392 



SIR A. GEIKIE ON THE TERTIARr 



[May 1896, 



all sizes from mere grains up to blocks a foot or more in length. 

 They are generally angular, like rock-chips from a quarry. The 

 granophyre here and there assumes a darker or greener tint, as if 

 it had dissolved and absorbed some portion of the older rock. 



Fig. 28. — Pale granophyre injected into dark basalt. 

 [From a photograph by Col. Evans.] 



St. Kilda. 





1 .j '^hmp-j 



Though closer in grain where it comes in contact with the gabbro, 

 the granophyre never assumes any vitreous or distinctly spherulitic 

 textures along its margin. A series of thin slices prepared from 

 my specimens has been examined for me by Mr. Harker, who has 

 furnished the following notes regarding them : — ' The basalt traversed 

 by the granophyre is a fine-textured variety, with small porphyritic 

 felspars. These latter seem to be usually unaltered, retaining the 

 glass-cavities which in some of the crystals are abundant. The 

 groundmass, however, shows minerals of metamorphic origin which 

 must be derived mainly from the original augite. A brown mica is 

 the most conspicuous : but with it are associated some brownish- 

 green hornblende and certain chloritic and perhaps serpentinous 

 substances. It is chiefly near the margin of a fragment of basalt 

 that the mica gives place to these minerals. The basalt still retains 

 plenty of unaltered granules of augite in the central parts of a 

 fragment. It is not certain that the secondary minerals named 

 come exclusively from the augite of the basalt ; judging from their 

 form and mode of occurrence I should say that they may in part 

 have replaced olivine or even rhombic pyroxene. 



