22 Prof. S. J. Shand — The Norite of the Sierra Leone. 



norite given by Giirich, but wbere the whole store of observations is 

 so meagre any addition to it ought to be useful. It is desirable in 

 any case that records relating to British colonies should be accessible 

 in the English language. 



The rock, as exposed about Freetown and on Wilberforce Hill, 

 shows the following textural facies. 



1. A coarse-grained variety, sometimes forming definite pegmatite 

 veins, composed mainly of felspar plates in sub-parallel arrangement. 

 Some of the crystals are nearly an inch in diameter, and their colour 

 is iron-greJ^ The coarser the grain of the rock, the smaller the 

 proportion of heavy minerals, which may sink to 5 per cent or less. 



2. A medium-grained variety which may be called the average 

 rock, in which the felspar crystals have diameters of 2 to 4 mm. 

 The arrangement of the crystals is still sub-parallel. Olivine grains 

 are prominent, and the heavy (or mafic) minerals constitute about 

 25 per cent of the volume of the rock. 



3. A fine-grained variety in which the crystals are equidimensional 

 and have an average diameter of less than 0'5 mm. The texture is 

 similar to that of an aplite, or what is sometimes called a " trap- 

 granulite ". Heavy minerals make up some 60 percent of the whole. 

 I believe this facies of the rock to bear the same relation to the last 

 that an aplite bears to a granite. 



The following minerals are present : plagioclase, olivine, hyper- 

 sthene, diallage, titan omagnetite. 



The plagioclase is entirely fresh and glassy, and invariably dark- 

 grey in colour. In general it forms pinacoidal tables with Carlsbad 

 and albite twinning, but in the aplitic rock it appears in anhedral 

 grains. The extinction angles show it to be an acid labradorite, 

 approaching Abg An^. No zonal structure could be detected. 



Olivine is an important constituent of the average rock, but is 

 entirely absent from the aplitic facies. It is perfectly fresh, or 

 shows only incipient hydration along cracks. The crystals are 

 moulded upon the felspars, and are sometimes mere skeletons 

 embracing numerous felspar laths. Giirich has illustrated this 

 feature in his paper. Olivine is commonly intergrown with diallage 

 and may be enclosed by hypersthene. Some varieties of the rock 

 contain little but felspar and olivine. 



Diallage of a smoky-brown colour is the common pyroxene in the 

 coarser varieties of the rock. It is charged with opaque ore- 

 inclusions which lie along planes of parting. It forms skeleton- 

 crystals surrounding felspars, and rounded grains of diallage are 

 often completely enclosed in hypersthene. 



Hypersthene is the only ferromagnesian mineral present in the 

 aplitic facies of the rock, but in the average rock it is associated 

 with, and generally subordinate to, diallage. In the former case the 

 hypersthene grains are equidimensional, and they form an even- 

 gi'ained aggregate with anhedral felspar grains of the same size. 

 Even here, however, the hypersthene has demonstrably been moulded 

 on the felspar. In the coarser varieties of the norite the hypersthene 

 occurs in anhedral plates which enclose all the other minerals of the 

 rock. The pleochroism is strong, indicating a high ii'ou-content. 



