334 MK. ALFKED HAKKEK ON THE GABBBO [Aug. 1 894, 



If it be granted that the mica in the gabbro is a phenomenon of 

 inverse contact-metamorphism, due to the absorption of a certain 

 portion of the enclosed lava by the gabbro-magma, an interesting 

 conclusion follows. It is clear that if such absorption had taken 

 place soon after the intrusion of the gabbro-magma, when that 

 magma was fluid enough, as we have seen, to permit free diffusion 

 throughout, the brown mica could not have been restricted, as it is, 

 to the immediate neighbourhood of the lava. The action must 

 therefore belong to a later stage, when the magma had attained a 

 considerable degree of viscosity, and must be connected with the 

 growing acidity of the magma in its central region due to concen- 

 tration of the basic constituents in the marginal parts. In point of 

 fact, it is iu the quartz-bearing varieties of the gabbro that these 

 phenomena of inverse metamorphism are observed. 



8. Conclusion. 



The last kind of modification of the gabbro to be noticed is seen 

 near the northern edge of the mass, at Furthergill and westward. 

 On comparing the dense iron-ore gabbros along this strip with 

 those on the southern border of the mass, certain peculiarities are 

 observed which can only be referred to the proximity of the large body 

 of granophyre intruded at a later time, when the gabbro was solid. 



Some of these peculiarities may be considered simply as 

 phenomena of metamorphism produced by the heat of the later 

 intrusion. Thus, instead of the usual uralitic alteration of the 

 augite, we find that mineral passing in a capricious fashion into 

 a compact brown hornblende, which probably indicates some 

 absorption of iron oxide from the ilmenite. Granular sphene has 

 arisen probably from reaction between the ilmenite and the felspar 

 [1866]. The felspar is much broken up into secondary minerals, 

 among which specks of a pale amphibole are conspicuous as well 

 as chloritic substances. Pale fibrous amphibole sometimes forms 

 a fringe in crystallographic relation with augite, but evidently 

 occupying the place of felspar [1525]. There are large patches 

 consisting essentially of matted tremolite-fibres, the origin of 

 which is not clear. With this may be associated a little brown 

 mica [1536], while small patches of this or of a deep brown horn- 

 blende have formed characteristically about some of the grains of 

 iron ore. The various changes observed, or at least some of them, 

 point to thermal metamorphism of the gabbro by the granophyre. 

 (See also PI. XVII. fig. 4, and explanation.) 



The rocks showing the above features may be regarded as the 

 margin of the gabbro proper. They are immediately succeeded by 

 rocks of a very remarkable character, which form a zone running 

 up Furthergill and onward nearly to Round Knott, dividing the 

 gabbro from the granophyre. Along this zone the gabbro has 

 been in great part actually re-fused, and has crystallized again as a 

 rock of strikingly coarse texture, with large idiomorphic felspars 

 and large well-built crystals of hornblende. The granophyre 



