PLATE xiv. 



FELSPAR -AUGITE ROCK. 



SOUTH DYKE IN SHANKHOUSE PIT, NORTHUMBERLAND. 

 Magnified 50 diameters. Fig. I. ordinary light ; fig. II. with crossed nicols. 



The substances represented are plagioclase (10), augite (7), magnetite and 

 interstitial matter. 



The felspar occurs in elongated lath-shaped sections, often showing ragged 

 terminations. The individuals are usually twinned. An examination of fig. II. will 

 show that the felspar must have crystallised before the augite. 



The augite is nearly colourless in thin section. It occurs in irregular grains, 

 sometimes showing a tendency to elongation in one direction. 



The magnetite is wholly present in the interstitial matter where it occurs as 

 minute crystals and rods. 



When the interstitial matter is examined with a high power it is seen to consist 

 of a base of perfectly colourless isotropic glass (the acid residuum of the magma,) 

 in which innumerable minute spherical bodies (globulites) and crystals and rods of 

 magnetite are scattered. 



The yellowish brown patches seen in fig. I. appear to represent portions of the 

 interstitial matter which have been more or less altered. The dyke is intrusive in the 

 coal measures of the district. Its precise age is unknown. 



