PLATE XXVII. 



FIG. 1. 

 PYROXENE-HORNBLENDE-MAGNETITE ROCK. 



SCOUBIE, SUTHERLANDSHIRE. 



Magnified 25 diameters. Drawn with polarizer only. Short axis N. it S. 



The minerals represented are hypersthene (2), diallageor pseudo-hypersthene 

 (7), hornblende (8), iron-ores (5), brown mica (9). 



The structure of the rock is granitic.. The different minerals interfere with 

 each other. Although massive in habit the rock is probably an integral portion 

 of the Hebridean gneissic system with which it is associated. The blue tint in 

 the diallage and pseudo-hypersthene is much exaggerated in the figure. 



FIG 2. 

 FELSPAR-PYROXENE-MAGNETITE-GARNET ROCK. 



SCOURIE, SUTHERLANDSHIRE. 



Magnified 25 diameters. Felspars represented under crossed nicols ; the other 

 minerals as they appear in ordinary light. 



The minerals represented are plagioclase (10), green diallage (7), iron-ores 

 (5), and garnet (20). There is also an undetermined mineral ,r. 



The green diallage is similar to that of the Baltimore gabbro described by 

 Mr. WILLIAMS. This diallage enters largely into the composition of the Hebridean 

 gneissic system of Sutherlandshire. The garnet occurs in zones surrounding 

 the iron-ore. That the mineral in question is garnet and not sphene is proved 

 by the fact that it is perfectly isotropic. 



The rock occurs under the same circumstances as the former. 



