The Rev. Prof. Blake — Base of the Sedimentary Series. 355 



the base of that series, and unconformable on all the beds below. 

 The Llaneiniolen conglomerate contains a different set of pebbles, 

 some of which correspond to the Twt Hill igneous rock, and the 

 neighbouring felsites. It runs obliquely to the Arenig conglomerate 

 in a N.N.W. direction, and is followed on the dip side by other con- 

 glomerates and ashy beds, with intervening compact halleflintas or 

 slates, as demonstrated by Prof. Bonney. It is thus entirely distinct 

 from and older than the Twt Hill conglomerate. Further north on 

 the same dip comes the Bryniau Bangor conglomerate in the midst 

 of similar halleflintas, and this therefore is a higher conformable bed 

 of the same series. But though of the same series, it is not identical 

 with the Llandeiniolen conglomerate, as is proved by the careful 

 stratigraphy of Professor Bonney against the statement of Prof. 

 Hughes, who acknowledges not to have examined the critical parts 

 of the area. The Bryniau Bangor conglomerate is not, therefore, in 

 spite of its large pebbles, the base of any series, but is due to the 

 denudation of some new mass of felsite which has previously been 

 covered. The only basal conglomerates as proved by stratigraphy 

 are the Twt Hill one, which is the base of the Arenig, and the 

 Llandeiniolen one, which is the base of some older series. These 

 results are in absolute accordance with the mapping of the Geo- 

 logical Survey. All therefore we know about the group of rocks 

 between Bangor and Carnarvon is that they are Pre-Arenig. 



The Pre-Arenig rocks are considered by the Geological Survey to 

 be altered Cambrian intruded upon by felspar porphyry. This last 

 conclusion cannot possibly be correct seeing that the bedded portions 

 contain fragments of the porphyries, etc., which are nowhere else to 

 be matched in the district. The tout ensemble of the bedded rocks is 

 exceedingly volcanic, being almost entirely conglomerates, agglome- 

 rates, ashes, and halleflintas. We must not assume that the Geo- 

 logical Survey has overlooked this fact, and has imagined that the 

 ordinary slates and grits could change into such as these. We must 

 read the words " altered, Cambrian " with a comma, and as intended 

 to indicate two distinct opinions, that they must be classified with 

 the Cambrian (though they may be lower beds than any elsewhere 

 seen), and that they are altered. Perhaps their only direct connection 

 with ordinary Cambrian rocks is the occurrence amongst them on 

 the west of Bangor and elsewhere of purple slates. They are 

 entirely cut off on the east by the overlap of the Arenig. Under 

 these circumstances all that can be said at present is that as the 

 Cambrians nearest on the east have no base, and these have no 

 summit ; it is highly probable that the two wants are satisfied together, 

 by these being considered as basal Cambrian. 



With regard to the felsites and Twt Hill rock, the fact of the 

 Llandeiniolen conglomerate containing their fragments shows that 

 they were in existence at the time of its formation, and must 

 therefore antedate it, but by how long an interval will depend on 

 their nature. It is well known that attempts have been made to 

 prove that they are bedded ; but such attempts, as in the case of 

 St. Davids, have been entirely unsuccessful. It was Prof. Bonney 



