and the Palceozoic St/stem of England. 3p5 



great inclination and pass under the regular terrace of thCiWeji^- 

 lock shale. j, ; 



An important negative fact is presented by this section. It 

 gives no trace of the May Hill sandstone; and there is no pass- 

 age between the soft earthy beds of the Mathyrafal group and 

 the beds of Wenlock shale which rest upon them, the upper 

 formation having been brought over the lower by an unconform- 

 able overlap. If, in any single section near the cuttings of the 

 new road, there be the appearance of such a passage, it is, I 

 believe, deceptive. 



The same remark I would now apply without hesitation to the 

 mistaken appearance of a passage between the Cambrian rocks 

 of Glyn Ceiriog and the overlying beds of Wenlock shale, as 

 seen a few miles to the south of Llangollen, where, in like man- 

 ner, the May Hill beds are entirely wanting. I may further 

 remark, that when two unconformable deposits are composed of 

 soft yielding materials (like those just alluded to), it is often 

 impossible, on the evidence of one section, to conclude that they 

 are the conformable portions of a regular and uninterrupted 

 sequence. The lower soft and yielding beds, when pressed down 

 by the superincumbent mass, may give a false appearance of 

 conformity, which may become still more deceptive in cases 

 where both the upper and lower masses have been afterwards 

 subjected to common movements of contortion. On the con- 

 trary, where the lower beds are hard and inflexible, a discordancy 

 of position becomes at once palpable and obvious. Thus, in the 

 neighbourhood of Welch Pool, we have, between the hard and 

 highly inclined Cambrian rocks (which have, I now think erro- 

 neously, been called Caradoc sandstone), and the overlying flag- 

 stones of the Wenlock age, very obvious cases of unconformity. 



In the hope of making these conclusions clear, I subjoin three 

 ideal vertical sections, two of which (figs. 2 and 3) show the 

 actual sequence of deposits to the east of the Berwyn chain {e. g, 

 at Mathyrafal and Glyn Ceiriog), while the other (fig. 1) shows 

 a similar sequence, among rocks of nearly the same age, on the 

 west side of the Berwyn chain on the line of the Holyhead road. 

 Of these sections, fig. 1 gives a fine exhibition of the May Hill 

 sandstone overlaid by the Denbigh flags, Wenlock and Ludlow, 

 and underlaid discordantly by a part of the great Bala group. 

 Fig, 2 represents a case like that of Mathyrafal, where the May 

 Hill sandstones and conglomerates disappear, and the Wenlock 

 groups rest, with a slight discordancy of position, upon the Cam- 

 brian rocks, which here belong to a high part of the Bala group. 

 Fig. 3 represents the case of Glyn Ceiriog, where there is no 

 apparent discordancy of position, but where the May Hill group 

 is entirely wanting. In one respect all these sections are imper- 



FUl. Mag, S. 4. Vol. 8. No. 52. Oct, 1854. X 



