ROOKS. 



[ 003 ] 



ROCKS. 



tion between crossed Nicols either coincides 

 with the edge formed by the union of the 

 basal plane and the clinopinakoid, or else 

 lies at right angles to this edge; while 

 in the trichnic system the maximum extinc- 

 tion does not coincide with, or stand 

 at right angles to the homologous edge , 

 formed by the basal plane and the brachy- 

 pinakoid, but occurs in some azimuth on 

 one or the other side of those directions, 

 the angle being determined by rotating the 

 sta^e upon which the preparation lies. To 

 facilitate the recognition of the precise 

 azimuth in which the axes of elasticity lie, 

 an axially-cut plate of quartz, having a 

 thickness of 1-6", is inserted between the 

 object-glass and the analyzer, and the 

 Nicols should be so adjusted that a uni- 

 formly violet light pervades the field. A 

 known crystallographic edge being made to 

 coincide with one of the spider-lines, the 

 section is then turned, until a tint is esta- 

 blished which exactly corresponds with the 

 tint visible in the field of the microscope 

 when the preparation is removed. Similar 

 determinations may also be effected by 

 stauroscopic methods, which are described 

 in the works already cited. These means 

 of ascertaining the position of the axes of 

 elasticity, are of course applicable to any 

 minerals which are translucent when cut in 

 sufficiently thin slices. Tables of the angles 

 of extinction in the different minerals will 

 be found in various works on mineralogy. 



Reverting to our section of granite, we 

 may notice that there is perhaps more than 

 one kind of mica present. One is probably 

 of a more or less deep-brown colour, and 

 the plane of the section may be either 

 parallel with the basal planes of the cry- 

 stals, thus affording six-sided forms, or it 

 may cut through the crystals at some angle 

 to the basal plane. lii the latter case it 

 will be found, that by rapidly rotating the 

 polarizing prism, the analyzer being re- 

 moved, a marked change of colour or tint 

 occurs, while, if the mica be colourless, 

 there will merely be a slight change in the 

 intensity of the light transmitted under 

 similar conditions. The brown, or in hand 

 specimens often black, mica is commonly 

 Biotite, the colourless mica, Muscovite"; 

 but there are many other species of mica, 

 and in order to know which species is pre- I 

 sent, it may be needful to have recourse to j 

 blowpipe analysis or to an examination of 

 cleavage plates, or of sections coincident | 

 with the basal plane, in convergent pola- I 



rized light. For this purpose two or more 

 strongly converging lenses are placed above 

 the polarizer, and are brought just beneath 

 the preparation. A wide-angle half- or 

 quarter-inch object-glass should be used in 

 conjunction with an eyepiece-fitting with- 

 out lenses, surmounted by an analyzer ; or 

 better, the arrangement introduced by 

 Swift, of University Street, Tottenham 

 Court Road ; in either case, the Nicols being 

 accurately crossed. To determine the posi- 

 tive or negative character of the crystal, a 

 quarter-undulation plate, or a quartz-wedge, 

 should be employed; but the manner of 

 using it and the phenomena observed under 

 these conditions must be looked for in 

 books specially devoted to such questions. 

 Again, in a thin section of granite, the 

 observer will also notice the presence of 

 more or less quartz. This will appear 

 colourless and limpid by ordinary trans- 

 mitted illumination, but in plane-polarized 

 light it will exhibit strong colours ; and in 

 convergent light it will, if of suitable thick- 

 ness, show a more or less perfect inter- 

 ference-cross. The rotatory polarization of 

 quartz cannot be observed in ordinary 

 microscopic rock-sections, which are far 

 too thin to permit this phenomenon. 



Quartz, when examined under moderately 

 high powers, is usually seen to contain 

 lacunas partially or wholly filled with fluid, 

 which is very commonly, but not always, 

 water. In the partially filled cavities, 

 bubbles of course exist, the size of which in 

 relation to the size of their containing cavi- 

 ties represents the condensation which the 

 liquid has undergone since its imprison- 

 ment. On the application of heat the 

 bubbles contract. The cavities vary greatly 

 in form ; those shown in PI. 42. fig. 1 are 

 unusually large. The bubbles in these lacunse 

 sometimes exhibit spontaneous motion when 

 high powers are employed. 



PI. 42. fig 10. represents part of a section 

 of Syenite. In this rock, hornblende is 

 one of the essential constituents. In one of 

 the patches of hornblende there figured, the 

 characteristic cleavages are shown, inter- 

 secting at an angle of about 125. These 

 cleavage-planes rim parallel with the faces 

 of the oblique rhombic prism. In augite, 

 the corresponding cleavage-planes intersect 

 approximately at right angles. Hornblende 

 may also usually be distinguished from 

 augite by its dichroism, which becomes 

 evident when the lower Nicol is rotated, the 

 analyzer being removed. Augite shows, as 



