MONT AN SYSTEM OP EOCKS. 



489 



a coral as Si/ringopora ; but whether these are really the remains 

 of corals with their internal structure obliterated it is impossible to 

 say. South of Cerrig Ceinwen I have found no jasper associated 

 with these limestones in mass, though most of the limestone is 

 "jaspery:" but in the masses to the north there are bands and 

 isolated pieces of red jasper, which behave towards the limestones 

 exactly as flint does to chalk, and a similar origin is at least 

 suggested. Nodules of similar jasper occur in beds of the same 

 series farther south. 4. Disturbed masses near Dinas Llwyd. In 

 this locality the intimate relations of slates, ashes, quartz, and lime- 

 stone with one another and with basic igneous rocks are beautifully 

 shown. On the east side of the headland are slaty rocks, but on the 

 west the rough irregular ashes and local breccias ; on the western 

 slope the following section is seen (fig. 7). The quartz here comes 



N.w. 



Fig. 7. — Section on west side of Dinas Llwyd, 



S.E. I N. 



1. Ashes. 2. Quartz. 3. Diabase. 4. Breccia. 5. Ashy grit. 



in as a thick tongue from above, and it dies out behind on the 

 surface. Near at hand is a diabase- dyke, probably belonging to 

 the series ; beyond this are some breccias, and then more gritty 

 rocks. Here, then, the quartz is certainly not a fragment of old 

 land, nor is it a contorted bed, but something that has formed 

 where now we find it. In the grits are found fragments of similar 

 quartz, which show the polygonal structure ; we may therefore even 

 conclude that the quartz was a contemporary product. To the west, 

 in the valley, lies a band of slate, and then comes a ridge of 

 remarkable diabase, which is repeated in other ridges near Bodowen. 

 Beyond this slates recur and form the headland of Trwyn-y-parc. 

 The succeeding hollow is occupied by a limestone somewhat similar 

 to those of Cerrig Ceinwen, and finally we come to the agglomerate 

 of Forth Trwyn Mawr, referred to by Dr. Callaway. Comparing 



