160 SCIENCE PROGRESS. 



large scale. Besides the dominant rocks, there are along 

 the whole stretch of country innumerable dykes and sills. 

 These are of two peculiar rock-types, camptonite and 

 bostonite, the one more basic, the other more acid than the 

 olivine-gabbro-diabase. The two are intimately associated, 

 often in the same dyke-fissure ; but where one cuts the 

 other, the bostonite is invariably the newer. The field- 

 evidence proving that these dykes are genetically connected 

 with the main basic intrusions, Brogger regards the two 

 rock-types as complementary products of differentiation of 

 the original basic magma, and he shows from the analyses 

 that nine parts of camptonite and two of bostonite give 

 almost precisely the mean composition of the olivine- 

 gabbro-diabase. This differentiation must have been 

 effected prior to crystallisation of any importance, as is 

 clear from the striking difference in mineral constitution 

 between the rocks : for instance, the ferro-magnesian 

 minerals in the main intrusions are olivine, pyroxene, and 

 biotite ; in the camptonite dykes, brown hornblende. 

 Camptonites and bostonites are known in other countries, 

 such as Canada, in connection, not with gabbro, but with 

 nepheline-syenite, and we learn that the same rock-types 

 may arise by differentiation from more than one kind of 

 parent-magma. Again, the large masses of olivine-gabbro- 

 diabase are found to pass frequently into special basic rocks, 

 such as pyroxenites, which are traversed by segregation- 

 veins of a more acid rock, akerite or augite-syenite. These 

 types also are regarded as complementary products of 

 differentiation from the original basic magma, and it thus 

 appears that a given parent-magma may be differentiated 

 into partial magmas in more than one way, with different 

 results. 



As an example of differentiation of a totally different 

 kind, we will take the case of certain muscovite-biotite- 

 gneisses in the highlands of Forfarshire, the peculiarities of 

 which have been described by Barrow. 9 The field-relations 

 of these rocks conclusively prove their igneous origin and 

 intrusive nature. They form innumerable thin sills, bands, 

 and veins in rocks highly metamorphosed by them, and 



