522 



REV. J. F. BLAKE ON THE 



coarser conglomerate, dying out above, as marked in the figure. 

 Before attempting to discuss this section, we must obtain the data 

 offered by the continuation of the series on the eastern side of the 

 Berth. Passing round the shore, we encounter several knobs of 

 quartz and limestone, which now add nothing to our information, 

 and then at Forth Pridd we find another of the Ordovician slices let 

 down by faults. This fragment consists of the ordinary black 

 shales with indurated bands, showing that the true Ordovician 

 remains constant here. The fault on the north is an overthrust 

 fault, and the rocks beyond it are the ashy shales, with fragments 

 of limestone cemented in the fault. In these shales we soon find 

 small quartz-knobs, and further east these form a well-marked crest. 

 This crest is so instructive that a plan of it is given in fig. 24. 



w. 



Pig. 24. — Plan of the HilUcrest, west of Forth Llechog, 



E. 



1. Laminated ashes. 2. Quartz. 3. Quartz-conglomerate. 



Here the direction of the lines of subdivision in the ashy slates, 

 which are parallel to the bedding, as indicated by the changes, is 

 along the length of the crest: there is no disturbance or irregularity. 

 The quartz-knobs may be seen on the lower slope and creeping up 

 the hill across the bedding, and then turning round and lying in 

 the direction of the bedding, the several beds coming directly 

 ao-ainst them. Beyond these quartzites, and therefore higher in 

 the series, yet closely associated with them, is a band of coarse 

 conglomerates whose pebbles have been derived from the quartzite 

 itself; they come in with the quartz, and they disappear with the 

 quartz, and the size of the pebbles diminishes as we pass upwards 

 from the quartz. The natural interpretation of these phenomena 

 is that the quartz has been forced across the edges of the ashes ; and 

 the simplest method of accounting for this is by hot siliceous springs, 

 the quartz of which has immediately consolidated, and has then 

 been broken up and rounded by the waves without any cessation of 

 the ordinary deposits. In structure this quartz-knob is one of the 

 purest, there being no recognizable rounded pebbles, but all being 

 composed of the polygonal network. 



