E. Greenly — Quartzite in Schists of Anglesey. 551 



apply tins excellent saying to the exaggerated form in which 

 land-ice theories are often presented to ns ? 



The phenomena of the Glacial Period in Britain contain some 

 of the most interesting problems it has yet been the lot of geologists 

 to attempt to solve. It therefore behoves us to approach the 

 subject in a spirit of humility. That such varied explanations have 

 been proffered from time to time, that most contradictory con- 

 clusions should be drawn from well - ascertained and generally 

 acknowledged facts, is curious and somewhat depressing. There 

 is, however, this reflection to comfort us : however strange, however 

 contradictory, however devoid of common-sense the various expla- 

 nations and theories of the Glacial Period appear to the various 

 observers and reasoners upon them, the total effect is, like that of 

 the hypothetical Ice-Sheet, a push forward. Without opposition, 

 observation stagnates, so that the first effect of enthusiasm, even 

 if directed in lines that afterwards prove to be mistaken, is to 

 advance the science we love so well. Even if a theory be utterly 

 false, it may prove of great educational value, for, until every 

 possible line of reasoning has been traversed, secure ground cannot 

 be reached. 



IV. —On Quartzite Lenticles in the Schists of South-Eastekn 



Anglesey. 1 



By Edward Greenly, F.G.S. 



THE prevailing type of schist in the south-east of Anglesey, for 

 some miles to the north and north-west of Beaumaris, is a wavily 

 foliated, pale-green, chloritic rock, 2 the quartz in which, though more 

 abundant than appears at first glance, is in a very fine state of 

 subdivision. In a common variety, especially prevalent about 

 Llanddona, quartz forms nearly the whole of the rock, but it is 

 still in very compact seams, separated by partings of chlorite 

 or mica. 



In none of these rock-types can original clastic grains be detected 

 by the naked eye or with the hand-lens ; but areas occur in which 

 they contain great numbers of lenticles of quartz, imbedded in 

 a finely foliated matrix of much finer quartz and chlorite. Many 

 of these lenticles have the aspect and fracture of vein-quartz ; but 

 a large number, and in some areas almost all, are fragments of 

 true granular quartzite, composed of clastic grains often well 

 rounded. The whole rock, indeed, might be termed a " quartzite- 

 augen-gneiss." With a few exceptions, these fragments range in 

 size from a quarter of an inch to three feet across, but they are apt 

 to be elongated, the axes being often as 8 : 2 : 1. Very frequently 

 they are ellipsoidal rather than lenticular, the bounding curves 

 being continuous all round. 



1 An abstract of this paper was read before the British Association, Liverpool, 

 September, 1896. 



2 There are other green schists in the same area, which are probably of igneous 

 or pyroclastic origin. 



