52 PKOCEEDINaS OF THE GEOLOGICAL SOCIETY. 



tals often associated with kaolin, but differing from it not only in 

 form, but also in having a much more powerful depolarizing action. 

 They are well seen in the pipeclay of Kingsteignton, and occur in 

 greater or less numbers in most fine-grained clays. 



Identification of some Minerals. 



In distinguishing from one another different Jiinds of small 

 laminar or prismatic crystals, I have found it most valuable, and, 

 indeed, indispensable, to make use of the quartz wedge inserted 

 into the eye-piece like a micrometer described by me in a paper 

 read before the Eoyal Microscopical Society*. Minute portions 

 of minerals which appear identical, when examined with a simple 

 polarizing microscope, may often be distinguished by it with great 

 ease. Thus, for example, if we have a more or less transverse 

 section of a laminar mineral like mica, and rotate it in polarized 

 light, at two different azimuths at right angles to one another it 

 does not depolarize. These directions may be very conveniently 

 called the " axes of depolarization." They are not exactly the 

 axes of elasticity, but are the direction along which these axes 

 intersect the section in the line of vision. Arranging the section 

 on the stage of the microscope so that one of these axes is at an 

 angle of 45° to the plane of polarization, we, of course, see to the 

 greatest advantage the colours due to depolarization. Pushing 

 the quartz wedge into the eyepiece, with the axis also at 45° to the 

 plane of polarization, the tints given by the object under examina- 

 tion are raised if its positive axis be parallel to it, but lowered 

 when it is the negative axis which is parallel. In this latter 

 case, when the part of the wedge which gives the same tint as the 

 section is over it, the object appears black. If the depolarization 

 is extremely feeble, it is difficult to see this, because the object 

 must be observed so close to the extreme thin edge of the plate. 

 It is then much better to notice whether the object lowers the blue 

 of the second order to the red-orange of the first order, or raises 

 the latter to the former. In the case of mica cut more or less 

 transversely we can thus easily see that the negative axis of depo- 

 larization is invariably at right angles to the laminas, whereas in 

 some other analogous minerals this axis is positive. In the case of 

 some prismatic or fibrous minerals, like hornblende, the positive axis 

 appears to be somewhat variably inclined to the line of the fibres. 

 This is because the true axis of elasticity is not parallel to the 

 axis of the prism, and thus the result depends partly on the 

 manner in which the crystal happens to be turned on its axis in 

 the plane of the section. This alone is a very important fact ; but 

 we should also observe v\^hether, when the axis of depolarization is 

 parallel to the fibre, it is negative or positive. Unlike hornblende or 

 transverse sections of mica, small prismatic felspar microliths 

 have their negative axis more or less nearly in the line of their 

 length. The angle at which it is inclined differs in different kinds 



* Monthly Microscop. Journ. 1877, Tol. xyiii. p. 209. 



