Dr. Johnston- Lavis — Eruptive Rocks of Oran, JSforivay. 253 



of where, with the exception of the MgO and CaO, there has really- 

 been loss or gain. 



Prof. Brogger tells us that the earlier rocks were the more basic, 

 but he hardly gives us any segregation explanation why this should 

 be. The osmotic theory affords very efficient reason. The first part 

 of an acid magma penetrating limestone, or other basic strata, comes 

 in contact with fresh unaltered rock, and can soon become basified ; 

 but in so doing it has formed a practically neutralized lining to 

 the igneous conduit, and consequently the paste that follows will be 

 gradually less and less aifected. This earlier basified rock has partly 

 or entirely cooled — in some cases by a re-extension upwards of the 

 igneous matter — this cooled rock is only cracked and reinjected by 

 the more acid paste that follows, just as Prof. Brogger describes as 

 so common at Gran. In other cases the fissure-filling fluid material 

 at a later date extends and breaks the earlier basic rock, enveloping 

 it just as ray Scandinavian friend describes the bostonites cutting 

 the comptonites and enveloping rounded, and therefore to me partly 

 eaten up, fragments of the latter, and also pieces of pyroxenite which 

 we are told were due to "earlier crystallizations of basic composition 

 in a paste, from which the distinct magma of the bostonite was not 

 yet separated as a final product of differentiation." 



Now how much simpler and more natural is my own explanation 

 of the state of things as given above. Curiously, that very same 

 morning, at Nottingham, when Prof. Brogger read his note, Prof. 

 Sollas brought forward an admirable example of an analogous case 

 of extraneous rock enclosures, and gave the right interpretation of it. 



In the bostonite breccias the included fragments of shale are 

 described by Prof. Brogger as being " surrounded by a basic green 

 mass, partly of comptonitic composition," and we are expected to 

 believe that the cooling eflfects of these fragments caused a more 

 basic segregation on their surface. This phenomenon is of every- 

 day occurrence, and well known to a geologist accustomed to active 

 volcanoes ; but so far from favouring the theory of chemical diffusion 

 and segregation, it is a striking example of the contact metamorphism 

 of a magma by its solid enclosure. As a rule, the smaller the 

 enveloped fragment, the relatively greater is the basic shell around 

 it ; but were the result due to the cooling eff"ect of the enclosure, 

 the larger this was the broader should be the segregation band. 



Certainly the analyses given by Prof. Brogger prove in a most 

 striking manner the acquisition of MgO and CaO of the comptonite 

 paste with the actual or relative loss of FeO, Alg O3 and the alkalies 

 from the bostonite paste. 



Another fact struck me as very remarkable : the two bosses of 

 Braudberget and Solvesberget are smaller and more basic than that 

 of Yiksjeldene, and this again is more basic than its larger rival at 

 Dignaes. This is just what we should expect, that the less the size 

 of the injection the greater would be the relative chemical change 

 produced in it by the surrounding sedimentary rocks. 



I am glad Prof. Brogger confirms what I have always maintained 

 to be a general principle, that the larger the mass of any given 



