MONIAN SYSTEM OF ROCKS. 525 



by the black shale. There cannot now be the slightest question as 

 to the character and age of this conglomerate. Unfortunately the 

 line is so much broken by small faults that it is unfitted to lead to 

 any stratigraphical conclusions. Still the basal conglomerate is 

 there ; on one side is a large area of Ordovician, on the other a 

 large area of another group of rocks. Thus, in horizontal dis- 

 tribution, the conglomerate is intermediate between the two, and it 

 seems impossible that the second group of rocks should be younger 

 than it. Thus there is reasonable ground for treating this district 

 as not " altered Silurian," but as Pre-Ordovician ; and if Pre-Ordo- 

 yician, from what we have elsewhere seen, then Pre-Cambrian. 

 But there are still some difficulties on this head to be overcome. If 

 we draw a line from Nebo quarries in the direction of the general 

 faulting, we find the conglomerate continued at least a little way 

 along it ; and where it emerges on Perth Lygan, there are strewed 

 on the shore immense blocks of the same kind of conglomerate, 

 hiding, with other blocks, the solid rocks below. At this spot, too, 

 Sir A. Eamsay states that there are 60 or 70 feet of black Ordo- 

 vician shales, not now exposed, and the valley, which here descends 

 to the shore, indicates soft rocks. There would thus appear to be a 

 band of Ordovician running through the heart of the district, and we 

 cannot wonder that Prof. Ramsay considered the granite intrusive 

 in its midst. But, as we shall see, the district is not all granite, and 

 such a band would not correspond with the lie of the other rocks. 

 We may therefore better account for this strip by another parallel 

 fault to bound it on the east. But, again, there is marked on the 

 Survey map a tongue of granite runiiing across the fault on the 

 northern side and into the Ordovician shales. If this were correct, 

 the granite, at least, would have to be late Ordovician in date. I 

 have therefore examined the ground with care. The supposed 

 tongue lies entirely in a large grass-field, and very little live rock 

 can be found. The only knob in which anything is seen consists of a 

 gritty band of the Ordovician, and not of granite. I conceive there- 

 fore that this tongue is simply an error, which would be of sl-i-ght 

 importance if the granite had been proved Ordovician elsewhere. 



Description of the Rocks. — At first sight the rocks of this district 

 seem entirely different from anything we have seen before ; but this 

 is because the examination naturally begins on the sea-coast, where 

 their true character is revealed by weathering. But, seen inland, 

 we do not lose all clue. At Dryslwyn, south of Parys Mountain, 

 and not much more than a mile from Pen-Ion, the aspect of the 

 rock is that of a pelite ; but when examined microscopically it is 

 seen to consist of tolerably clean foliated quartz and felspar with 

 mica in parallel lines ; in other words, it corresponds to the ordinary 

 grey gneiss, differing only in not being quite so clean. Then by 

 the outer rows of evaporating pools in the low ground we find 

 granite passing into mica- schist, reminding one of the district 

 south of Traeth Dulas ; and the low parallel mounds yield a dark 

 foliated rock, which is micaceous diorite, only differing in the 



