536 EEY. J. r. BLAKE 0^ THE 



but why ? Certainly not on tlie principle of identification by 

 organic remains ; for what fossils there are in the Bray-Head rocks, 

 omitting the doubtful Oldhamia, are not those of the Cambrian rocks 

 of Wales, or of any other place where such rocks lie conformably 

 beneath Ordovician. jN'or can they be correlated by similarity of 

 lithology with true Cambrian rocks. jSTo doubt they are more like 

 Cambrian than Ordovician, but they are most like the rocks of 

 Anglesey, and this is, in fact, all that geologists have meant by their 

 identification. 



AEEAIsrGEME:N^T OF THE GrOTJPS Ilf THEIK RELATIVE OrDEK. 



This is by no means an easy task, nor is it possible that conclusions 

 should be final. The first point to determine is, which group of rocks 

 we are to consider the lowest. The choice lies between the grey 

 gneiss and the bedded quartzite. In the Western District we have 

 the quartzite passing up into the chloritic schist, and in the Eastern 

 we have the grey gneiss behaving in the same way. In the Central 

 District there is a break in the succession above the grey gneiss ; but 

 yet there is an isolated mass of bedded quartzite in Bodafon moun- 

 tain. This therefore is the only district in which the two occur 

 together, and here they are not in association. The more quartzose 

 variety of the grey gneiss seen at Gualchmai and referred by 

 Dr. Callaway to the quartzite is not worth considering, — it is an 

 unimportant accident. We are thus left to general considerations. 

 From these it seems to me most satisfactory to conclude that the 

 grey gneiss is the basal rock. 



The reasons are as follows : — In the Central District the grey 

 gneiss makes a long band in the south until it meets the granite, 

 and even after that we get traces of it north of Llanwyllog, possibly 

 at Tafarn-y-botel, at Llanerchymedd, in the district south of Traeth 

 Dulas, and even perhaps in that to the east of Parys Mountain ; 

 all this apparently consecutive series of exposures leads past the 

 crest of Bodafon mountain, and the general lie of the rocks in the 

 district does not afi'ord much expectation of finding it on the east of 

 that mountain. Thus, in geographical position, the quartzite here 

 lies between the grey gneiss and the higher chloritic rocks, and if 

 the geological arrangement were different, the method of transpo- 

 sition would be very difiicult to conceive, and the conception would 

 be highly improbable. Again, a quartzite is exactly the stjie of 

 rock we might expect in an episode, and the masses themselves are 

 characteristically limited. If, then, we consider them to have this 

 character, the absence of any representative in the Eastern District 

 is not remarkable, especially as we have there a group of very 

 quartzose mica-schists which occupy their place, and in the heights 

 of Mynydd Llwyddiart even approach them in appearance ; on the 

 other hand the absence of so important and wide-spread a group as 

 the grey gneiss in the Western District, between the quartzite and 

 the chloritic schist, can hardly be accounted for, the fault which Dr. 

 Callaway thought he had discovered being actually non-existent. 



