part 3] LOWER PALEOZOIC of artiiog-dolgelley. 271 



marked contrast to the ashes of the [Upper Acid Series of 

 Cader Idris. 1 



In thin section the massive rocks of the Lower Acid Series prove very 

 disappointing. When viewed in natural light sections appear quite featureless ;. 

 while, as seen between crossed nicols, the rocks are uniformly cryptocrystalline 

 with only occasional quartz and felspar phenocrysts, which are more abundant 

 but less idiomorphic in those rocks that are obviously ashes than in those 

 that may represent flows. Faintly-pleochroic chlorite, occasional leucoxene, 

 and minute zircons may be present as accessories. No trace of perlitic or 

 spherulitic structure has been observed. It is also worthy of note that none 

 of these rocks appear to show the ' aschen-struktur ' that is so frequently 

 found in quartz-keratopbyre tuffs. They probably consisted originally of the 

 very finest dust of glassy material. 



A noteworthy feature of the Lower Acid Series in the field 

 is the persistency with which, throughout its outcrop, the strata 

 have been invaded by diabase intrusions. The intrusions, while 

 generally sill-like in form, often bear rather peculiar relationships 

 to the stratified rocks, the details of which will be described below 

 (p. 281). Where the volcanic series is comparatively thin, as in 

 the area east of the Gwynant, there ai-e usually a number of 

 separate but small intrusions. Individual intrusions may be only 

 a few feet thick, and yet may be followed along the strike over- 

 distances surprisingly great. 



(ii) The Pont Kings Slates, and the age of the Lower 

 Acid Series. — Interbedded among the rhyolitic rocks in the 

 western part of the area are two bands of slate, here termed the 

 ' Pont Kings Slates.' The slate-bands are separated one from 

 the other by typical rhyolitic ash, while the upper slate-band itself 

 contains thin bands of rhyolitic ash, and is followed by massive 

 ashes. This sequence can be followed over a distance of 2 or 3. 

 miles, and in numerous places all the junctions are well exposed. 

 There is, then, no room for doubt that it is a true stratigraphical 

 sequence, and that the slates are interbedded among the rhyolitic 

 rocks. 



The slates are of a rather dark blue, and they weather somewhat 

 greyish. Cleavage is well developed, though not sufficiently so to 

 render the beds workable for slates. A sort of incipient con- 

 cretionary structure is common, and the manner in which the slate 

 splits into large but thin sheets with rather irregular surfaces,, 

 recalls closely what is seen in the Tetragraptits Beds of South. 

 Wales and Shropshire. 



Fossils are scarce as a rule, though locally certain species may 

 be very plentiful. The general scarcity of fossils is the more 

 unfortunate, since these slates appear to contain the junction of the 

 Arenig with the Llanvirn Series. Numerous graptolites were 

 obtained from the lower slate-band where it gives rise to a small 

 tump on the 1000-foot contour-line, due south of Garth Angharad 

 and about a quarter of a mile west-north-west of Llyn Pen-Moelyn. 

 They included Didymograptus nitidus (Hall), small forms of" 



1 A. H. Cox & A. K. "Wells, ' The Ordovician Sequence in the Cader -Idris, 



District (Merioneth)' Eep. Brit. Assoc. (Manchester, 1915) 1916, p. 424. 



