238 ORIGIN OF THE ROCKS AND SCENERY OF NORTH WALES. 



The order of succession in its main features had, however, been 

 determined by Sedgwick before the surveyors examined the district. He 

 found that the conglomerate beds and banded slates of Llyn Padarn are 

 inferior to the purple slates of the Llanberis and Penrhyn quarries, and 

 that these again underlie the great mass of felspathic rocks of which the 

 Snowdon group is composed. That this succession is correct, any visitor 

 to the beautiful district of Llanberis may verify in a few hours by first 

 working along the northern shores of the lake, from the quartz felsite to 

 the purple slates of Yr-Alt-Wen, and then by crossing the stream joining 

 the two lakes by the bridge, and continuing the section up the pass of 

 Llanberis, casting a glance now and then to the heights of Derwyn and 

 Yr Tryfan on the right, to keep the eye familiar with the general dip of 

 the beds. Sedgwick, however, did more than this. He examined the 

 rocks on the south-east of Snowdon, and proved that the average dip was 

 exactly opposite to that on the north-west ; that Snowdon, in fact, like 

 many other mountains, forms the centre of a great synclinal trough. 



The Survey verified Sedgwick's conclusions, and also mapped the 

 district, so that the exact distribution of every important rock is represen- 

 ted on their maps. By comparing the dip of the beds with their surface 

 distribution they have succeeded in constructing sections along certain 

 lines which represent the arrangement of rocks beneath the surface. 



An examination of these maps and sections shows at once that the 

 prevalent trend of the various beds of rock is N.E. and S.W. ; that this 

 trend is determined by the folding of the beds along axes running in the 

 same direction ; that the most important of the foldings in the district we 

 are considering is the synclinal which runs from Moel Hebog through Snow- 

 don to Carnedd Llewellyn and Carnedd Davyd ; and that this great 

 synclinal is not simple in structure, but contains in itself, so to speak, 

 a number of minor anticlinal and synclinal folds, examples of which may 

 be actually seen in the almost vertical cliffs of Lliwedd, and inferred 

 from the peculiar oval outcrop of certain well marked beds of igneous 

 rock in the valley above Capel Curig, and in the neighbourhood of Llyn 

 Ogwen. 



The courses of all the more important faults are also indicated on 

 these maps, and the actual extent of the disturbances, amounting in 

 some instances to thousands of feet, are shown in the sections. 



The structure of the district, as thus determined, enables us to trace 

 the history of events in North Wales. Within the last two or three 

 years some most important additions to the early history of North Wales 

 have been made by Profs. Hughes and Bonney and by Dr. Hicks. The 

 Geological Surveyors believed that there was no satisfactory evidence of 

 any rock older than the conglomerate, described at the commencement of 

 this paper. This opinion you will find recorded in the maps and publica- 

 tions of the Survey, and it has been again reiterated by Prof. Ramsay 

 in his most philosophical book on the Physical Geology and Geography 

 of Great Britain, published only last year. Thus on page 67, speaking 

 of the conglomerate, he says : — " The country from which these pebbles 



