MONIAN SYSTEM OF ROCKS. 543 



to pass into any other group. Thcj^ have been referred to the Cam- 

 brian simply on account of their infra-position and their unfossi- 

 liferous nature. The fossils which actually occur, being found also 

 at Bray Head, correlate the Longmynd rocks with these. They 

 will therefore be Upper Monian. The Volcanic group of St. David's 

 belongs entirely to the Middle Monian, and the occurrence there of 

 the same kind of granite, which is probably also intrusive, seems to 

 indicate that this rock is to be regarded as essentially part of the 

 series. The *' Uriconian " rocks of the Wrekin, with the granites 

 and altered rocks of Primrose Hill, form another isolated mass of 

 Middle Monian rocks. The Charnwood-Forest rocks do not show 

 sufficient similarity to any rocks in Anglesey to aflford any great 

 certainty in their correlation. If they are really of Monian age, the 

 Volcanic portions will belong to the Middle Group and the slates 

 of Swithland to the Upper. What further rocks may in future be 

 found comparable with these in the Highlands or elsewhere, cannot 

 be foretold ; but if massive quartzites with little bedding and great 

 irregularity of development are taken to characterize the system, it 

 is probable that some, at least, may be found to belong to it. The 

 occurrence of Oldhamia (thou^jh it may not be a fossil) in the so- 

 called Cambrians of the Ardennes may indicate the extension of 

 the Upper Monian at least as far as Belgium. 



Physical Histoey of the Monian Rocks of Anglesey and 

 THE Neighbourhood. 



In conclusion, I may attempt the difficult task of interpreting the 

 succession of the rocks as indicating the history of their formation 

 and subsequent alterations. 



When a series is fully known, we expect to find somewhere a 

 conj^lomerate at its base ; but if I am right in placing the grey 

 gneiss at the base, or even if i;he quartzite were the true base, we 

 find no such conglomerates in Anglesey. Whether any such rock 

 as the Torridon Sandstone will ever be recognized as the true base 

 can scarcely be foretold ; but at present there is no base to be 

 found, and the lowest rock yet recognized indicates a rather remote 

 source, being metamorphosed fine sediment. The Archaean rocks, so 

 far as we know, lie to the west, and this must have been the 

 direction whence the materials were derived. The cause of the 

 episode, of which the quartzites are the result, is very difficult to 

 conceive. The remarkable accumulation of so much quartz in one 

 place may suggest a siliceous spring, whose deposits were first 

 produced and then broken up and stratified in the neighbourhood. 

 An early sandbank is the only alternative hypothesis. After this 

 the deposits were made in shallower water, and subject to variations 

 in the source, which caused them to be laminated, but soon the 

 material became finer. The subsequent history is diff'erent in the 

 various parts of the island, according as volcanic eruptions intervened 

 or not. Where they did not, the varying deposits of quartz and 

 slate in the ^ South-Stack Series indicate a shallow sea subject to 



Q. J. G. S. No. 175. 2 o 



