240 ORIGIN OF THE ROCKS AND SCENERY OF NORTH WALES. 



Many of the rocks show a brecciated character, and Prof. Hughes 

 regards them as consisting of volcanic agglomerates and ashes. 



The full working out of these Pre-Cambrian rocks will require much 

 labour and perseverance. At present, however, we may make the 

 following statements with regard to them. The areas coloured as 

 intrusive rocks on the Survey maps in the county of Caernarvon consist 

 largely of stratified formations, partly of aqueous and partly of volcanic 

 origin. The volcanic rocks are partly of the nature of ashes and 

 agglomerates, as determined by Prof. Hughes in the Bangor area, and 

 partly true lava flows, as determined by Prof. Bonney for the Llyn 

 Padarn district. 



The Cambrian conglomerate, hitherto regarded as the oldest rock in 

 North Wales, was formed along the shore line of a land surface com- 

 posed of these older foundations. This is proved by the occurrence of 

 fragments of the older rocks, as pebbles in the Cambrian conglomerate 

 and by evidence of unconformabihty. 



These newly-recognised Pre-Cambrian rocks furnish the first chapter, 

 in the geological history of our country. Whether they belong to one 

 epoch, or to three as Dr. Hicks maintains, I am not prepared to state ; 

 but one thing is clear, they were produced by the destruction of still older 

 rocks by aqueous and igneous agencies. Speaking of the Cambrian con- 

 glomerate, Professor Kamsay states that it must have been derived from 

 the destruction of pre-existing land, of which we have no trace, except the 

 pebbles in this rock. We have now discovered a fragment of this old land. 

 We may stand on the surface of quartz-felsite hills about the lake of Llyn 

 Padarn and feel sure that we are on a land surface which existed before 

 one fragment of the rock that now forms the majestic range of Snowdon 

 and the Glyders had been laid down. But has this discovery given us a 

 definite halting place in our attempts to trace back the history of our 

 globe ? Not in the least ; for the materials of which this old land surface 

 is composed are but the fragments of an earlier world, of which Professor 

 Bamsay's remarks may be again repeated, to be again proved erroneous, 

 it may be, ty a race of future geologists. 



But to continue our geological history. The Pre-Cambrian rocks were 

 formed and then fashioned into a land surface before the deposition of 

 the conglomerate commenced. Of the real extent and character of this 

 land surface we know very little. It probably formed an extensive 

 continent having its surface diversified by high mountain ranges and 

 deep valleys. Into the sea of that period the river drainage of this 

 continent ran, carrying down immense quantities of sand and silt. The 

 sand would fall to the bottom in the neighbourhood of the shore, while 

 the mud would be carried further out to sea, as it is at the present day. 

 From these materials the grits and slates of the Cambrian period have 

 been formed, and the ripple drifts in these deposits give evidence of the 

 currents that flowed in this ancient sea. 



The entire thickness of the sedimentary deposits is many thousand 

 feet, and since we cannot suppose that any portion of this series was 



