Vol. 65.] HAKTFELL-V ALEUTIAN SUCCESSION ABOUND PLYNLOIOX. 509 



graptolites appear. Fossils were collected from several places 

 along the road down into the valley from Yspytty Cynfyn, the 

 following being identified : — Monograptus becki, M. marri, 31. nudus 

 var. variabilis, M. resurgens, and M. nmcinatus (?). 



At the point where a small stream crosses the road Rastrites 

 linnaii, Monograptus exiguus, and M. nudus var. variabilis were 

 collected (F. 45) ; this is the lowest horizon at which I have found 

 M. exiguus, a species which becomes so abundant in higher strata. 

 Nearly opposite this locality the Afon Myherin turns almost at 

 right angles, from an east-and-west course in the upper part to a 

 north-and-south course. A very fair section is exposed along the 

 steep slopes and numerous crags on both sides of the upper portion 

 of the valley, which is nearly 2 miles long. 



In the first part of the section the beds are somewhat similar to 

 those already passed over ; but the grit-bands are noticeably fewer 

 and thinner, being often mere stripes. The intervening mudstones 

 accordingly appear more massive: their prevailing colour is a pale 

 greenish blue. Some poorly preserved graptolites were obtained 

 from the north side of the valley about half a mile east of the bend : 

 Monograptus nudus var. variabilis was the commonest form. 



The rocks appear to dip eastwards at about 30° to 50° along 

 the whole length of the valley, and this seems to point to an 

 enormous thickness of strata. It is, however, doubtful whether 

 the easterly dip is as persistent as it appears, for gaps fre- 

 quently occur where little or no rock is exposed ; and, as we have 

 seen, such gaps usually coincide with an undulation of the strata 

 or with a belt of compression. If, as I have reason to believe, this 

 is a fairly general rule, it greatly reduces the thickness as calculated 

 from the visible dip and the length of the exposures. 



Despite the folding, however, the section is an ascending one 

 towards the east, for any compression which may take place at the 

 gaps is not sufficient to counteract the visible easterly dip. Of 

 course, this argument would be of no value in the presence of 

 strike-faulting : but, so far as I am aware, such faulting is rare in 

 these rocks, and its effect may therefore be ignored. Moreover, 

 certain changes in lithology become apparent in ascending the 

 valley, which are consistent with the view that one is dealing with 

 a rock-sequence. Gritty bands almost disappear, and mudstones 

 become dominant; the colour of the rocks gradually darkens, and 

 frequent bands of dark shale weathering in pronounced rusty-red 

 and brown tints occur among them. Most of the shaly bands con- 

 tain graptolites ; but the persistent cleavage makes their extraction 

 and subsequent identification somewhat difficult. 



About halfway up the valley the Myherin is joined from the 

 north by Nant-y-Creiau, which emerges from a gloomy ravine 

 where some mining operations were carried on not long ago. 

 About a quarter of a mile to the east the south side of the Myherin 

 Valley is furrowed by a small gully exposing a section in soft, 

 pasty, blue-hearted mudstones which are intensely cleaved. They 



