Physical Geography. 67 



Leaving these details of stratification, I will now 

 endeavour to catch a glimpse of the physical geography 

 of our area during the time that the Cambrian, Lin- 

 gula, and Tremadoc rocks were being deposited. I have 

 already stated that the purple strata of the Cambrian 

 series seem to me to have been deposited in inland 

 fresh waters, subject to influxes of the sea, probably 

 owing to occasional subsidence of the land ; in the same 

 manner, for example, that in Tertiary times the Miocene 

 strata of Switzerland and the Ehine were deposited in 

 great fresh-water lakes, in areas that underwent local 

 temporary submergence. The thick strata of sandstones 

 in the Cambrian rocks of Merionethshire, indicate the 

 neighbourhood of land, and in Caernarvonshire the 

 numerous beds of sandstone and coarse conglomerate 

 interstratified with mud deposits now slates, point not 

 only to the proximity of land, but even give a clear idea 

 of the kinds of rock of which that land was made. 

 ' In the 8,000 feet of these rocks in Merionethshire 

 there is very little slate, and even the 700 or 1,000 

 feet of interstratified slaty beds in Caernarvonshire are 

 quite subordinate to the grits and conglomerates. * * * ' 

 ' The structure of this land may be partly inferred 

 from the nature of the pebbles in the conglomerate, 

 which are water-worn, and consist of purple and black 

 slates, quartz-rock, felspathic traps, quartz-porphyries, 

 and jaspers.' 



The country from which these pebbles were derived 

 must, indeed, have physically resembled North Wales 

 of the present day, ' but except these pebbles no 

 trace of that land remains in or near North Wales/ 1 

 Fragments of this old continent, however, probably 

 still exist in the Laurentian rocks of the Outer He- 

 1 The Geology of North Wales,' p. 160. A. C. Ramsay. 

 F 2 



