Or THE XALVESX HILLS. 



501 



directions of the lamellae. The orthoclase also occurs in large 

 crystals. One of those present in the section is twinned on the 

 Carlsbad type. The mica is of a green colour by transmitted light 

 and the crystals are much smaller than those of the felspars. The 

 quartz encloses great numbers of fluid-lacunas containing bubbles. 

 These cavities are mostly ranged in lines or streams -winch appear 

 to follow two general directions crossing at a high angle, which, 

 however, varies, as the lines are rather wavy and sometimes converge. 

 By carefully traversing the preparation under the microscope it is 

 seen that the directions of these streams of cavities are tolerably 

 persistent in patches of quartz more or less widely separated, and it 

 is possible .that the}' may be approximately normal to two different 

 directions of stress. The large patches of quartz are seen in 

 polarized light to be made up of smaller patches, each being an 

 individual quartz- crystal ; yet the same stream of cavities will pass 

 unbroken through many of these crystals, until, joining with other 

 streams of cavities, it can no longer be traced, or until it reaches 

 the opposite side of the composite area of quartz. 



None of the component minerals exhibit any definite crystalline 

 forms. iSo apatite is present, but a few granules, apparently of 

 epidote, are visible. 



The rock is a coarse-grained granite or pegmatite. 



No. 23. SwinyarcVs Hill. Highest point of the ridge. — A coarsely 

 crystalline rock composed apparently of dark-green hornblende and 

 pinkish-grey to greyish-white felspar, with a few minute scales of 

 a silvery mica. The chief constituent of the rock, however, appears 

 to be hornblende in large crystals. 



Under the microscope the constituents are seen to be hornblende, 

 triclinic felspar, a colourless mica, epidote, natrolite, and magnetite. 

 Of these, hornblende is by far the most important, constituting 

 probably more than three fourths of the rock. The felspars approxi- 

 mate in their extinction-angles, in some cases to andesine, in others 

 to labradorite ; but the lamellae are often bent. They are by no 

 means numerous, and exhibit no well-defined crystalline form. 

 The mica is colourless or of a very pale greenish tint when viewed 

 in thin section by transmitted light. Both epidote and magnetite 

 occur in small irregularly- shaped grains. The natrolite is chiefly 

 met with in the triclinic felspars, in small prisms which polarize 

 in brilliant colours, and, between crossed nicols, undergo straight 

 extinction. Here and there minute scales of specular iron of a 

 bright orange-red colour may be seen in basal sections of the mica. 

 In these sections only a portion of an hyperbola can be seen in 

 convergent polarized light. 



The rock is a diorite, and was described as such by the late 

 Dr. Holl *. The Eev. J. H. Timins has also described it in his 

 paper f as " containing hornblende, white felspar and silvery mica. 

 The more micaceous and felspathic portion of the rock was- 

 analyzed.'"' The analysis is subjoined. 



* Op. cit. p. 77. Where he refers the felspar to oligoclase or andesine. 

 t Op. cit. pp. 363-364. Analysis lx. 



