294 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Names of Genera and 

 Species. 



East of the Axis, Northern part of the Trough. 



West of the Axis, South 

 portion of the Trough. 



Holyhead Road. 



Dolwyd- 

 delan 



Valley. 



Chertv bed of 

 Yr Arddu. 



42i 

 miles. 



43 

 miles. 



Bettws- 

 y-Coed. 



Water- 

 loo 

 Bridge. 



miles. 



Trinucleus Caractaci 



Spirifer plicatus 



Orthis canalis 



+ 



+ 

 + 



+ 



+ ? 

 + 



+ 



+ 



+ 



+ 



+ 



+ + 

 + 



basalis .'(Da/m.) 



expansa (or "I 



0. pecten) J 



flabellulura — 



Leptsena sericea 



Porites pyriformis . . . 



Middle and eastern side of the Carnarvonshire trough. — The beds 

 in the middle and on the eastern side of the Carnarvonshire trough, 

 extending from the synclinal axis to the Bryn-y-ddinas fault, consist 

 of slate and schists, alternating occasionally with hornstone. The 

 author has heard of only two places where limestone has been found ; 

 the first to the west of Gwydir Park, on the west side of Nant 

 Gwydir ; the other half a mile to the west of Trefriew. 



The whole district is intersected by numerous dikes or beds of 

 green.'jtone, which strike with the beds, and are apparently interstra- 

 tified with them. They are found however, when minutely ex- 

 amined, to thin out irregularly, and occasionally to cut through the 

 beds: they usually make but little disturbance in the strata; but 

 the greenstones on the lines of the synclinal axis and of the Bryn-y- 

 ddinas fault are exceptions to this rule. The mineral character of 

 the beds however is much altered by these igneous rocks : thus 

 the hornstone is converted, in many instances, into semi-crystalline 

 quartz-rock ; and probably the quality of the slates depends very 

 much on the distance or proximity of the greenstones. 



The slate quarries in this district are very numerous, and are only 

 second in importance to those on the western flank of the Snowdon 

 chain ; but on the eastern side of that chain scarcely any two quar- 

 ries are worked on the same bed, and few beds are worked to any 

 great distance : for many a bed, which in one part of its course fur- 

 nishes excellent slate, in another part, not remote from the former, 

 is found to be so altered in quality as not to be worth working ; still, 

 regarded geologically, and without reference to their economical 

 value, the beds of slate are found to be persistent. 



Since the direction of the synclinal axis, for the greater part of 

 its course, is north-east and south-west, and that of the Bryn-y-ddinas 

 fault N.N.E. and S.S.W., the breadth of the space included between 

 the axis and the fault increases considerably as you proceed south- 

 wards. The prevailing strike of the beds is parallel to the synclinal 

 axis, and consequently, on the eastern side of the trough, the Rhaiadr- 

 cwm section presents a much longer series of beds than the section 



