Vol. 52.] LINGULA-FLAGS AND IGNEOUS ROCKS OF D0L6ELLY. 517 



On the southern side of the diabase-mass in the north of the 

 map, west of the Dolgelly fault, a similar series of volcanic rocks is 

 exposed, together with a certain amount of slate. They dip towards 

 the south, and appear to be the continuation of the sequence of 

 which the lower members are exposed on the northern side of 

 the diabase. The thickness of the black slate visible above the 

 lava-band in the Bryn-y-gwin woods is very much less than above 

 the equivalent band on Mynydd Gader; bat probably the upper 

 beds of the slates are concealed beneath the diabase. 



A series of ashes and tuffs of rhyolitic character is exposed in 

 the road and copses north of Ehydwen ; and a belt of ashes, 

 agglomerates, etc. runs from east to west between the two patches 

 of diabase immediately south of Dolgelly. The general dip in this 

 belt is southerly. Similar rocks form the bed of the Afon Aran 

 near Dolgelly and part of the eastern bank of the stream above 

 Pandy'r-odyn. 



It is only on Mynydd Gader that the age of these volcanic beds has 

 been proved by palaeontological evidence. There they undoubtedly 

 lie above beds which contain Dictyograptus flabelliformis, and 

 therefore they cannot be older than Tremadoc. 



But we cannot be far wrong in assigning a similar or later age 

 to the other patches, for nowhere in this district have we found 

 any evidence of so great a series of ashes and lavas among the beds 

 below the Tremadoc. 



It may be remarked here that in Aber Gwynant, a stream which 

 lies beyond the western boundary of our map, there is a sequence 

 not unlike that on the slopes of Mynydd Gader ; and here again 

 .it is not till we pass the Dictyograptus-sl&tes that we reach any 

 extensive series of volcanic rocks. 



So far as we have examined with the microscope the rocks here 

 spoken of as the Upper Volcanic Series, we find that they are 

 jhyolitic lavas, ashes, and tuffs ; while the band of lava in the 

 Lingula-be&s is an andesite. 



V. The Intrusive Diabases 1 and their Relations to the 

 Stratified Deposits. 



The most important areas of diabase in the district examined are 

 the Mynydd Gader mass, and the large patches which occupy so 

 much space in the northern half of the map. The latter, as will 

 be shown, are probably parts of a single mass. 



Besides these there are a number of smaller masses or sheets, the 

 positions of which are sufficiently indicated upon the map. 



The Dolgelly mass. — There can be no doubt that the more southerly 

 of the two patches of diabase which lie immediately south of Dolgelly 

 is continuous across the Dolgelly fault with the great mass which 



1 [Some account of tbe petrological characters of these diabases will be found 

 in Messrs. Cole and Jennings's paper (Quart. Journ. Geol. Soc. vol. xlv. 1889, 

 p. 431). We hope, at some future date, to deal more fully with the petrology 

 of the igneous rocks to which we have referred.] 



