248 MESSES. T. C. CANTRILL AND H. H. THOMAS ON THE [May 1906, 



west separates the latter from the Bala Beds (Dicranogrcijptus-Shales- 

 and Bala Limestone), affects the base of the Old Red Sandstone ; 

 these faults are probably both normal faults. Other faults, how- 

 ever, clearly involve the Old Red Sandstone, as, for instance, those 

 which bound the basal green beds east of Moelfre, and that which 

 separates the same beds at Coomb from the Tetragra-ptus-Beds of 

 Llangynog Church. 



VI. Sequence and Aoe op the Igneous Rocks. 



On the eastern side of Coomb Dingle the sequence is quite clear,, 

 and may be made out with ease. The oldest rocks exposed are those 

 forming the southern mass of augite- an de sites and their associated 

 f ragmen tal rocks ; next in succession follows a series of perlitic and 

 spherulitic rhyolites, of which the uppermost member is a rhyolitic 

 fluxion-breccia of some importance. This breccia in turn was 

 followed by another set of augite-andesites, exactly similar to those 

 below the rhyolites. 



It is a misfortune that the relations of the rocks on the two sides 

 of Coomb Dingle are so obscure, for not only is there a discrepancy 

 between the two sides, but the andesites on the west are themselves 

 probably faulted off from the rhyolites of Castell Cogan. However, 

 these andesites contain fragments of devitrified rhyolite, and it is 

 highly probable that they were extruded later than the rhyolites. 



Although there are some slight differences to be observed between 

 the rhyolites on the east and those on the west of Coomb Dingle, 

 we think that there is sufficient similarity in composition and mode 

 of occurrence to warrant us in considering them as marking one 

 geological horizon. 



With the exception of the occurrence of a small thickness of 

 hornblende-andesite (probably of extrusive origin) in the lower 

 part of the western andesitic series, these rocks are identical in 

 lithological characters and composition with those of the upper 

 andesitic series on the eastern side of the dingle. As no hornblende- 

 andesite has been met with on the eastern side, we suspect that the 

 augite-andesite to the north of Castell Cogan may be the highest 

 member of the series. 



We therefore suggest that the sequence was, first, augite- 

 andesites followed by rhyolites, and then another series of augite- 

 andesites, which included a small outpouring of a hornblendic rock. 

 The whole series was, at a later date, intruded into by diabase. 



There is not much evidence to prove that the igneous rocks of 

 Coomb and those of Capel Bethesda are on the same horizon, 

 excepting perhaps their close proximity and the evident lithological 

 similarity which exists between the rocks of the two areas. The 

 rhyolites of Coomb and those of Bethesda are identical in character, 

 and both are presumably followed by intrusions of diabase : although 

 the latter rock in the Bethesda area is so rotten, that it has been 

 found almost impossible to compare it with that of Tre-hyrn or 

 Pentre-newydd. 



