29 



to occur along one, two, three, four, or even more planes. Where five planes 

 of inclusions occur they correspond to the macropinacoid, the brachypinacoid, 

 the two prism faces, and the basal plane. Sometimes the dimensions of the 

 inclusions are such as to enable them to be observed with low powers ; at 

 other times they can only be resolved by the use of the highest powers ; and 

 in some cases there is reason to believe that they are ultra-microscopic. 

 The play of colours, so characteristic of the well-known labradorite, from 

 Labrador, is due to the presence of these minute inclusions. 



The pyroxenes, both monoclinic and rhombic, sometimes contain 

 inclusions which remind one, so far as their arrangement is concerned, of 

 those of the felspar, already described. (See figs. 13 and 14.) 



Fig. 13. Magnified 75 diameters. Fig. 14. Magnified 225 diameters. 



Fig. 13. Inclusions in augite, from olivine-gabbro, Loch Coruiskh, Skye. The inclusions 

 appear to have been developed along lines of cracks. 



Fig. 14. Pseudo-hypersthene, from olivine-gabbro, Loch Coruiskh. Two sets of inclusions 

 lying in planes at right angles to each other, are well marked. A third set, probably 

 inclined to the plane of the section, is also distinctly represented. 



It is difficult to determine the nature of these inclusions. They are 

 regarded by some observers as crystals of hematite, magnetite, brookite, 

 augite, or other minerals; and by others, including PROFESSOR JUDD, as 

 mixtures of various oxides, in a more or less hydrated condition ; such as 

 hyalite, opal, gothite, and limonite. According to the latter view their 

 definite form is explained by the assumption that they are negative crystals. 

 It is the presence of these inclusions along definite planes which gives the 

 " schiller," or sheen, so characteristic of diallage, pseudo-hypersthene, 

 bronzite, and hypersthene (paulite). In diallage the inclusions are developed 

 along one set of planes, parallel, or approximately parallel, to the ortho- 

 pinacoid. In pseudo-hypersthene (Dana), they are developed along two or 

 more sets. Bronzite and hypersthene proper (paulite) are the "schiller" 

 varieties of the rhombic pyroxenes which correspond respectively to 

 diallage and pseudo-hypersthene of the monoclinic group. 



The olivines which occur associated with felspars and pyroxenes having 

 the characters above described also exhibit peculiarities which differentiate 

 them from the olivines of volcanic rocks. They contain peculiar and often 



