490 



ME. F. HUXLEY OK THE KOCKS 



Carlsbad type is also present, and, judging from the angles of 

 extinction, there is more or less andesine. Apatite crystals are 

 numerous, and magnetite is also plentiful, occasionally in octahedra, 

 hut mostly in irregularly shaped patches. Quartz is plentiful, and 

 it contains great numbers of fluid lacunae which frequently lie in 

 more or less well-defined hands, an arrangement which is probably 

 due to pressure, as pointed out by Prof. Judd in the quartz-grains 

 of certain crushed quartzite-pebbles ; but in this rock the stresses 

 have been exerted in so many different directions, owing to the 

 coarsely crystalline and mixed mineral constitution of the rock, that 

 the bands of lacunae seldom exhibit any approximate parallelism, 

 except in one and the same crystal. The bubbles contained in the 

 liquid of many of these lacunae exhibit spontaneous movement. A 

 bubble in one of the largest lacunae shows this spontaneous motion 

 very perfectly, and the drawing (PL XXI. fig. 8) roughly indicates 

 the course which it followed while watched for about a minute. 

 Occasionally well-formed tabular crystals of specular iron occur in 

 the quartz. The apatite crystals lie within crystals of hornblende 

 and quartz and also within patches of magnetite, and evidently 

 represent the first-formed constituents of the rock. 



PI. XIX. fig. 1 shows the coarsely crystalline character of this rock. 

 On the right, a portion of a large crystal of hornblende is represented, 

 with a few included crystals of apatite ; the remainder of the figure 

 shows portions of felspar- crystals and some interstitial quartz. The 

 rock is a foliated quartz-syenite or hornblendic granite. The paucity 

 of mica, however, hardly entitles it to the latter name. Gneissic 

 quartz-syenite is, perhaps, the most appropriate term to apply to it. 



Xo. 2. North Hill. Largest quarry. North face of the hill. — A 

 rather finely crystalline dark greenish-grey rock, with pale greenish- 

 grey streaks varying from a millimetre to a centimetre in breadth. 

 This banding is sometimes very even and parallel, the rock splitting 

 more or less readily along the pale greenish bands. Tn places the 

 bands are less regular and have a tendency to branch. 



Under the microscope the pale greenish streaks are seen to con- 

 sist in great part of epidote, which has probably resulted from the 

 alteration of hornblende. Some green magnesian mica is also 

 present. The section contains one crystal in which the change of 

 the hornblende has only been partially effected, portions still showing 

 the characteristic cleavage and strong pleochroism. 



Much of the felspar appears, from the extinction-angles, to be 

 labradorite. Quartz is present in irregular patches, and contains 

 numerous fluid lacunae, generally ranged in lines, which, however, 

 do not correspond in direction in the different crystals. The section 

 also shows some good crystals of apatite. There are some portions 

 of the preparation, consisting possibly of cordierite, which have been 

 altered into a mass of minute fibrous crystals, irregularly felted, and 

 showing, although colourless, strong absorption of light. Their 

 extinction indicates that they are rhombic, and it is probable that 

 they are sillimanite or natrolite. Magnetite is present, but only 

 in small quantity. 



