PLATE x. 



FIG. I. 

 OLIVINE-DOLERITE. 



TOBERMORY, MULL. 







Magnified 50 diameters. Ordinary light. 



This figure shows augite (7), olivine (1), plagioclase (10), and magnetite. 



Olivine occurs in more or less irregular grains, and is sometimes included in the 

 augite. 



The augite occurs in large ophitic plates. 



The felspar occurs in lath-shaped sections which frequently penetrate and are 

 sometimes completely enclosed in the augite. It is often rendered more or less cloudy 

 by alteration 



Magnetite occurs in crystals and grains. 



The rock is one of the Tertiary dolerites of Scotland. It is identical in structure 

 and composition with the Carboniferous dolerite of Tideswell Dale, in Derbyshire, 

 and closely resembles many of the altered dolerites (diabases) associated with the 

 Lower Palaeozoic rocks of Wales. The last-mentioned rocks are, however, as a general 

 rule more altered, and olivine appears in many cases to have been absent. 



FIG. II. 

 OLIVINE-DOLERITE. 



PORTREE, SKYE. 

 Magnified 50 diameters. Crossed nicols. 



This figure shows the same constituents as the preceding. 



The olivine sometimes occurs as fairly well marked crystals. The two crystals 

 near the centre of the figure give the red of the first order ; whereas the augite in 

 the same portion of the slide gives only the neutral tints of the same order. 



The portion of the slide here figured illustrates the transition from the ophitic type of 

 texture to that characterised by granular augite. The somewhat parallel arrangement 

 of the plagioclase crystals, seen in a certain portion of the . figure, has evidently been 

 determined by differential movement in the mass. In these portions the ophitic plates 

 have been broken up into granules. 



In the left-hand portion of the figure the ophitic character of the augite is still 

 preserved, and in other portions of the same slide the rock is a normal ophitic olivine- 

 dolerite. 



The association of the two types of texture in the same rock-mass may 

 be observed also in the Carboniferous olivine-dolerite of Tideswell Dale. 



