﻿558 DE. T. GEOOll AND MB. P. LAKE OK THE [NoV. I908, 



others are more irregular, and they are of all sizes up to at 

 least 6 inches across. In the quarries on the right bank of the 

 Ceiriog, the fragments of slate show a distinct tendency to lie 

 obliquely to the bedding, the inclination being downwards from 

 south to north, apparently indicating that the current in which they 

 were deposited flowed from north to south. In the upper portion of 

 the gritty series numerous fossils have been found, and among the 

 chief localities may be mentioned the northern end of the quarries 

 on the right bank of the Ceiriog at Pandy ; the old tramway-cutting 

 along the southern bank of the Teirw ; and some small openings 

 farther up this stream. Elsewhere in the area represented on the 

 map (PI. LlII) there are no very good exposures of these beds. 

 jS'ear Spring Hill, as has already been mentioned, the ash is overlain 

 immediately by sandstone ; while at Llechrhydau the beds that 

 immediately overlie the ash seem to have been cut out by a fault. 

 The sandstones have been extensively quarried near Pandy for 

 paving-stones, road-metal, and concrete. 



The beds that rest upon these sandstones and grits are fairly 

 exposed in the woods of Craig-y-gelli and Craig-y-Pandy, and also, 

 less perfectly, on the right bank of the Ceiriog below Pandy. They 

 consist chiefly of slates, with more or less numerous bands of 

 sandstone. The sandstone-bands vary in thickness, from an inch or 

 less up to about 4 feet. 



The Teirw Beds are characterized especially by the abundance of 

 gasteropods and lamellibranchs. Many of these pass up into the 

 succeeding groups, but Bellerophon nodosus, Cydonema crebristrict ^ 

 Euomjphalus corndensis (?), and Modiolopsis M'-Coj/i are, so far as our 

 observations go, confined to this series. Lingula tenuir/ranulata and 

 Asaphus Poivisi are also characteristic, and do not occur in the beds 

 above ; L. tenuigranulata, however, is rare or difiicult to find. It may 

 be remarked here that the greater number of these characteristic 

 forms occur only in the sandy and ashy beds at the base of the series. 



Fossils are sometimes abundant in layers so rich in volcanic 

 material as to deserve the name of ash. Thus, in an old roadside 

 quarry east-south-east of Llechrhydau, thin layers of ash are 

 crowded with complete specimens of Plectambonites sericea and other 

 fossils. 



On the following page (559) is the list of the fossils that we have 

 identified from the Teirw Beds. 



(3) Craig- y -Pandy Ash. — In the area of our map (PL LIII) 

 the westernmost exposure of this ash occurs in the neighbourhood 

 of Ty'n-y-pistyll. Prom the farmyard it runs down into the Teirw, 

 and then slants gently up the northern bank of the stream towards 

 the east. Owing to a small fault, it terminates abruptly in a 

 prominent crag, which rises through the wood about 400 yards 

 below Ty-du ; but it reappears a little farther north in the rugged 

 eminence near Ty-isaf. Here again it is displaced by another fault, 



