Yol. 62.] IGNEOUS AND SEDIMENTAKY KOCKS OP LLANGYNOG. 225 



1 mile east of Llangynog village, but where no igneous rock is 

 now to be seen. Strangely enough, the Lambstone porphyry is not 

 represented on his map. 



Of his three patches of ' volcanic grit/ one appears to be the 

 Didymograptus-bifidus grit of Castell-y-garthen ; a smaller patch 

 to the north-east of this is probably a grit of like age, which 

 crops out 400 yards west of Lambstone Farm ; while the third is 

 evidently the Tetragraptus-gr\t of Pen-y-Moelfre. 



He represents the sedimentary rocks associated with the igneous 

 rocks as ' Llandeilo Elags ' and ' Upper Silurian ' rocks : the ' Llandeilo 

 Flags' aveTetragrap>tus-Beds (Arenig); the supposed ' Upper Silurian ' 

 rocks are certain green marls, cornstones, and sandstones, which 

 form tbe basement-beds of the Old Red Sandstone of the district. 



On his section (pi. xxxiv, fig. 11), which is drawn along a 

 north-north-west and south-south-east line through Pen-y-Moelfre, 

 Murchison represents the [Lower] Silurian rocks of that hill as 

 undulating, not vertical, and as containing bands of volcanic grit ; 

 and a boss of trap is shown as bursting up between the Lower 

 Silurian rocks, and what (from the colour on the map) he appears to 

 have meant for Upper Silurian, but which are now known to be 

 Old Red Sandstone. These supposed [Upper] Silurian rocks are 

 represented as faulted off from the Old Red Sandstone. The last- 

 mentioned boss of trap does not appear on the map, and does not 

 indeed occur on the ground through which the section is drawn. 



De la Beche l refers to the presence of conglomerates among the 

 Lower Silurian [Ordovician] rocks of the district. The original 

 Geological Survey-map, Sheet 41 (published in 1845), shows the 

 igneous rocks at Coomb and Lambstone as ash-beds, but the 

 rhyolite of Capel Bethesda is included in the diabase north of 

 Pen-gelli-uchaf and coloured as a greenstone -dyke. The diabase of 

 Tre-hyrn is not shown, and much of the ground occupied by 

 andesites near that place is thrown into the sedimentary series. 



The horizontal section, Sheet 2, Section 6, published by the 

 Geological Survey in 1844, is drawn along a north-west and south- 

 east line through Pen-y-Moelfre ; it represents that hill and also 

 Moelfre Wood as anticlines, with a syncline between them. The 

 Lambstone igneous mass is shown as ' trap-rock,' and bleached 

 shales are noted near the ' trap/ 



The late Thomas Roberts 2 noted that ' the rock marked Fsb 2 

 on the Survey-map, near Llangynog, appears to be a diabase.' It is 

 probable, although not certain, that in this passage he referred to 

 the diabase of Tre-hyrn. 



We have given a brief account of the igneous rocks in the 

 ' Summary of Progress of the Geological Survey ' for 1904. 3 



1 ' On the Formation of the Kocks of South Wales & South- Western England ' 

 Mem. Geol. Surv. vol. i (1846) p. 29. 



2 Quart. Journ. Geol. Soc. vol. xlix (1893) p. 170. 



3 Mem. Geol. Surv. 1905, p. 37. 



