PLATE xix. 



FIG. 1. 

 PLAGIOCLASE- PYROXENE ROCK. 



DYKE. NKAH SCOUHIK, SUTHKKLANUSHIKK. 

 Magnified 25 diameters. Ordinary light 



The figures in this and the next plate are intended to illustrate different stages 

 in the metamorphism of a basic igneous rock. The minerals represented are plagioclase 

 (10), pyroxene (7), titaniferous iron ore (o), and hornblende (8). 



The plagioclase is, as a rule, somewhat cloudy, owing to the presence of excessively 

 minute inclusions. It gives broad lath-shaped sections which sometimes penetrate 

 the pyroxene, thus showing that the rock approaches the ophitic dolerites in structure. 



The pyroxene is not deeply coloured. It occurs in grains and irregular plates. 

 In some places it is free from inclusions, and shows only the prismatic cleavages : in 

 other places it is rendered cloudy by minute inclusions in the form of dots, rods or 

 plates. Where the minute dots alone occur they are generally distributed irregularly 

 through the crystal-substance, giving rise to cloudy patches or zones. Where the rods 

 or plates occur we have the structure of diallage or pseudo-hypersthene (see page 29). 



The titaniferous iron-ore appears in the section in the form of irregular plates. 

 Green secondary hornblende is very sparingly represented in this section. 



The microstructure of this rock is essentially that of an igneous product. This 

 is shown by the form of the felspar, and by its relation to the pyroxene. Whether 

 the rock should be called a dolerite or a gabbro is a matter of little importance. The 

 structure (ophitic or sub-ophitic) is that of a dolerite, but the occasional occurrence of 

 diallage serves to link the rock with the gabbros. 



The rocks represented in this and the next plate were taken from a dyke in the 

 Hebridean gneissic system of Sutherlandshire. They are different portions of one and 

 the same rock-mass. 



FIG. II. 

 PLAGIOCLASE-PYROXENE-HORNBLENDE ROCK. 



( I'ltoTKKOUASK . 



Portion of same rock-mass as Fig. [. 

 Magnified 25 diameters. Ordinary light. 



The constituents are the same as in the preceding figure. The secondary 

 hornblende is present in much greater quantity, and the pyroxene in proportionately 

 smaller quantity. The plagioclase is partly cloudy and partly colourless. The lath- 

 shaped form of the sections is less pronounced than in Fig. I. The cloudy portions 

 often preserve more or less the forms of the original felspars. Under crossed nicols 

 they often give evidence of considerable mechanical deformation. The green hornblende 

 has been especially developed round the margins of the pyroxene. The two most 

 important points illustrated in this figure are: (1) the increase in the hornblende 

 relatively to the pyroxene, and (2) the partial obliteration of the original microstructure. 



