338 DE. F. DIXEY OX Tli£ [vol. Ixxviii,. 



of xenocrysts. Tlie results obtained confirm the remarks made above as to 

 the importance of assimilation in certain of the Sierra-Leone intmsives, and 

 are. moreover, of especial interest in that they reveal the mechanism by which 

 an intrusion may, in certain cases, absorb large quantities of the rock which, 

 it invades. No xenocrysts have been found, however, in the norite-aplite and 

 the dolerites ; consequently, alteration due to these intrusions has to be 

 looked for in those portions of the norite which are near the contacts. 



(6) Character. — The intrusions of the second group, particularly the 

 norite-aplite, were relatively more acid in composition than those of the fijst 

 group. Hence the contact-effects of the second group are locally due to the 

 infusion of new material, even more than to the application of heat ; this has 

 resulted near the margins of these intrusions in a more intense alteration,, 

 combined with the development of new minerals in the norite, especially 

 hornblende and biotite. The alteration produced by the younger norite and 

 the beerbachite, on the other hand, may be ascribed almost entii'ely to the- 

 effect of heat, since any infusion that might have taken place could not have 

 altered the composition of the invaded rock very appreciably. 



(1) A. Alteration of norite bv voiinger norite. — The 

 Yarioiis minerals of the normal norite have been affected as 

 follows : — 



(a) Felspar. — This mineral shows corrosion and recrystal- 

 lization, distortion and irregularitr of twin-lamellse; and also a 

 patchy, irregular extinction. Cleavage and other cracks are abun- 

 dant. The magnetite-microliths of the felspar are sometimes 

 replaced by exceedingly minute, pale-green, rounded grains, and 

 m.uch micrographic felspar and augite are developed, both fine and 

 coarse. The coarse micrographic augite is enclosed in clear felspar,. 

 in Avhicli twin-lamellse may sometimes be distinguished. A string 

 of augite-granules, associated with a little magnetite and biotite, 

 frequently runs along the contact between adjacent felspars, and on- 

 each side of the string is a narrow zone of clear recrystallized 

 felspar. The fine variety of intergrowth generally originates at 

 the contact of a ferromagnesian mineral with felspar, and then 

 spreads fanwise into the felspar. It should be noticed that the 

 above two intergrowths of felspar and augite occur also in the 

 unaltered norite (see p. 332), in which the coarse variety can 

 frecj[uently be demonstrated to be of primary origin ; whereas- 

 the fine variety is often secondary (that is. formed shortly after 

 the main period of consolidation of the rock). In the thennally- 

 altered rock, however, these structures are abundantly and charac- 

 teristically developed, probably because the gentle heat- gradient 

 set up at comparatively low temperatures gave the minerals a 

 better opportunity for growing together in graphic fashion than 

 thev had had in the cooling of the main bodv. 



o 



(b) Augite. — Outlines are embayed and irregular; there are 

 lines of granules of felspar and magnetite apparently due to 

 crystallization along cracks and in pores ^C 120_. A granular recry- 

 stallization of the augite may occur. Magnetite is thrown down 

 in abundance ; much of it takes the form of long narrow lenticles 

 generally parallel to the cleavage, with numerous thread-hke 



