112 PROCEEDINGS OP THE GEOLOGICAL SOCIETY* 



part of the group to much more acid masses in the higher part. In 

 the Survey map numerous sheets of intrusive " greenstone " arc 

 shown traversing the Lingula Flags, Tremadoc slates, and lower 

 part of the volcanic group along the northern slopes of Cader 

 Idris. The true intrusive nature of much of this material is clearly 

 established by transgressive lines of junction and by contact-meta- 

 morphism, as well as by the distinctive crystalline texture of the 

 rocks themselves. But the surveyors were evidently puzzled by 

 some parts of the ground. Sir Andrew Eamsay speaks of " the 

 great mass of problematical vesicular and sometimes calcareous 

 rock which is in places almost ashy-looking." After several 

 oscillations of opinion, he seems to have come finally to the con- 

 clusion that this vesicular material, which occurs also in the upper 

 part of the mountain, passes into and cannot be separated from the 

 undoubted intrusive "greenstones " *. 



The true solution of the difficulty will be found, I believe, in the 

 recognition of a group of scoriaceous lavas among these greenstones. 

 The presence of a cellular structure is not unknown among dykes, 

 but, so far as my experience goes, it is comparatively rare among 

 sills, and when met with in these, occurs rather locally. That 

 some of these Cader-Idris amygdaloids were really poured out at the 

 surface can be demonstrated from the volcanic conglomerates 

 associated with them. Below Llyn y Gadr — the dark tarn at the 

 foot of the vast wall of Cader Idris — the beds of coarse volcanic con- 

 glomerate, to which I have already alluded, are largely composed of 

 blocks of the vesicular " greenstones " on which they lie. These 

 " greenstones," moreover, have many of the most striking cha- 

 racteristics of true lavas. They are extraordinarily cellular, their 

 upper surfaces sometimes present a mass of bomb-like slags with 

 flow-structure, and the vesicles are not infrequently arranged in 

 rows and bands along the dip- planes. 



A microscopic examination of two slides cut from these rocks 

 shows them to be of a trachytic or andesitic type, with por- 

 phyritic crystals of a kaolinized felspar embedded in a microlithic 

 groundmass. The rocks are much impregnated with calcite, which 

 fills their vesicles and ramifies through their mass. 



A few miles to the east some remarkable felsitic rocks take the 

 place of these vesicular lavas immediately below the pisolitic iron- 

 ore. I have not determined satisfactorily their relations to the 

 surrounding rocks, and in particular am uncertain whether they 



* Mem. GeoL Survey, vol. iii. 2nd ed. p. 36 ; see also pp. 31, 32. 



