112 



HARDWICKE'S SCIENCE-GOSSIP. 



•well-known features are distinctly visible. To the 

 left is a glimpse of the sea high up in the horizon. 

 Moel Hebog, the Glaslyn and pont Aberglaslyn are 

 also distinguishable. The Glaslyn runs like a silver 

 .streak through a mass of green fields to the Traeth 

 Mawr. Beyond is the embankment across the marsh 

 for the road and railway terminating in Port Madoc, 

 the houses showing distinct and clear with Moel-y- 

 geist in the back-ground, and in the far distance 

 -stretches the long promontory of Caernarvonshire till 

 almost lost in the blue haze of the distant sea. The 

 Rivals showed like little cobs on the relief map of 

 Caernarvonshire. Further to the west shone the 

 brilliant orange sands of the estuary of the Dwyryd 

 below Tan-y-Bwlch, and beyond this was the sea with 

 its shore sweeping round to Harlech Castle, which, 

 with its towers, appeared as a little group of dots. To 

 the south could be seen the Rhinogs, and the long 

 scarped face of Cader Idris beyond, and, to the east, 

 appeared a sea of mountains out of which arose the 

 Arenigs and the Arrans. To the north the land rose 

 and fell in billowy swells till lost in the grey haze. 



The immediate foreground of the view over Snow- 

 donia is occupied with the remarkable mountain 

 called Cynicht. From the road between Tan-y-Bwlch 

 and Bethgelert this mountain looks like a pyramid ; 

 but it is there seen in profile. From the summit of 

 Moelwyn we see it as a long ridge with its flanks 

 scored with gullies and talus, which traverse its steep 

 sides like streams till they become confluent in the 

 talus cones at the foot. Immediately below, to the 

 left, were the rocky north-west precipices of Moelwyn. 

 The day was a perfect day, the clouds floating high, 

 •the air clear and exhilarating, yet warm. 



I have dwelt upon this view perhaps more than a 

 geological article warrants ; but let us pause and 

 consider if it will yield us any scientific information. 



The traveller about Ffestiniog will soon find out, if 

 he carries a compass with him, that the general strike 

 of the rocks is from south-west to north-east. At 

 right angles to this the strata have been thrown into 

 a series of anticlinal and synclinal folds, broken up, 

 and, to some extent, obscured, by faults, it is true. 

 Perhaps this feature in the structure of the country 

 ■can be best appreciated in the general view of the 

 mountains of Snowdonia obtained on the coast road 

 between Maentwrog and Harlech. It can, however, 

 be observed on Moelwyn itself. A slate quarry on 

 the back of Moelwyn shows the rock to dip rapidly 

 to the north-west. Without going into details, the 

 structure of the mountain is a series of shales and 

 slates, with an interbedded massive series of felstones 

 and felspathic ashes. 



It is these hard massive beds which form the grand 

 ■scarp in which lies Llyn Trwstyllon. The whole of 

 these beds belong to the Lower Silurian series, 

 commencing with the Lingula beds in the Ffestiniog 

 and Tan-y-Bwlch valley, and terminating in the Bala 

 .beds at the summit of Cynicht. The slates of 



commerce are interbedded in the series, and as the 

 beds dip steeply to the north-west the quarrying 

 operations have to be mostly followed by galleries, 

 and not in great cuttings open to broad daylight, as 

 is the case with the quarries at Penrhyn, near 

 Bangor, which lie in the older Cambrian slates. To 

 the north, in the valley of Dolwyddelan, the cal- 

 careous ashes there largely developed are the actual 

 representatives of the Bala limestone and the Caradoc 

 sandstone of Shropshire, and the vast masses of ashes 

 that crown the felstones of Snowdon and Moel 

 Hebog are but an enlarged development of the same 

 strata.* 



To understand the present surface form of the 

 country, it is requisite to keep in mind the great fact 

 that the whole of the Upper Silurian strata which 

 formerly covered Merioneth and Caernarvonshire has 

 been entirely removed by denudation. It is only 

 when we get as far to the south-east as the river 

 Vyrnwy, where the great reservoir to supply Liverpool 

 is being constructed, that we come upon the remains 

 of the Upper Silurian, here preserved in a synclinal. 

 A general glance at the geological map of North 

 Wales shows the persistent strike of all the rocks 

 from south-west to north-east. It is along these 

 lines that the denudation has principally acted, 

 many of the main valleys possessing the same paral- 

 lelism of direction. The hard beds of felstone and 

 ash, and the intrusive greenstones and other igneous 

 rocks, have helped to preserve that peaked and ridgy 

 character which here gives the distinguishing beauty 

 to the scenery. 



A walk down the north-west slope of Moelwyn 

 brings us to Bwlch Cwm Orthin, a pass between 

 Cwm Orthin and Cwm Croesor, which lies between 

 Cynicht and Moelwyn. Here we may stop to 

 examine some slate works. The slates are gener- 

 ally of small size, but beautifully true and fine. 

 Descending the path to Cwm Orthin, we get a good 

 view of the Llyn below, now being rapidly filled up 

 with the debris from the Cwm Orthin slate quarries. 



At the entrance to the cwm may be seen those 

 well-rounded rocks specially noted by Ramsay as 

 good instances of roclie moutonnce glaciation. Beyond 

 these we may again examine slate works. Here some 

 of the slate is of that peculiarly fine and soft nature 

 which fits it for manufacture into school slates, the 

 process of which may be watched. I impressed on 

 my mind the view of Cwm Orthin looking towards 

 the glaciated rocks, in the best possible way, by 

 sketching it. It is a true rock basin, the dip of the 

 strata to the north-west and the hardness of the 

 felspathic rocks at the outlet, no doubt being deter- 

 mining causes, together with ice, in producing this 

 form of denudation. A steep down-hill walk brings 

 us to Tan-y-Grisiau station, but we may pause a 

 moment to look at the waterfall. The stream from 



* Ramsay, " Memoir of the Geology of North Wales," ist 

 ed. p. 95. 



