18 MB. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [vol. lxxvi, 



The most important change that can, I think, he referred to 

 the first of these stages, is the complete recrystallization of many 

 of the component minerals. This has resulted in the development 

 of a deep-green strongly-pleochroic hornblende, in idiomorphic 

 crystals remarkably fresh in character, and quite distinct from the 

 pale yellowish-green variety found in the normal granodiorite. 



The felspar, too, has been entirely recrystallized, and occurs in 

 large clear crystals, in which are scattered abundant inclusions of 

 magnetite in quadrate grains, and minute granules and prisms 

 of hornblende. This new felspar appears to have the optical cha- 

 racters of andesine, and we may therefore infer that there has not 

 been much change from the acid labradorite of the original rock. 



In this connexion it should be noted that there is not very much 

 epidote present here, whereas in the bulk of the granodiorite which 

 has been acidified by admixture this mineral is abundant. Biotite 

 in fresh unaltered crystals occurs in association with the hornblende ; 

 the latter, however, is usually idiomorphic to it. It is doubtful 

 whether its origin should be attributed to the first stage of meta- 

 morphism, although it has the appearance of having been entirely 

 recrystallized in common with the felspar and hornblende : for, in 

 the hybrid rock produced by admixture of the normal granodiorite 

 with the residual acid magma, it is seen to become, more and more, 

 the dominant ferromagnesian mineral at the expense of the horn- 

 blende, from which, indeed, it appears in great part to have been 

 formed. Moreover, where the latter rock has been crushed, horn- 

 blende entirely disappears, whereas biotite is persistent, a fact 

 which suggests that its development may have taken place pari 

 passu with the structural change. 



Saussuritization of the outer zones of some of the felspars, 

 together with fracture and dislocation of many of the hornblende- 

 crystals, is a proof that they were formed prior to the development 

 of the earth-stresses, and affords an indication that these were 

 already beginning to make themselves felt. 



From this stage, where no marked structural changes are visible 

 in a hand-specimen, a complete transition can be traced into a fine- 

 grained, intensely -crushed and banded gneiss, in which the effects 

 of dynamic metamorphism have almost completely obliterated the 

 original characters of the rock. 



Before describing this change, however, attention must be 

 directed to the contact-metamorphism of the normal gi'anodiorite 

 by the invading residual magma of highly-acid charactei". As has 

 been mentioned above, it is not easy to distinguish the effects of 

 contact from those of admixture. 



The hybrid rock is characterized by an abundance of free silica, 

 in the form of a coarse quartz-mosaic, of dusty appearance (owing 

 to numerous inclusions), and by the presence of biotite in the form 

 of aggregates of fresh unaltered crystals. 



The felspars vary according to the proportion of the invading 

 magma present in the rock. In acid types they are chiefly alkaline, 



