277 



it is often almost destitute of twinning. In the present state of our knowledge 

 it is not safe to regard the untwinned felspar as orthoclase. As a rule the 

 felspar does not show crystalline form. It occurs as grains of tolerably equal 

 dimensions in the different directions. In some of the rocks, however, dis- 

 tinct indications of porphyritic crystals, having more or less definite outline^ 

 occur. These are almost always more or less cloudy in consequence of the 

 presence of vividly polarizing scaly (micaceous) or granular matter. In the 

 foliated varieties they are more or less replaced by a felspar-mosaic. In 

 short, the foliated diorites bear the same relation to the massive diorites, 

 that the foliated gabbros do to the massive gabbros, and the foliated 

 granites to the massive granites. Indeed it is extremely interesting to 

 note how similar are the structural characters, both microscopic and 

 macroscopic, of the foliated representatives of the different types of 

 massive plutonic rocks. The biotite of these rocks is remarkable for 

 its richness of colour (see Fig. 1, Plate XXXII.). It occurs in somewhat 

 irregular plates. The pleochroism is as follows : a pale yellowish 

 brown, /3 and 7 a rich reddish brown. It varies in amount in different 

 specimens, sometimes being almost the only ferro-magnesian mineral 

 present. The hornblende is the green variety. It occurs for the most part in 

 irregular grains. In the more massive rocks which occur as veins in the 

 gabbro and in which no interstitial movement has taken place either 

 during or subsequent to consolidation it is sometimes seen to form ophitic 

 plates containing inclusions and penetrations of felspar. In the foliated rocks 

 this mode of occurrence has not been observed. Sphene is almost invariably 

 present. It occurs as irregular grains and also as exquisitely-formed 

 crystals. In some varieties the characteristic lozenge-shaped sections 

 bounded by faces of the form (123) are tolerably abundant. Notwithstanding 

 the perfection of form in so many of the sections the mineral appears to 

 have been developed, in part at any rate, after the felspar for it contains 

 inclusions of the latter mineral. The irregular masses of sphene bear 

 the same relation to the felspar as those of the diorite or granite of 

 Ballachulish. Iron-ores are very sparingly represented in these rocks. 

 They occur as irregular grains and are seen in some varieties to be 

 surrounded by a narrow border of sphene. 



The structural and mineralogical changes accompanying the plastic 

 deformation of masses of diorite under the stresses involved in mountain 

 making have not as yet been worked out in detail. It will doubtless be 

 found that the diorites like the gabbros and granites pass over into gneissose 

 and schistose rocks. Slides of the quartz-diorite (tonalite) of Mount Adamello 

 illustrate certain phases of dynamic metamorphism in a very perfect manner. 

 The plates of biotite may be seen to have been crumpled and puckered and 

 the crystalline grains of quartz and felspar to have been strained, broken 

 and even reduced to a micro- or crypto-crystalline aggregate. This aggregate 

 winds in and out amongst the larger crystalline particles and sometimes 

 forms a kind of matrix in which these lie embedded. In its relation to the 

 other constituents it exhibits in great perfection what may be aptly termed 

 micro-flaser structure. 



