542 J. R. Daliijns 8f E. Greenly — Fehitic Slates of Snowdon. 



in 1903, saying that he thought the volcanic eruptions in the West 

 Indies threw some light on their origin. Hence this paper, in which 

 he sets forth his views on the subject. He proposed that we should 

 write a joint paper, but I did not see my way to doing this, as I had 

 no definite area mapped, and had not separated the cleaved felsitic 

 rocks of doubtful character from the undoubted lavas and ashes. 



I ought to say that the rocks like felsites in which I found fossils 

 in 1900 occur in the calcareous felspathic ashy series near the base 

 thereof ; but are coloured pink on the Geological Survey map as part 

 of the lower Snowdonian felstones. I have since then found fossils 

 in another locality in an area so coloured, but not in a rock looking 

 like a felsite. The fossils which I found in such a rock in 1900 

 were kindly named for me by Mr. E. T. Newton. They were 

 StropJiomena and Orthis ; but the specimens were too poor for 

 specific determination. The day after finding these fossils I 

 discovered a small area of slaty I'ocks containing fossils, from 

 1,000 to 1,300 yards west of the summit of Moel Meirch, and 

 entirely surrounded by the felstone of which that hill is composed. 

 The relation of the fossilifevous rocks to the felstone is not clear. 

 I suppose it to be an outlier, possibly faulted, of the calcai'eous ashes 

 which, according to the Geological Survey map, overlie the felspar 

 of Moel Meirch. 



Though I have not finished mapping any well-defined area 

 occupied by the cleaved felsitic rocks which form part of the lower 

 Snowdonian felsites, I may point out some of the places where they 

 occur and also places where they are of a fragmentary character. 

 Cleaved felsites occur in Cwm Llan, extending from Lliwedd past 

 Geuallt and along the lower slopes east of Graig Wen. Their 

 fragmentary chai'acter is well seen in Cwm Llan near the Gladstone 

 monument. How like these rocks are in some places to the over- 

 lying ashes is shown by the fact, mentioned in the Geological 

 Survey Memoir on North Wales (p. 151 of the second edition), 

 that Mr. Selwyn had great difficulty in drawing a line of demarcation 

 between them on the side of Cwm-y-llan near Yr Aran. It seems 

 to me that the chief distinction in many places between the two sets 

 of rock is that one is generally bedded and tlie other not. Though 

 I have failed to find anything like stripe or lamination due to 

 bedding among the cleaved felsites of Geuallt, yet I have noticed 

 a set of features, roughly parallel to the bedding of the ashes, 

 running across the southern face of the hill. These are very 

 conspicuous in a good light, and are suggestive of bedding; but 

 they may be due to jointing. Cleaved felsites also extend from 

 Bryn Gwynant, past Llyn Du and Hafod Owen. Their fragmentary 

 character is well marked in Coed Eryr and Coed yr Odyn. After 

 crossing the line of the north-westerly fault at Perthlwyd, the rock 

 forming the mountain between Llyn y Ddinas and Nant y Mor is 

 felstone of the compact type, like that of Crib Goch, and is in places 

 spherulitic. Cleaved felsites also occur in the outlier of felstone 

 above Gelli lago. The greater part of this outlier is compact 

 uncleaved felstone with lines of viscous flow ; but a small portion of 



