Vol. 50.] BASIC ERUPTIVE ROCKS OF GRAN. 29 



60 per cent, of the rock, while olivine and brown mica are often 

 wanting, and pyroxene is invariably subordinate. 



Wo have consequently in the basic rocks of Gran a remarkable 

 example of the fact that one and the same magma partly without 

 essential differentiation has been pressed up to a higher level, and 

 there has crystallized out as large boss-masses (in the form of 

 olivine-gabbro-diabase), partly has been differentiated at a deeper- 

 seated level into a basic magma (which by its outburst has formed 

 sheets and dykes with porphyritic structure : caraptonites), and into a 

 more acid residuary magma (which in the final eruptions has given 

 rise to sheets and dykes of bostonite). This differentiation (into 

 camptonites and bostonites) has partly also taken place in the dyke 

 and sheet-fissures themselves after passing up into a higher level. 



In order to explain this fact, that one and the same magma has 

 in part been differentiated, in part not, it seems, in my opinion, 

 necessary to assume that in the latter case the essential cooling of the 

 magma has first taken place in the bosses themselves ; while, in the. 

 former case, even before the final pressing-up of the magma, an 

 essential decrease of temperature and pressure along the contact-plane 

 of the magma must have taken place in the magma-reservoir. In 

 this cooling and diminishing of the pressure the magma must have 

 been subject to conditions necessary for producing a tendency to 

 crystallization of the brown basaltic hornblende, although in all 

 probability actual crystallization did not take place. Along the 

 contact-p^ane there must then have been concentrated by diffusion 

 a liquid stratum essentially containing the components of the brown 

 hornblende. This may be deduced from the fact that the composition 

 of the camptonite derived from this differentiated magma differs but 

 slightly from the probable composition of the brown hornblende, l 

 and from the other fact that brown hornblende is actually the 

 essential mineral of the camptonites. These facts then favour the 



1 Tbe analysis of the brown hornblende in the camptonites of Gran is not yet 

 completed. The difference between that and the composition of the camptonite- 

 hornblende published by Hawes is probably unimportant. I refer, therefore, 

 provisionally to his analysis for comparison with the average camptonite- 

 composition : — 



Table VII. 



SiO...... 





Average q 



Analyses 



Basaltic Horn 



39-88 



4'8fi 



14 -S3 



12-60 



12 27 



12-68 



3-39 



ften 



of 



blende. 



Camptonitc-Hornblcnde 



(after Halves). 



40-79 



(not determined) 



17-36 



20-85 



6-97 



10-83 



4-17 (diff.) 



Averar/e of eight 

 Analyses of 



Camptonite, 



TiO., 



Al.,0, 



Fe,0 3 



MgO' 



CaO.... 





4-63 

 16-29 

 14-76 



5-96 

 1016 



K 2 . 



Na„0 . 



::} 



4-55 



100-51 100-97 100-00 



The average of 10 analyses of basaltic hornblende is calculated from those 

 published by Schneider, Zeitschr. f. Krystallogr. u. Min. vol. xriii. (1S91) p. 579. 



