H. C. Sargent — The Penmaenmawr Intrusions. 17 



After a careful examination of the ground, the writer submits that 

 this extension of area is not supported by field-evidence. All the 

 valleys in the area covered by the map (Fig. 1) are so choked with 

 drift that accurate mapping of the boundaries is impossible. The 

 streams have nowhere cut through the drift down to solid rock, 

 except at one point where the Afon Maes-y-bryn traverses a dyke. 

 Here the valley is suddenly constricted so as to form a narrow gorge, 

 and it widens out again immediately below. 



The drainage system appears to have been initiated in pre-Glacial 

 times, since the valleys were carved out pre-glacially, and it is 

 suggested that the three streams, which unite at the foot of Dinas 

 to form the Afon Llanfairfechan, would scarcely have cut deep 

 channels through a particularly hard igneous rock, when in other 

 directions a choice of soft beds was readily available. Further, the 

 rock at the foot of Dinas on the south and south-east sides of the 

 mountain, that is to say not far from the centre of the mass as 

 mapped by the Geological Survey, is distinctly of the marginal type 

 referred to below, and this is sufficient to show the improbability 

 of Dinas having originally extended to any considerable distance 

 beyond its present limits, in the direction of Carregfawr or of the 

 Bala lavas on the south-east. The surface of the ground in the 

 neighbourhood of the intrusions is strongly glaciated, in places up 

 to 1,100 feet above O.D. ; and around Dinas and Carregfawr, owing 

 to the covering of drift, it is only here and there that the Ordovician 

 beds are exposed. 



The summit of Penmaenmawr, an abrupt conical peak, appears 

 at a distance to rise from a level table-land at the height of about 

 1,100 feet above O.D. On the ground it is seen that this table-land 

 has a very uneven surface. An abrupt ridge varying from 200 to 

 300 feet in height above the surface of the table-land runs round the 

 south and east sides from Clip yr Orsedd to Graig Lwyd (Fig. 2). 

 The central portion of this eastern lobe of the outcrop is occupied 

 by a depression filled with drift, some of which appears to be of 

 local origin from the east. 



Around Penmaenmawr the slates are frequently exposed, and 

 a study of their behaviour towards the intrusive rock is not without 

 interest. Contact- between the two may be seen at the entrance to 

 the most easterly of the Graig Lwyd quarries, and the sedimentary 

 beds have here been so intensely baked to the thickness of 4 or 5 feet 

 that it is hard to determine in the field the precise line of demarcation 

 between them and the intrusive rock. 



In the same locality, that is to say on the east of the intrusion, the 

 slates are more perfectly cleaved than elsewhere, with nearly vertical 

 cleavage-dip, and as shown on the Map (Fig. 1) a close parallelism 

 is preserved between the cleavage-strike and the boundary of the 

 igneous rock. On the southern side the sedimentary beds are 

 imperfectly cleaved, the dip is lower than on the east, and the 

 parallelism is lost. 



Two inferences appear to be allowable from these considerations : 

 (1) that the pressure which produced the cleavage acted here in 

 a more or less east to west direction with the intrusive mass as 



DECADE VI. — VOL. II. — NO. I. 2 



