528 "DE. F. DIXET OX THE [vol. IxxVlii, 



ISTormally. the olivine is remarkably fresh ; aj)art from j^^le- 



. green and vellow'-green serpentine, the most important alteration- 

 product is iddingsite. This mineral replaces the olivine first along 



■tracks and then in the intervening spaces, nntil it forms large 

 plates, varying in colour from deep brownish-red to yellow and 

 green. It has strong double refraction, and is distinctly pleochroic, 

 the colours ranging from red-brown to a yellowish tint, vibrating 

 respectively parallel with, and at right angles to, the short axis of 



-the polarizer. In the hand-specimen it has a good cleavage in one 

 direction ; the iiakes, which can be separated with a knife, are 

 somewhat brittle. It occurs most commonh" in norite relatively 



irich in iron. The mineral bears a close resemblance to that 



described by Prof. AV. S. Boulton from an intrusion of monchiquite 

 in the Old Red Sandstone of Monmouthshire.^ Well-marked 



.reaction-rims are sometimes developed between the olivine and 

 the felspar. The olivine frequently encloses beautiful dendritic 



: magnetite. 



Iron- Ores. — The chemical analyses of the norite indicate that 

 the iron-ore contains a considerable percentage of titanium ; 

 examination of thin sections shows, however, that the amount of 

 titanium varies in different specimens of the ore. Lateritic iron- 

 ores derived from, the norite have been found to contain as much 

 as 20 per cent, of titanium dioxide. The ore apparently consists 

 of a mechanical mixture of magnetite and ilmenite ; for, when 

 ,a saiuple is powdered, the two minerals can readily be separated 

 by means of a magnet. ~ It is thus a titaniferous m.agnetite or 

 * titanomagnetite.' The luagnetite (titanomagnetite) is present 

 in all phases of the norite, but it is not always developed to 

 the same extent. It occurs generally as irregular grains, some 

 ■ of which are rounded while others are more or less idiomorphic. 

 Tt is often interstitial to other minerals of the rock, showino- that 

 its crvstallization extended to a A'erv late stas^e ; on the other 

 '.hand it not infrecjuently forms, as in the Skye gabbros, large plates 

 in which all other minerals are embedded.-^ It enters into intimate 

 intergrowth with other minerals (see below, p. 332), and even 

 : appears in some cases to be replacing them ; similar relations 

 have been noted in the Dulutli gabbro, concerning which M. L. 

 IsTebel states that 



' Magnetite is often later than angite and olivine, as well as plagioclase, as 

 it is found surrounding and penetrating tliem in stich a manner as to suggest 

 corrosion of the older mineral and partial replacement by the magnetite 

 (pi. xiii, c): Econ. Geol. vol. siv (1919) p. 372. 



^ ' On a Monchiqmte Intrasion in the Old Eed Sandstone of Monmouthshire ' 

 • Q. J. G. S. vol. Ixrii (1911) p. 472. 



- See J. H. L. Yogt. ' On Labradorite-Xorite with Porphyritic Labradorite- 

 ' Crystals, &c.' Q. J. G. S.toI. Ixv (1909) p. 87 : and also'j. J. H. TealL ' The 

 •Geology of the Glasgow District ' Mem. Geol. Surr. Scot. 1911. p. 128. 



^ A. Geikie & J, J. H. Teall, ' On the Banded Structure of some Tertiary 

 'Gabbros in the Isle of Skye' Q. J. G. S. vol. 1 (1894) pi. xxviii. 



