ON THE OLDER EOCKS OF ANGLESEY. 385 



When, however, a complex of small elements, whether original or pro- 

 duced by the disintegration of a larger crystal under pressure, is similarly 

 affected, the colour of each small element changes towards the edges and 

 thus becomes confused with that of its neighbours. Hence, instead of 

 each element being distinctly marked off, the colour gradually fades on all 

 sides into that of the others, and a general haziness and indefiniteness 

 of the complex mass is the result. This phenomenon is immediately 

 recognised on slightly rotating, or even simply examining a slide affected 

 by it, and it is proposed to refer to it as microspectral polarisation (see fig. 

 14). It is the most faithful indication of the previous stress the rock 

 has been subject to. It is sometimes referred to as a quartz-felspar 

 mosaic, but it differs essentially from such a mosaic as is produced by the 

 growth of small defiuite crystals, and should not therefore be designated 

 by the same word. Numerous instances of this occur in the rocks 

 already referred to. Thus the quartzites of Bodafon (109) and the 

 cleaved rocks of Roscolyn (66, 67) show it in a high degree, while the 

 cases in which individual crystals show ordinary spectral polarisation are 

 too numerous to mention. 



The two cases, however, must be distinguished in which, on the one 

 hand, a complex of small elements is the primary subject of pressure, and, 

 on the other, when the pseudo-elements are the results of the disruption 

 of a larger one. In the latter case the boundaries are much more in- 

 definite. In the cases above referred to the elements were originally 

 small ; and of this kind another example may be quoted — viz., the rock 

 near the river Cefni (102), which has a slaty aspect on the large scale, in 

 which not only the quartzose or felspathic elements are affected, but also 

 the abundant mica which had previously filled the cracks. In an ex- 

 ample, however, from Bodlew (148) both varieties are seen side by side. 

 In this the original small elements produced by primary metamorphism 

 are indicated by fine interstitial flakes of mica, while in the midst is an 

 infiltrated quartz vein which is broken up into optical elements, so that 

 the greater indefiniteness in the latter case can be easily appreciated. 

 When such a rock is examined with the paraboloid, the elements which 

 have thus optically broken up are seen to possess a peculiar structure.. 

 They are affected by a number of curved or crinkly cracks, like the sur- 

 face of crape, which are sometimes parallel throughout the original 

 element, and sometimes radiate from the apex of an adjoining crystal. In 

 the rock at Bodlew they are seen to occupy the intervals between the 

 broken fragments of a large microcline crystal, and cannot therefore be 

 in anything but vein-stuff. These cracks are very minute, about '0005 

 inch apart, and can only be seen with the paraboloid. Such a structure 

 may be called crape structure (see fig. 15). It is necessarily accompanied 

 by microspectral polarisation, but the converse of this does not hold. 

 In the case above quoted both the original small elements and the later 

 vein are microspectrally polarised ; but in another case in which the ele- 

 ments of primary metamorphism are larger — e.g., at Ty Mawr, Llandaniel 

 (149) — they are unaffected, while the interstitial segregation quartz shows 

 the crape structure. This would seem to suggest that the elements of 

 primary metamorphism were formed under so great a pressure that the 

 later pressure, producing the new phenomena, did not reach their limit of 

 elasticity, and consequently they were unaffected. If this be the case, 

 since small elements are affected they must have been produced under 

 less pressure. 



1888. c c 



