308 DE. E. DIXET ON THE [vol. Ixxviii,. 



Sometimes it becomes granular, and its texture then approaches 

 that of a coarse dolerite. The rock is characteristicall}^ banded,, 

 but to a variable degree, and it usually exhibits a parallelism of 

 the felspars which is often obvious even when the banding is 

 obscure. Finally, all minerals are beautifully preserved, in 

 common with those of succeeding intrusions. 



The banding of the norite occurs to a greater or less degree 

 throughout the Peninsula, and is essentially a flow-banding pro- 

 duced during intrusion. Individual bands are generally constant 

 in thickness, and their strike remains uniform over distances o£ 

 several hundred yards ; any contortion of the banding is quite 

 exceptional, but it has been observed in one case around the margin 

 of a pegmatite intrusion. The bands become more obvious with 

 weathering, on account of varying resistance or colour : the colours 

 of weathered bands range through dark-grey, brown, red-brown 

 to almost black ; whereas the slight differences in composition 

 between one band and the next are but faintly indicated on a fresh 

 surface. 



Locally the fluxion -structure is more pronounced ; individual 

 crystals within the bands are broken down and rolled out, and the 

 bands themselves, while keeping a more or less uniform strike, 

 tend to form lenticles which sometimes thin out completeh^ in 

 about half a dozen yards. Some of the bands contain streaks and 

 seams of magnetite, or of magnetite and felspar, ranging up to- 

 3 inches in thickness. Where a marked difference in composition 

 of neighbouring bands occurs, it is due to varjdng proportions of' 

 the felspathic and ferromagnesian constituents, leading in a few 

 extreme instances to bands of coarse granular olivine. The bands, 

 vary in texture as well as in composition ; the coarser of them are 

 generally dark, and they thus bear a close resemblance to the coarse 

 norite which was intruded at a slightly later date (see below, 

 p. 31.0). The darker tinge of the coarse bands is seen in thin 

 section to be due to the abundance of minute opaque inclusions in 

 the felspars, as well as to the greater proportion of ferromagnesian 

 constituents. 



The relation between the banding and the separation of the 

 constituents of the magma is clearly displayed at a point on the 

 foreshore near No. 2 Town. The normal norite assumes a patchy 

 character, due to the separation of the ferromagnesian constituents 

 (chiefly pyroxenes) ; these constituents segregate into numerous- 

 dark clots, up to 2 inches in diameter, which remain enclosed in a 

 pale felspathic matrix. Where movement occurred at a late stage, 

 tlie clots are elongated or drawn out into more or less parallel 

 streaks, and in some cases the structure can be seen to pass into 

 regular banding of the characteristic type. 



The banding of the Sierra-Leone norite is similar to, but 

 better developed than, that observed in the St. David's-Head 

 norite b}^ Dr. J. Y. Elsden.^ A banding with many parallel 



1 ' The St. David's Head Eock Series (Pembrokeskire) ' Q. J. G. S. vol. My 

 (1908) p. 277. 



