\):\ 



the elimination of iron and the development of magnetite along the cleavage 

 planes. Mr. AViLi.iAMs refers to a bleaching of biotite in his description 

 of the rocks near Peekskill, X.Y. 



The spiiiellidx, inat/nctite and ilinenite. In the typical peridotites the 

 dunites, saxonites and Iherzolites the minerals of the spinelle-group, 

 chromite, pieotite and pleonaste, commonly occur. The two former are 

 indistinguishable under the microscope, and usually appear a deep brown 

 colour. Sometimes the colour is so intense that the sections appear opaque 

 except at their margins. Pleonaste is distinguished from chromite and 

 pieotite by its green colour when viewed in thin sections. 



The magnetic and titaniferous iron ores occur in the picrites. The latter 

 may frequently be recognised by the characteristic alteration product, 

 leucoxene. 



Fehpar. This mineral is entirely absent from the typical peridotites. 

 Sometimes it occurs sparingly, however, in rocks which approach very closely 

 to these types. In the picrite-group felspar frequently occurs as ari accessory 

 constituent. It usually shows the characteristic lamellar twinning on the 

 albite-plan and sometimes also that of pericline. In all cases it appears, 

 from the high extinction angles, to belong to one of the more basic species ; 

 labradorite, bytownite or anorthite. It may occur in forms Avhich give 

 lath-shaped sections, or as large irregular plates of secondary consolidation 

 (Fig. 2, Plate III.) The felspar of the picrites appears to be peculiarly 

 liable to alteration. In the Tertiary picrites it is usually fresh, but in 

 the palieozoic picrites it is frequently represented by zeolitic pseudomorphs 

 in which analciinc, with its characteristic anomalous double-refraction, may 

 sometimes be recognised. Natrolite also occurs. In the Meiiheniot picrite 

 needles of colourless hornblende occur in the serpentinous pseudomorphs 

 after olivine. It seems probable that the lime necessary for the formation 

 of this secondary hornblende has been supplied by the decomposition of the 

 felspar which unquestionably occurred in the original rock. 



BRITISH PERIDOTITES. 



The rocks which have been so frequently referred to as the typical 

 peridotites, the dunites, saxonites and Iherzolites, are rarely found in Britain 

 in the unaltered form. Ultra-basic rocks are largely developed in the Island 

 of Rum and in the Shiant Isles to the North of Skye. (1) They are here 

 associated with gabbros and dolerites. Some varieties consist almost wholly 

 of olivine enclosing rounded grains of chromite and pieotite ; others consist 

 mainly of olivine, augite and ferriferous enstatite, and must be classed 

 therefore with the Iherzolites. In the majority of cases, however, a certain 

 amount of felspar appears to be present, and such rocks must be termed 

 picrites. 



(i) JUDD Q.J.G.S., Vol. XLL, p. 389. 



