part 4] NOEiTE OF sieeea leone. 315 



rock has, like that of the Lizard, incorporated fragments of the 

 gabhro, softened and disintegrated them, and isolated their felspars 

 into clusters and strings. 



The exposures of the Sierra-Leone beerbachite are generally 

 small, and often are onh" a few yards across. Nevertheless, those . 

 which occur on Wilberforce Spur, on Mount Aureol, and in the 

 Congo Valley respectivel}'', corresponding to as many sei^arate in- 

 trusions, are much more extensive than the others, and probably 

 range up to 150 yards in length. The first of these intrusions 

 shows interesting contact-phenomena with the younger norites, 

 while the second displays a wide zone of hybrid rock produced by 

 interaction with the older norite. 



The Wilberforce Spur intrusion crosses an ill-defined junction 

 between the older and younger norites over part of its outcrop, 

 but elsewhere it is confined to the younger norite. It affects the 

 two invaded rocks in the same manner. The beerbachite-magma 

 possessed a great power of corroding and even of assimilating the 

 norite, and all stages in the process from a fairly sharp junction to 

 complete permeation can be observed in the field. The beerbachite 

 contains blocks and fragments of norite, some of them having 

 clean outlines and traversed by veins, others with deeply-indented 

 margins showing where they have been attacked; yet others, 

 more completely altered, are merely ' ghosts,' in that (although 

 their original outlines are still faintly defined) they actually 

 consist of mixtures in which corroded crystals and small fragments 

 of the original rock are enclosed in a mass of beerbachite. This 

 alteration has proceeded on a much larger scale at Mount Aureol, 

 where the intermixture of normal noiite and beerbachite has 

 produced a hybrid rock. This h3'-brid rock is exposed along the 

 sides of the unfinished motor-road, which runs around the flank of 

 the hill from a kind of barrack- square to the Gr.O.C.'s house on 

 the top. The road passes in turn over normal norite, beerbachite, 

 the hj^brid rock, and, finally, near the G.O.C.'s house, normal 

 norite again. The hybrid rock forms an ill-defined zone about 

 150 yards wide, and, although it is better developed on Mount 

 Aureol than in any other known locality, the actual exposures are 

 poor, and consist chiefly of small masses and residual boulders more 

 or less enveloped in laterite. The hj^brid rock is darker than 

 either of the parent rocks ; it usually contains numerous small 

 spots and streaks, most of Avhich are dirty brown to black, while 

 many others are pale grey. The rock is tough, and possesses an 

 irregular fracture. 



The invading magma continued to flow after the enclosed norite- 

 fragments had become plastic and more or less disintegrated ; 

 these fragments were consec|uently so drawn out into streaks that 

 many of their crystals became isolated, or were broken down into 

 * augen.' Specimens collected show strings of augite-fragments- 

 and of white lenticles that represent felspars, and also the attenuated 

 remnants of sponge-like crystals of magnetite (see PI. XVII, fig. 2).. 



Q. J. a. S. No. 312. z 



