﻿Vol. 66. ~] TREMADOC SLATES OF SOUTH-EAST CARNARVONSHIRE. 175 



Y Castell. Farther south-east, beyond Tremadoc Church, shivering 

 shales with graptolites are seen in Ynys-fadog, and are overlain by 

 the vesicular rock and a couple of ash-bands. At Bodawen gate a 

 flaggy ash-band underlies the Diplograptus-he&ring slate, which is 

 overlain first by the 30-foot mass of vesicular andesite of Ynys-hir,. 

 and then by the massive ashes and agglomerates of Y Nursery y 

 which are at least 50 feet thick. The rocks above these are dark 

 needle-slates, very like the crush-rock at Ynys-galch. At Ynys 

 Cerrig-duon, east of Portmadoc, another mass of ash is underlain by 

 an andesite the larger vesicles of which, not having come within 

 range of the dolerite metamorphism, still contain some zeolitic 

 minerals. 



The third range of andesitic rocks is much more massive, and 

 forms the hill-ridges of Y Fedw and Y Glog, as well as the out- 

 standing promontory on which the Roman Altar was built. Though 

 massive to a thickness of at least 100 feet, just south of Llyn Cwm- 

 bach, the outcrop thins to vanishing point at the western end of 

 that lake. It is also completely pinched out for a considerable 

 distance in the woods of Tan-yr-allt, between Y Glog and the foot 

 of the hill. West of Y Fedw, above Cwm-mawr House, the 

 slates for 150 feet or more below the agglomerate are very flaggy, 

 and enclose strings of thin ash-lenticles. These continue past the 

 Cwm-bach farmhouse, and approaching the Y Glog ash-mass are 

 separated from it only by a vesicular andesite. The section through 



Y Glog is, therefore, identical with that at Bodawen and Ynys-hir 

 mentioned above, and it is satisfactory to find that the slates among 

 the flaggy ashes at both places yield specimens of Diplograptus. 

 The small outcrop of ash in the wood, 150 yards north of Tan-yr- 

 allt House, is also underlain by vesicular rock. 



A fourth range of andesitic ashes and agglomerates forms the 

 culminating ridge of Pen-yr-allt. There are, however, no vesicular 

 rocks ; and, as the rocks below the ash-bands show a complete 

 and gradual passage down into slate, I wait for palaeontological 

 evidence before venturing to suggest that these rocks also belong 

 to the same horizon with the three outcrops below the dolerites 

 just discussed. 



A diagrammatic section from Tyddyn-dicwm to the corner of the 

 map passing through Y Glog and Pen-yr-allt is appended (PL XV, 

 section B). On this section it will be noted that the reduplicating 

 thrust-planes are occupied by the dolerite sills ; but, with this inter- 

 pretation, though I am unable to find any other explanation of the 

 phenomena, I am not altogether satisfied. 



The Banded Slates. 



Having now seen something of the structure of the country, we 

 are in a position to discuss the age of the grey and banded slates. 

 Since the latter overlie the graptolite-bearing Climacograptus-jpeltifer 

 beds of Tyddyn-dicwm, there is no reason to believe that they are 

 older than these ; but, pending the discovery of more fossils, we must 



Q. J. G. S. No. 262. n 



