60 REPORT — 1841. 



hills on the south bank of the Dee between Llangollen and Corwen, and in a large 

 portion of Denbighshire west of the vale of Clwyd. 



3. Slates and flags. — This is by far the most important and valuable portion of 

 the North Wales Silurians, since it contains numerous quarries that are extensively 

 worked in the hills north and south of the vale of Llangollen, and in the district be- 

 tween the rivers Clwyd and Conway. They are not however so durable as the true 

 Carnarvonshire or Cambrian slates, becoming brown and rotten on long exposure, like 

 the " mudstones " of Shropshire, of which they are proved to be the equivalents, both 

 by their place in the general section, and by the large Orthocera and Cardiolae found 

 upon the surface of some of the flags. The lowest beds rest upon true fossiliferous 

 lower Silurian rocks. 



In the chain of hills north of the great Holyhead road between Cerrig-y-Druidion 

 and Pentre Voilas, rocks of this section assume a new and very peculiar character. 

 They consist of numerous streaked beds of soft ferruginous schist, which soon crumbles 

 down into small fragments, and of intermediate beds of hard amorphous greywacke, 

 ten or twelve feet thick and several hundred feet apart in the upper part of the series, 

 forming the summits of the hills, and gradually diminishing in thickness downwards. 

 Collectively thej' cannot be less than 1200 or 1400 feet thick. As they occur on the 

 confines of two great centres of volcanic eruption to the south and west, and as the 

 beds along their whole line are everywhere thrown off from one or other of these cen- 

 tres, the author supposes that the greywacke beds have been formed by the mixture of 

 volcanic ashes with the sedimentary deposits ; and that after consolidation the schists 

 have been altered by long contact with heated matter, which in its struggles to escape 

 has first upheaved, and then burst through the surface ; those portions which covered 

 the immediate seat of the eruption having been altogether swept away, and the hills in 

 question forcibly tilted aside. This altered character gradually disappears to the 

 eastward of Cerrig-y-Druidion. All the lower division of the Denbighshire upper Si- 

 lurians is remarkable for the singular uniformity, parallelism, and streaked appeai'ance 

 of the beds ; for their freedom from twists and curvatures, and from cleavage ; for their 

 low angles of inclination, and for the general absence of organic remains. Their 

 total thickness is estimated at 3700 feet, and their parallelism and freedom from con- 

 tortion through so great a depth is attributed to the cessation of subterranean disturb- 

 ances through a long continued epoch ; a fact that would scarcely have been suspected 

 at so low a point in the scale of the sedimentary deposits. The streaked character is 

 supposed to be due to a slight admixture of felspar in some of the beds, which on 

 weathering acquires a paler hue, and which may have been derived from some distant 

 vent and diffused through the waters. Only one instance was met with of the beds 

 being thrown up into high angles. This was in the rugged district between Llangol- 

 len and the head of the vale of Clwyd, near the north-east end of the Berwyn range, 

 and exactly in the common line of strike of that and of two other independent moun- 

 tain chains. Here they converge as to a focus, and appear to have been upheaved 

 by so many distinct masses of pent up elements in their struggles to escape, which, 

 meeting in a common centre, have by their combined forces eiiected this tremendous 

 convulsion. 'J'he rocks are thrown into inconceivable disorder, the beds dipping at 

 all angles; along the whole extent of one mountain 1200 feet high and above half a 

 mile long, they are absolutely perpendicular, and seem to have been rifted and tossed 

 on end ]}\ one entire mass. It is in the midst of this scene of devastation that the 

 Dee " winds her wizard stream " through the vale of Llangollen ; but the harsher 

 features are worn down and concealed under the green slopes and hanging woods 

 which compose the rich scenery of this celebrated spot. The author then compared 

 the Denbighshire upper Silurians with their soft equivalents, the " mudstones " of 

 Montgomeryshire ; and showed their connection by instances of rocks of similar age, 

 but of an intermediate character, from other localities. The chief difi'erence is in the 

 lower division. The same species of fossils occur in the contemporaneous beds of 

 each portion in both districts, and in the same geological sequence, but very sparingly 

 in Denbighshire ; also the same general parallelism and low angle of dip ; the same 

 character of the jointed structure; and the same tendency to concretions both in beds 

 and in detached hailstones. 



The paper concluded by showing, from the altered appearance of the upper Silurian 

 group in North Wales compared with those of Shropshire, the still greater differences 

 that may be expected to occur in distant countries ; that little or no stress can there- 

 fore be laid on mere lithological structure ; but that as beds of the same age in widely 



