384 REPORT— 1888. 



The best examples of elemental orientation are seen at Abersant (19), in 

 the Western district — the only locality in that district where coarse mica- 

 schist occurs ; also near Bodafon (106) and at Tros-y-gors (155), near Menai 

 Bridge. All these are composed of elements which in an ordinary section 

 polarise in neutral tints, whose long diameters are in general about twice 

 the short ones, and whose boundaries are sutaral ; the mica is, of course, 

 parallel to these, but varies in amount, most of it occurring at Abersant 

 and least at Tros-y-gors. 



Confused orientation is seen in the rocks at Llanflewin (210), Roscolyn 

 {67), and south of Traeth Dulas (141), only some of the elements being 

 orientated, and all containing numerous original fragments. In these 

 the orientation is also assisted by lines of epidote which, as in the case 

 of the Roscolyn rock, is aggregated along certain lines and produces 

 cleavage. At Y Foel, Llanerchymedd (80) the rock has no crystals of 

 mica, as that mineral has passed over into fine granular sericite, and the 

 principal orientation is due to the other elements. 



There are two other crystalline rocks which cannot well be included 

 in any of the above groups ; they difTer widely from the gneisses, yet 

 have no resemblance to the mosaic rocks. One of these is a highly mica- 

 ceous rock in the railway between Llanerchymedd and Llangwllog (93), 

 containing both derivative and authigenetic mica, while the quartz and 

 felspar have sutural boundaries. It looks like a peculiarly altered vol- 

 canic tuff. The other is from Gorse Mill, Gwyndy (81). It also has 

 large derivative mica, but the authigenetic elements arc much decomposed, 

 and only occasionally show elemental orientation. 



We have now to consider the alterations of a mechanical nature. It 

 is by no means assumed that mechanical forces have played no part in 

 bringing about the chemical changes already described, but the effects in 

 these cases are not direct. The phenomena now to be dealt with are the 

 immediate results of mechanical forces observable in the rocks as they 

 now are. The results of these forces will be different according to the 

 power of the rock to resist disintegration. If it possesses that power the 

 effects, if any, will be observable in the elements themselves ; if not, their 

 relations to each other will be affected. 



The effect of pressure on a crystal or an isotropic substance is well 

 known. When an ordinary unpressed crystal is viewed between crossed 

 Nicols it will show a definite colour according to its position relatively to 

 the planes of vibration in the Nicols, and as it is rotated the colour will 

 change equally all over the crystal, and every part in a certain position 

 will extinguish at the same time. When, however, such a crystal has 

 been subjected to pressure sufficiently great to have an effect on its mole- 

 cular constitution it will show unequal polarisation. Hence the colour of 

 any such crystal will not be the same throughout, but will vary from 

 spot to spot, and when it is rotated different parts will extinguish at 

 different times, and the darkness will pass over it like a wave (see fig. 13). 

 Such an extinction has been called undulose extinction. As, however, 

 the com-esponding phenomenon is seen without rotating to extinction, 

 and the colours pass over the crystal like a shadow, a more general title 

 to use would be spectral folarisation, without any reference to extinction. 

 This phenomenon has also been beautifully referred to as ' strain-shadows.' 

 The mode of description here proposed, however, lends itself more easily to 

 grammatical inflexion. Such a phenomenon, then, may be taken to prove 

 that the crystal has been strained beyond its limit of perfect recovery. 



