THE FELSPARS. 



to observe that when they do not become dark at the same time the 

 two parts do not extinguish when the dividing line makes equal angles 

 on each side with the principal sections of the Nicols. 



We shall often find in these sections a very indefinite extinction — 

 i.e., the whole surface does not become equally dark all over at any 

 possible position of the section, but takes a curious mottled look. The 

 cause of this is not certainly known-, but it has been suggested that it 

 is owing to the crystal being compound instead of simple, built up of 

 an extremely large number of very minute ones of another of the 

 group, a potash felspar, but differing from Orthoclase in some impor- 

 tant particulars. 



Now we shall probably soon find among the colourless, but often 

 rather cloudy grains, which we are inclined, at the first glance, to 

 take for the mineral we have been studying, some which, under the 

 searching test of polarised light, show themselves to be made up of a 

 vei'y large number of fine bands, alternately light and dark, or if the 

 section is thick enough, alternately differently coloured. On rotating 

 this section between crossed prisms alternate bands will be found to 

 extingui-^h together and at a considerable angle at times from the 

 position of extinction of the other ones, and this, even when the lines 

 of junction make at the extinctions equal angles with the principal 

 planes of the prisms. This shows that we have to do with a mineral 

 which is not symmetrical (physically) with regard to the plane of 

 composition of the twinning ; in fact it will be found that we have 

 here a crystal of what is called the Trlcliiiic system, in which, that is, 

 the three axes are all inclined to each other, in which, therefore, no 

 plane can be found wliichwill divide the form into two exactly similar 

 parts — no plane of symmetry, in fact. If the block of which I spoke 

 earlier had not had any of its edges rectangular, it would be seen that 

 it was impossible to find any position in which there was not an 

 apparent break between the object and its reflected image when it was 

 placed on the looking glass. 



The mineral which we have thus detected is one of the Tricliuic or 

 Plagioclase felspars, so called because if we examine the cleavages in 

 a crystal of one of them we shall find that these are no longer at 

 right angles to each other, but enclose an angle, measured over the 

 edge in which they meet, of about 86^°, varying slightly with the 

 different sorts. 



The rough models before you are attempts to give visible proof of 

 some of the exterior properties of these minerals, especially in regard to 

 that by which we have detected them in the rock, the multiple twinning. 

 In the first place, we find that, seeing that no face of the crystal is a 

 plane of symmetry, we may, by simply turning one component through 

 half a circle, keeping the corresponding planes together, produce a 

 twinning possible according to the laws of crystals ; and, indeed, this 

 IS the commonest of all among the Triclinic felspars, and fi'om its 

 prevalence in Albite is called the Alhite twinning. In most cases this 

 is repeated very many times, and the models show that the result is 



