280 PEOF. A. IT. COX AND ME. A. K. WELLS OX THE [vol. lxxvi T 



sill. On the south side of the sill the agglomeratic beds reappear 

 on Myn} r dd Caer-go, only to be cut out by a normal fault near Cefn 

 Owen, so that their outcrop does not reach the Grwernan Fault. 

 South of this fault, however, and outside the area at present con- 

 sidered, the ashes have been followed along the flank of Cader Idris 

 as far as the Aran Valley. Their lithology is so characteristic that 

 they furnish a most useful datum-line in mapping the country. 



Coming immediately underneath a thick volcanic series — the 

 Lower Basic Series — it may appear that the Cefn Hir Ashes should 

 be grouped with this volcanic series, especially since some of the 

 ash-beds in the Cefn Hir Ashes are similar to ash-beds intercalated 

 in the Lower Basic Series. But there is a general predominance 

 of somewhat acidic material in the Cefn Hir Ashes, as opposed 

 to the thoroughly basic character of the overlying lavas. More- 

 over, there is the distinct slaty zone between the Cefn Hir Ashes 

 and the lavas, although this has, perhaps, no great significance, 

 since slate-bands also occur intercalated within the Lower Basic 

 Series. But, taking all the facts into consideration, we have 

 deemed it advisable to separate the ashes as a distinct formation, 

 particularly since they have proved so useful in mapping the country. 



(4) The LoAver Basic Series. 



The Lower Basic Series consists of a great thickness of basic 

 lavas, Avith various ashy and slaty intercalations, and with numerous 

 intrusions of basic material. Manj r of the lavas are typical spilites. 

 As the series is more completely deA r eloped on the northern slopes 

 of Cader Idris, it is proposed to defer a more detailed account until 

 the geology of the Cader-Idris range is described. It may be 

 mentioned, hoAvever, that the pilloAV-lavas and associated ashes may 

 be seen at various points between Llynau Crogenen and Hafotty 

 Fach, and also on the dip-slope of Mynydd Caer-go. 



The exact age-limits of this basic volcanic series have not yet 

 been definitely settled. As already stated, the slaty beds (of the 

 Cefn Hir Ashes) immediately underlying it contain Didymograptus 

 bifidus. On Cader Idris a band of slate that appears to belong to 

 the volcanic series has yielded graptolites Avhich, although the 

 exact species are unfortunately indeterminable, appear to be of 

 Llandeilo age. The strata that immediately overlie the lavas 

 probably belong to a high horizon in the Llandeilo. 



III. The Intrusive Rocks. 

 (A) The Diabases, 



(a) Field -relations. — As will be seen from a glance at the 

 map (PI. XX), sills and laccolitic masses of diabase are extremely 

 abundant throughout the district. It is also apparent that the 

 intrusions are not limited to any particular stratigraphical horizon, 

 but that they occur throughout the Avhoie range of formations in 

 the area mapped, Avith a maximum development at the horizon of 



