part 4] NOEiTE OF sieeea leone. 389' 



microliths projecting at right angles. The magnetite occurs also 

 as numerous crystallites and clots ; individually these are generally 

 parallel to the cleavages, but they are more closely packed along 

 certain twin-lamellse. All these features are confined principally 

 to the inner portions of the crystals, since there is generally a 

 narrow outer zone free from them; this outer zone points to 

 renewed crystallization (see PL XIX, fig. 4). The normal faint 

 pleochroism of the augite is sometimes increased. A patch}" 

 polarization may be produced, generally in bright colours, but 

 sometimes in grevs. Flakes of brown biotite appear in some cases 

 [C172Z^]. 



(c) H^q^ersthene. — Magnetite thrown down, both as clots and 

 in coarse dendritic form. The hj^persthene is less susceptible to 

 alteration than the augite is. 



(f?) 1 i V i n e. — Deeply emba^^ed outlines are common ; cracks are 

 unusually numerous, and manj^ of them show ferruginous stains. 

 Much serpentine is produced, particularly near the margins of the 

 crystals. In some cases numerous flakes of biotite are formed 

 around the margins [C 170] ; in others, where the olivine has 

 recrystallized in granular form near the edges, small cr3''stals of 

 augite^ and of hjq^ersthene may be included [C 172 &]. Magnetite 

 is frequently tin-own down as clouds of dust or in dendritic 

 fashion. 



(1) B. Alteration of norite by beerbachite. — The most 

 important feature of this alteration is the extensive corrosion of 

 xenocrj^sts ; olivine and h3'persthene, in particular, have been 

 attacked so vigorously that they have been reduced to mere 

 spongy skeletons (see PI. XIX, fig. 3). Augite also has been 

 activel}^ corroded, with deposition of much iron-ore. Where cry- 

 stallization has recommenced, however, the augite-crj'-stals have 

 grown again ; the}^ have thus acquired wide clear zones free from 

 inclusions of magnetite (see PI. XIX, fig. 4). 'No biotite has been 

 detected in this type of alteration. Apart from these features, 

 the contact-action of the beerbachite has been similar to that o£ 

 the younger norites. 



From consideration of numerous xenoliths in the beerbachite^ 

 it appears that the various minerals of the norite were dissolved in 

 the following order : — magnetite, felspar, augite, hypersthene, and, 

 finally, olivine. Large magnetite-xenocrysts are rarely seen — they 

 disappear at an early stage ; numerous small and more or less- 

 rounded xenocrysts of augite may be recognized, and the felspars 

 are in due course reduced to the average size of the beerbachite- 

 felspars ; hypersthene and olivine are the most resistant, and large 

 spongy masses of these minerals, with their rounded pores occupied 



^ Prof. C. Doelter lias shown that the formation of augite from oHvine and 

 felspar is a reversible action; see A. Harker, 'The Natural History of Igneous 

 Eocks' 1909, p. 168. 



