GRITS OF CORWEN, NORTH WALES. 



209 



the direction of the second pressure may coincide with that of the 

 first ; and though perhaps the result may be more intense cleavage 

 in the previously cleaved rocks, no crossing of cleavage-planes could be 

 produced ; and, moreover, in cases where the second cleavage is very 

 intense, or the rock very susceptible of cleavage, this would entirely 

 obliterate all previously existing divisional planes, as notably in the 

 volcanic slates of the Lake-district, where we find joints and faults 

 quite healed, and the rock splitting along the cleavage-planes only. 



The beds below the Grits are, as a rule, bluish rab, i. e. a shivery 

 mudstone breaking along bedding, joints, and imperfect or double 

 cleavage into irregular, often somewhat prismatic fragments. Fre- 

 quently, however, these beds have, especiall}- where much weathered, 

 a pale grey colour, much resembling that of the beds above the Grits. 

 This may perhaps be because they are derived directly from the fel- 

 spathic ash of some volcanic district, while the Silurian beds above 

 the Grits have been formed later on from the waste of such beds. 



These pale beds above the Grits pass up into the " Pale Slates " of 

 the Survey, which in turn pass up into the striped flaggy Slates of 

 Penyglog, on the top of which come Grits to be referred to the true 

 Denbigh Flag- and Grit-series, and which I traced for about two miles 

 to the N. side of Moel Ferna. In the flaggy Slates I found Grapto- 

 lithus priodon, Cyrtograpsus Murchisoni, and JRetiolites, sp., with 

 Orthoceras pinmcevum and 0. subundulatum. I again found some 

 Graptolites in a small watercourse N.E. of Moel Ferna, on what 

 seemed to be the same horizon. The dip is here about 10°, ISLE, to 

 N.N.E., and the cleavage 30° N. I would here acknowledge much 

 useful information which I got from Mr. Phillips, of the Penyglog 

 Quarries. 



I then tried to apply this key to other districts. 



First, then, there is a patch of Bala coming in south of the fault 

 near Bryn Gorlan, at the south end of the Yale of Clwyd, as shown 

 in the section (fig. 2). 



Fig. 2. Section at Bryn Gorlan, Vale of Clwyd. 



N. Llanfair- 

 dyffrynclwyd 



i 



Bryn 

 Gorlan. 



c ~*7 e -f i tn. Pit 7i- 



a. New Red. b. Basement bed of New Eed, sometimes conglomeratic. 

 c. Carboniferous Sandstone, generally stained red. d. Mountain Limestone. 



e. Basement bed of Mountain Limestone (shale, sandstone, and conglomerate). 



f. Denbigh Flags. 



k. Pale slates (including part of what was previously called Bala). 

 I. Corwen Grit. on. Bala beds. 



Here I found at the north end the Denbigh Flags. These may 

 be examined along the road to LlanfairdyfErynclwyd, where Grapto- 

 lites (chiefly G. priodon) occur. South of the Bryn-Gorlan fault we 



Q. J. G. S. No. 130. p 



