368 MF^SKS. A. V. JENNINGS AND G. J. WILLIAMS 



22. Manod and the Moelwyns. By A. Vaughan Jennings, Esq., 

 F.L.S., F.G.S., Curator of the Eton-College Museum, and 

 Griffith J. Williams, Esq., E.G.S. (Head March 11, 

 1891.) 



Contents. 



Page 



I. Introduction 3(i8 



II. Topography of the District SttU 



III. Succession of Strata on the Moelwyns ^{iy 



(a) The Treinadoc Series ,*3()9 



(5) The Garth Grit 373 



(c) The Arenig Series 374 



{d) The Llandeilo Series 370 



IV. Correhition of the Strata between Moelwyn and Manod 377 



V. The Intrusive Rocks 379 



I. iNTRODrCTION. 



On the north and west of the great mass of Harlech Grit which 

 forms the centre of the Merioneth anticlinal, the Lingula andTrema- 

 doc strata dip under the mountains of the Moelwyns and Manods, 

 and are there overlain by the Arenig ashes and slates. The beds 

 exposed on the southern slopes of these mountains have therefore 

 the same general slratigraphical age as those cropping out on the 

 northern slopes of Cader Idris and on the western aspect of 

 Arenig and the Arans. 



The general relations of the rock-masses of this difficult district 

 have been admirably elucidated by the officers of the Geological 

 Survey, and the results of their work published in Sir A. llamsay's 

 invaluable memoir on the Geology of JS^orth Wales. 



In the present paper, Avhich is the result of observations carried 

 on interruptedly for several years, our objects are : — 



To offer conclusive evidence as to the intrusive nature of the 

 great crystalline mass known as the Syenite of Tan-y-grisiau. 



To correct what seems to us an inaccuracy of some importance in 

 the correlation of beds in different parts of the range, as interpreted 

 in the map and memoir of the Geological Survey. 



To trace with greater completeness the position and constancy of 

 the beds of slate in the Arenig series ; a subject of considerable local 

 and practical importance, when we remember that those who are 

 engaged in slate-quarrying have so far had to rely mainly on their 

 personal and unaided observation. 



In order to make the conclusions arrived at on these points more 

 readily intelligible, the writers have worked over all the ground from 

 Dduallt on the south to Tal-y-waenudd * on the north, and from 

 Moelwyn Mawr on the west to Manod on the east, inserting the 

 geological details on the six-inch Ordnance-Survey map. The 

 structure of the Moelwyn range itself is illustrated by a series of 



* Spelt * Tal y Waunedd ' on the map. 



