284 



PEOF. J. F. BLAKE ON THE CAISIBKlAN 



observable in the other specimen, where the felsite runs in veins 

 into the slate. 



In the road outside the quarry, beneath some cottages, there are 

 again phenomena which point to the same conclusion (see fig. 7). 

 In the mass of rock, on which a brick waU is built, are seen the 

 felsite and baked slate running into each other in a way which 

 proves intrusion, in this case, of course, on the part of the felsite. 



Fig. 7. — Section in Road near Bryn Efail. 



I i I I I i 1 1 I I I I 1 I 1 I I M I I 1 1 11 M 1 I I 



i ,1 , i , -. , .^ 



! i 1 i 1 :Br Lck 



1 I ; 1 u'oai 



1. Porphyry. 



2. Baked Slate. 



This spot is so im^Dortant in its teaching as to the age of the 

 felsite, that I can only suppose that it has escaped the attention of 

 previous observers. It provides a crucial test, and thus a decisive 

 proof, that this great flow of acid lava was, like the 99 per cent, of 

 which Prof. Sedgwick speaks*, contemporaneous with the Cambrian 

 rocks amongst which it is found. The Cambrians to the west are of 

 earlier, and those to the east of later date than this. The question 

 being thus apparently settled, it is necessary to see how this con- 

 clusion fits with other facts that may be observed in the neighbour- 

 hood. In the first ^Dlace, the conglomerates ought to contain not only 

 felsites, but also fragments of the Cambrian rocks to the west. At 

 Clegyr, where the top of the felsite is an agglomerate of felsitic frag- 

 ments (and nothing would be easier than to round them into pebbles), 

 the conglomerate is mostly, though not entirely, made of such stones. 

 But at Moel Tryfaen, where the top of the felsite, for some reason, 

 is clean, the majority are slate. On this, Prof. Eamsay saysf, " the 

 ])ebbles of purple slate resemble those of the verj^ strata amidst 

 which they are found ; the quartz-rock and jaspers resemble some 

 of the metamorphic rocks of Anglesey." It is remarkable that 

 Dr. Hicks t, who thinks this conglomerate is derived from Pre- 

 Cambrian rocks alone, omits all mention of the purple slates, and 

 enumerates only " quartz, quartz-felsites, porj)hyTitic rocks, and 

 schists." Xow these fragments of slate are large and scarcely 



* Quart. Journ. Geol. Soe. vol. iii. 1847. 

 t Ibid. Tolix. 1853. 



Ibid. vol. xxxiv. 1878. 



