356 The Rev. Prof. Blalce—Base of the Sedimentary Series. 



who first made the attempt in the case of Twt Hill. Admitting that 

 he " saw nothing absolutely irreconcileable with an igneous origin,' 

 his doubts were set at rest by the observation of a conglomerate— 

 the Arenig conglomerate— which was apparently part of the mass. 

 These doubts should be raised again, now that the error of this 

 observation has been made clear. There is, in fact, no sign of true 

 bedding anywhere in the hill ; there are only segregation bands of 

 more felsitic-looking rocks, and a certain amount of brecciation in 

 the neighbourhood of the fault. The impossibility of finding any 

 is shown by the fact, that while Prof. Bonney states that the Twt 

 Hill group is the highest of the metamorphic series, Prof. Hughes 

 places it at the base, with the " Crug beds" above. It pan only 

 be theory indeed which prevents any one from recognizing the 

 former as granitic, and the latter as granophyric, and both there- 

 fore of igneous origin. The same is the case with the Dinorwic 

 felsites. °Tbough called "beds" by Prof. Hughes, they are as 

 good volcanic rocks (very possibly flows, as there are occasionally 

 ashes with them) as any of more modern date, as has been well 

 shown by Prof. Bonney. This being the case, they need not be 

 separated by any long interval from the detrital rocks which 

 contain their fragments. It may be said, perhaps, that the Twt 

 Hill rock, being granitic, must have been buried deeply when formed, 

 and have required a long time for the denuding forces to reach it. 

 But this would not be a just inference, when we remember that 

 quite as granitic a rock is found in the Ponza Isles in association 

 with surface volcanic materials, which are probably exceptionally 

 bad conductors of heat. 



The association of these three types of rock in one small area, 

 granite, quartz felsite, and stratified volcanic materials, calls to mind 

 very forcibly the similar association at St. Davids, and the upper 

 part has by Professors Bonney and Hughes and Dr. Hicks been 

 referred to the Pebidian. The actual similarity of the individual 

 rocks is not very striking, but it is certainly greater than the 

 similarity between the Pebidian and any rocks of volcanic^ origin 

 in Anolesey. My correlation of the Pebidian with the latter instead 

 of with these depended upon the general stratigraphical evidence 

 that the Pebidians could not be Cambrian, and that these rocks 

 partly are. In this I was probably wrong; but if so, then, as it is 

 impossible to create a separate "system" for these Carnarvonshire 

 rocks, jambed in as they are between higher Cambrian and Monian, 

 both they and the Pebidian will have to be brought into the Cambrian 

 as basal deposits. 



The great porphyry rib from Moel Tryfaen to Llyn Padarn must 

 now be" considered. Here again it was Prof. Hughes who first 

 suggested in 1877 that the beds passed through in an adit on Moel 

 Tryfaen might be Precambrian, and accordingly the locality was 

 examined in the following year by Dr. Hicks, who adopted the 

 suggestion. It should be noted that there is abundant evidence in the 

 cleavage of the Penrhyn slates, which extends in a remarkable manner 

 into the felsites, that there has been enormous pressure here at work, 



