28 



granitoid rocks which he has termed Dimetian. He calls attention 

 to the fact that one and the same plane of inclusions may be traced 

 through two or more grains of quartz ; a phenomenon which is entirely 

 inexplicable on the assumption that the inclusions are original. 



Professor JUDD (1) argues in favour of the secondary origin of the fluid 

 inclusions in the felspar of the gabbro of the Western Isles of Scotland. 



MINERAL INCLUSIONS. Inclusions of one mineral in another are by no 

 means uncommon amongst the constituents of igneous rocks. Thus, apatite 

 is constantly seen to be enclosed in other minerals. Olivine is frequently 

 enclosed in felspar, hornblende and augite. Augite is sometimes seen to be 

 enclosed in felspar, and sometimes the relation of these two minerals is 

 reversed. Augite is often enclosed in leucite. 



In all such cases as those referred to we have direct evidence of the 

 order in which the different constituents of the rock have crystallised. The 

 enclosed mineral is necessarily the earlier of the two. The only exceptions 

 to this rule that can possibly occur are cases in which crystallisation has 

 taken place in a portion of the liquid magma, enclosed in a growing crystal, 

 after the enclosure has been effected. These cases, however, do not present 

 any serious difficulty in actual practice because the enclosed liquid has not 

 a definite composition, and we accordingly find, in addition to the mineral, 

 some other substance ; such, for instance, as glass. A study of the 

 phenomena of mineral inclusions shows very clearly that the minerals of 

 igneous rocks do not necessarily separate in the order of their fusibilities ; a 

 conclusion long ago arrived at by BuNSEN. (2) 



Under the present heading we may refer to those singular inclusions to 

 which PROFESSOR JUDD has recently called special attention, in his paper 

 on the "Tertiary and Older Peridotites of Scotland." < 3 > In the 

 minerals of certain rocks, especially those which there is reason to believe 

 have consolidated at considerable depths and therefore under great pressure, 

 we find minute rods and plates, often exhibiting a more or less definite 

 external form, arranged in planes which correspond to natural faces, or to well 

 marked structural planes of the crystal itself. Thus, in the felspar of the 

 Gabbro of the Cuchullin Hills, Skye, minute black rods and plates are seen 



Fig. 12. Magnified Q'25 diameters. 



(1) On the Tertiary and Older Peridotites of Scotland. Q.J.G.S., vol. XLL, p. 375. 



(2) Z.D.G.G., vol. XIIL, p. 62. 1861 



(3) Q J.G.S., vol. XLL, p. 354. 



