144; Geological Society. 



down through a thick bed of this diluvium. It occurs, not only in the 

 valleys, but on the sloping sides and summits of hills, sometimes en- 

 tirely covering the hills, at others accumulated around small project- 

 ing crags. It is spread indiscriminately, and vpith little reference to 

 the rivers that now intersect the country: its greatest observed thick- 

 ness is about 140ft. 



A large proportion of this gravel is composed of pebbles and blocks 

 of various sizes, derived from rocks that occur in Caernarvonshire ; 

 many of these are less rolled than pebbles of another class that are 

 mixed with them, and which have come from a greater distance, and 

 must have been drifted upwards by some violent inundation, in a 

 direction contrary to that of the rivers which descend from Snowdonia 

 into the Menai. Among these pebbles are several which can be iden- 

 tified with the granite, sienite, green-stone, serpentine and jasper of 

 Anglesea : other granite pebbles agi-ee with no rock in Anglesea or 

 Wales, and resemble the granite rocks of Cumberland ; some may 

 have come from Ireland or the S.W. extremity of Scotland. 



There are also chalk flints, which can have come from no nearer 

 source than the chalk of the county of Antrim. 



This diluvium occurs in great thickness in the lower region of the 

 valley of the Ogwen, usually from 60 to 100ft ; forming its bed, and 

 often occupying both sides of the valley through which it flows. These 

 sides, for a considerable distance, afford indications of having received 

 their last form from the bursting of a lake higher up in the valley of 

 the Ogwen. 



Shells, and fragments of shells, like those on the shores of the ad- 

 jacent sea, are reported by the workmen to have been found in the 

 sand and gravel at an elevated spot near Moel Taban, on the right 

 bank of the Ogwen, nearly opposite the quarries of Penrhyn. Mr. 

 Trimmer did not see them here ; but on the summit of MoelTryfane, 

 on the south of Caernarvon, towards Bethgellert, in a sinking made 

 through sand and gravel, in search of slate, at about 20ft below the 

 surface, he found marine shells in a bed of sand ; they were for the 

 most part broken, resembling the broken shells on the adjacent beach; 

 when dry, they adhere to the tongue : the fragments are too indistinct 

 to identify species; the genera Buccinum, Venus, Natica and Turbo 

 occur among them. Mr. Trimmer found similar broken shells also 

 in the diluvium of the low cliff" near Beaumaris. 



Beneath the diluvial deposits of this district, when the surface of 

 the slate-rock is newly laid bare, it is found to be covered with 

 scratches, furrows and dressings, like those observed by Sir James 

 Hall on the summit of the Costorphine and other hills near Edin- 

 burgh. These furrows and dressings were noticed several years ago 

 by Mr. Underwood : they are referred to the action of the diluvial 

 currents which overspread the country with gravel : some of the 

 larger blocks amid the gravel have also deep scratches upon their 

 surface. 



Where the diluvium is argillaceous, the surface of the subjacent 

 slate has been so protected by it, as to remain sound and fit for use 



as 



