184?6.3 SEDGWICK ON THE FOSSIL SLATES OF N. WALES, ETC. 145 



higher mountains of the Rhinog Fawr chain. The same beds after- 

 wards turn round (whether gradually or by a succession of faults 

 I have not determined) and acquire the ordinary strike of tiie 

 Merioneth chain, at the same time dipping to a point considerably 

 south of east: and the junction of these beds with those of the 

 neighbouring mountains no longer has the character of a fault, but 

 becomes a true synclinal line, which may be traced into the Bar- 

 mouth estuary. 



The anomalous position of the Tremadoc rocks, the great inter- 

 ruptions to the continuity of the strata manifested by Traeth Mawr 

 and Traeth Bach, and the dislocation of the beds above noticed 

 along the Merioneth coast, were probably all produced by the same 

 set of disturbing forces. ; „ 



The section I am about to notice commences among these trem- 

 bled beds of the coast*. Near Harlech they are composed of very 

 coarse grits, which may be compared with the coarse grits (No. 2) 

 of the preceding (Festiniog) section. These coarse grits are over- 

 laid by bedded masses of roofing-slate and some contemporaneous 

 bands of porphyry. The line of section then crosses the chain 

 of Rhinog Fawr, and descends to the Merioneth anticlinal, a few 

 miles south of the point where the former (Festiniog) section 

 commenced, and the details of the two sections (after we cross 

 the above-mentioned line of fault) are in perfect general accord- 

 ance. Following this line of section beyond the Merioneth anti- 

 clinal to the south-east, we first meet with bands of roofing-slate 

 and porphyry, and then pass over a ridge of mountains in which we 

 trace the coarse grits (No. 2 of the previous section), in a somewhat 

 degenerate form, alternating with masses of contemporaneous igne- 

 ous rock. These again are followed by great masses of contempo- 

 raneous porphyry, trap-shale, beds of slate, &c. Then comes a series 

 of rather brown pyritous slates, much intersected with mineral veins, 

 and often of very complicated mineral structure. Among them were 

 beds resembling the Tremadoc or Festiniog flags, but 1 found in 

 them no traces of fossils ; but I thought it probable that they repre- 

 sented the Festiniog flags, partly from their structure, and still more 

 because they were nearly at the same geological elevation above 

 the beds on the Merioneth anticlinal. 



For the present, leaving the discussion of this point, we may fol- 

 low the section over some ridges of gnarled hills in which the por- 

 phyries and associated trappean breccias and shales almost exclude 

 the appearance of slates. Among these masses are, however, some 

 bands of slate and a few irregular and highly-mineralized masses of 

 limestone. I believe these great mineral masses, considered as a 

 whole, represent the groups S.W. of Great Arrenig, among which 

 we also find bands of a mineralized limestone. The enormous excess 

 of igneous rocks (most of which are unequivocally contempora- 

 neous) easily accounts for the absence of fossils ; but I have little 

 doubt that the calcareous bands in this part of the section (as well 

 as those near Great Arrenig, and I may add also some similar 

 bands at the east end of Cader Idris) are about the age of the fossil 



* See the westeni end of the Section No. 5. 



