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ON MANOD AND THE MOELWYNS. 377 



IV. Correlation of the Strata. 



After examining the succession of strata as seen on the mniiiitaiii- 

 sido under Moelwyn Mawr or Foel liydd, the fj;eologist would natur- 

 ally turn his attention to the outcrops north of Cwm Orthin, in 

 order to make sure that the beds so far observed are continuous. 



Direct observation in the valley itself is difficult, and for the 

 most part impossible, owing to the enormous accumulation of slate- 

 rubbish from the quarries above. There is, however, no indication 

 of faulting or dislocation of the strata, and the Garth Grit is found 

 in its natural position on either side of the valley. On Moelwyn, 

 as already noticed, the upper and lower igneous masses separated 

 by the slate correspond to well-marked physical features on the 

 mountain-side ; and the reappearance of those features on Craig 

 Nyth-y-Gigfran, which continues the range to the north, at once 

 suggests that the strata correspond on the two sides of the valley. 



On examining the Geological-Survey Map, on the other hand, it 

 will be seen that the lowi')- igneous beds of Moelwyn are marked as 

 crossing upward across Cwm Orthin and forming the tojj of Craig 

 Nyth-y-Gigfran, while the upper agglomerates of Moelwyn and lower 

 agglomerates of Xyth-j'-Gigfran thin out suddenly at the same place. 

 This could only be the case if there existed a fault, with a down- 

 throw to the south of more than 600 feet, along the Cwm-Orthin 

 valley ; but the presence of such a fault has never been suggested, 

 and is, in fact, disproved by the continuity of the Garth Grit. More- 

 over, this interpretation would make the Lower-Slate bed of Moelwyn 

 the equivalent of the bed worked at the Xyth-y-Gigfran Quarry ; and 

 owing to the variation in quality of the different slate-beds, the 

 (]uestion becomes one of considerable local and practical interest. 



lleference to the Survey Memoir shows that it is not a question of 

 mis-drawing of the lines on the map, for the map embodies the 

 opinion of the surveyors. Sir A. Ramsay states that the "por- 

 phyry abutting against the fault at Tal-y-waenudd " (that is, the 

 upper igneous series) "crosses the valley of Cwm Orthin a little below 

 the lake and rests directly on speckled flags of the Arenig beds " *. 



In order to discover what evidence there was to support this view, 

 we have carefully examined the cliffs between Cwm Orthin and 

 Tal-y-waenudd, and made a section of them passing through the 

 Xyth-y-Gigfran Quarry, for comparison with those farther south. 

 AVe tind that the Lower-Slate bed of Moelwyn occurs here in its 

 natural position ; that is, it is lower than below Foel Rydd, owing 

 to the northerly dip. It is not represented on the map, but lies 

 at the bottom of the Lower Agglomerate, which is doubtless the same 

 as the Lower Agglomerate on Moelwyn, and, like it, contains a fel- 

 stone band at the base. 



On the other interpretation this Lower Agglomerate not only thins 

 out suddenly at Cwm Orthin, but disappears inexplicably at the fault 

 on the north, and cannot be correlated with any beds except the 

 " Lower Ashes " on the farther side of xArenig. 



* Ramsay, op. cit. p. 94 and foomote. 

 Q.J.G.S. No. 187. 2d 



