-B40 DR. F. DiXEY OX THE [vol. Ixxviii, 



by gi'aniiles of felspar and augite, are a common feature of the 

 beerbachite near its contact with the norite. This interesting 

 snceession corresponds fairly well with the order of fusibility 

 of these minerals as determined experimentally,^ and may be 

 •compared Avith similar i-esiilts obtained by Pi-of . Lacroix - from 

 observations on the behaviour of olivine-nodules and other basic 

 fragments enclosed in coarse basic dolerites. 



Lacroix's observatirins on the manner in which the individual 

 minerals of basic xenoliths are affected by inclusion in a hio-hly 

 heated basic magma do not differ in any important respect from 

 those given above ; the chief differences noted in his work are as 

 follows : — 



(a) Felspar. — Micrographic intergrowths of felspar and aiigite are not 

 developed ; these minerals recrystallized in granular form. 



(/3) Hypersthene. — In general, this mineral was conrerted into granular 

 augite ; but sometimes it recrystallized into both augite and olivine. 



()') Olivine. — Commonly converted into granular augite and magnetite, 

 or into granular olivine; no biotite was developed. A notable 

 feature in the alteration of this mineral was that it frequently 

 recrystallized into a form quite unlike that habitual to it : it assumed 

 the habit of long-bladed crystallites which, elongated in the direction 

 of the C-axis, locally gTew parallel one to the other so as to form 

 the skeleton of a large crystal of olivine. 



These differences, particularly the gi-eater frequency of fine 

 granular forms in the minerals described by Lacroix, may be 

 ascribed to the fact that the xenoliths studied by this author had 

 been incorporated in a magma crystallizing under volcanic or at 

 most hypabyssal conditions ; whereas those considered in the pre- 

 sent paper were attacked by magmas subjected wholly to plutonic 

 ■conditions. Further, the phenomena observed in the Sierra-Leone 

 rocks took place on a much larger scale. 



(2) A. Alteration of norite by norite-aplite. — The 

 zone of alteration produced in the norite by a narroAV yein of 

 norite-aplite is generally many times wider than the vein itself. 

 The zone may be regarded as consisting of an inner zone, in which 

 the alteration is most intense, and an outer zone, with less 

 intense alteration. The minerals of the two zones are affected as 

 follows : — 



Felspar: inner zone. — The crystals are rendered clearer; 

 this is probably due to loss of magnetite-microliths, to the intro- 

 duction of silica and orthoclase. and also to a certain amount of 

 recrystallization. Further, the twin-lamellse of the felspars become 

 yague or even indistinguishable, the outlines of the crystals become 

 sinuous, and a carious patchy extinction is developed. The refrac- 

 tive index of the felspar does not, however, fall below that of 

 Canada balsam. Numerous shreds of hornblende are developed. 



^ A. Harker, ' The Xatural History of Igneous Eocks ' 1909, p. 156. 

 - ' Les Enclaves des Roches Yolcaniques ' Ann. Acad. Macon, ser. 2. vol. x 

 <1893) pp. 483 et seqq. 



