part 4] is'OiiiTE of sierra leone. 323 



extinction-angles, is determined mainly as oligoclase ; its twin- 

 lamellse are exceedingly fine. The felspar o£ the micropegmatite 

 is frequently seen to possess twin-lamellse. The enstatite is pale 

 green, and generally non-pleochroic ; it occurs as skeleton-crystals 

 and also as rods and grains, all associated with numerous grains of 

 iron-ore. Some cr3"stals; densely crowded with minute rod-like 

 inclusions, exhibit a slight greenish pleochroism. Certain other 

 vague pyroxenic patches occur in the slide, crowded with grains 

 of iron -ore. These, like most of the ferromagnesian minerals in 

 the gabbro-aplites of Skye,^ are probabl}^ the remains of pyroxenes 

 caught up b}^ the norite-aplite during intrusion. A variety of 

 medium-grained norite-aplite is seen as an inclusion in a dolerite- 

 dyke, which has forced its Avay along the same course as an earlier 

 aplite-vein. This variety is more acid ; it consists chieliy of 

 granular quartz and orthoclase ; an acid plagioclase occurs as well, 

 and generally takes the form of relatively large crystals. The 

 following minerals are also present : — pale-green pyroxene, brown 

 and green hornblende, brown biotite, magnetite, and apatite, all 

 showing much the same characters as in the first variety. The 

 fine extremities of the veins are rich in quartz and micropegmatite, 

 l)ut the other minerals are generally represented in them also. 



(2) The Dolerites. 



The dolerites are not often wdiolly of the ophitic tj^DC ; frequently 

 they show both ophitic and intersertal structure in the same slide, 

 due to the variable habit of the pyroxenes. 



The ophitic structure is developed when the felspars penetrate 

 irregular plates of pale-brown augite ; these augites, commonly 

 about 1/8 inch in diameter, are scattered through the rock in great 

 numbers, and, owing to the numerous grains of iron-ore and small 

 flakes of brown biotite associated with them, they impart to the 

 slice a markedly patch}^ appearance. They are sometimes almost 

 opaque with inclusions. In the remaining parts of the slice, the 

 pj^'oxene is chiefly a colourless to pale-green enstatite, which is 

 slighth^ pleochroic and of granular habit ; grains of this mineral 

 are often involved in the areas of clear felspar interstitial to the 

 common felspars of the rock. 



The finer varieties of dolerite, occurring near chilled margins 

 and in thin dykes, consist of minute laths of felspar, with inter- 

 stitial granules of augite and iron-ore. The coarser types, 

 developed in the large dykes, sometimes contain phenocrvsts of 

 felspar possessing fine twin-striation and occasional signs of cor- 

 rosion [C 125]. In one other variety [C 121], from the middle of 

 a dyke 18 feet wide, the pyroxenes w^ere developed in the form 

 of coarse grains showing a tendency to idiomorphism ; a number 

 of these grains belonged to the rhombic sj^stem, and possessed a 

 pale-brownish pleochroism. This rock is of especial interest, as 

 showing an intermediate stage between dolerite and norite. 



1 ' The Tertiary Igneous Eocks of Skye ' Mem. Geol. Surv. 1904, p. 119. 



