desig- 

 nated as magma has, so far as I can at present form an 

 opinion, been the oldest product of differentiation from the general 

 magma-reservoir of the sunken tract defined by me as the Chris- 

 tiauia region, this magma-reservoir itself having been, perhaps, a 

 product of differentiation, in yet more remote ages, from a universal 

 earth-magma. Magma was then, if my deductions be correct, 

 the source of all the different basic eruptive rocks in the Christiania 

 region, especially also of the bosses, dykes, and sheets in Gran and 

 the neighbouring parishes, and their equivalents, the effusive basic 

 rocks of the series of augite- and labrador-porphyrites, etc. 



In studying these rocks we have found examples of general 

 relations of some importance : — ■ 



(1) That we can, with great certainty, connect a series of dif- 

 ferent dyke-rocks (camptonites and bostonites) with an exactly 

 defined boss-rock of, per se, different mineralogical and chemical 

 compositions. 



Similar connexions are known from other regions. I need only 

 call to mind the connexion of lamprophyric minettes with granites, 

 often pointed out by our great master in general petrology, Prof. 

 Ilosenbusch, and recently so well described by Marr and Harker. 1 

 But in the present case I think that a connexion of this kind has 

 been more definitely proved than in previously published instances. 



(2) That the dyke-rocks in question — the camptonites and the 

 bostonites — have probably been produced by differentiation in an 

 abyssal magma of a certain chemical composition, which we have tried 

 to calculate exactly from sufficient data. A calculation of this kind 

 has not, so far as the writer is aware, been previously published. 



(3) That the calculated basic mother-magma, or magma 0, 

 has partly consolidated in the bosses in Gran without being differ- 

 entiated, as olivine-gabbro-diabases (type: Solvsberget), and has 

 partly been differentiated into camptonites and bostonites, but 

 partly also into other kinds of rock : i. e. into pyroxenites, horn- 

 blendites, and more acid augite-diorites, etc. VVe have, then, here 

 an example of the remarkable fact that one and the same magma 

 under different conditions has been differentiated in different ways, 

 and separated out so as to form different groups of rocks with different 

 chemical compositions in the individual members of each group : we 

 must above all remember that here is a question not only of dif- 

 ferent mineral-aggregates, but also of different chemical compositions. 



It is thereby proved that the differentiation of a magma depends 

 not only on the given chemical composition, but also essentially on 



1 Quart. Journ. Geol. Soc. vol. xlvii. (1891) p. 266 ; Geol. Mag. for 1S92. p. 199. 



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