THE GLACIAL DEPOSITS OF WESTERN CARNARVONSHIRE. 45 



quartz showing fine pyramidal forms, the reddish-streaked rhyolite, Millstone Grit, and 

 quartzites probably from Anglesey. 



The reddish-streaked rhyolite again occurred on Mynydd Annelog, near the 

 extremity of Lleyn, at a height of 350 feet, together with a boulder of Millstone Grit. 



Between Llanllawen and Aberdaron many boulders of diabase are dotted about. 

 These are probably mostly of local origin. Blocks of jasper and of serpentinous 

 rocks also occur which are derived from the neighbourhood of Careg, not far away 

 to the north. 



On Mynydd Cilan, lying between Porth Nigel and Porth Ceiriad, and at heights 

 of about 300 feet, the following erratics were found : — Several boulders of a coarsely 

 crystalline diabase, the reddish-streaked rhyolite, Carboniferous Limestone with crinoid 

 stems, a coarse grit quite unlike the rock of the district, light-coloured felspar porphyry 

 derived from the masses of the rock which are found in situ to the north-east, Millstone 

 Grit, and a reddish agglomerate. 



On Mynydd Mynytho, which consists of felspar porphyry, several erratics of diabase 

 were found, also a boulder of a dark felsitic rock at a height of 400 feet on the south side 

 of the mountain, and near it a small boulder of the reddish-streaked rhyolite. 



The reddish-streaked rhyolite is perhaps the commonest of all the boulders occurring 

 in the Drift of Lleyn. It is found on the surface, in gravel pits inland, in the coast 

 sections, and as pebbles on the modern beach. Its parent rock cannot be very far away, 

 and probably lies to the north-east in Anglesey or Carnarvonshire. The boulders of 

 this rock have been submitted to several petrologists, but no one has yet located its 

 parent rock. 



IV. Summary and General Conclusions. 



In the western part of the Lleyn peninsula the Lower Boulder Clay is seen at some 

 places to be separated from the bed-rock by an accumulation of angular detritus having 

 all the characteristics of a " Eock Rubble " or " Head " (fig. 4). This rubble is disclosed 

 in some of the coast sections towards the extremity of Lleyn, as, notably, at Porth Oer 

 and at Aberdaron. It rests on the rocky slopes of the valley sides, where the valleys 

 terminate seawards, and with these slopes it descends on either side below the level of 

 the modern beach. This "Head" consists of angular blocks and fragments of all sizes 

 packed close together or lying in an earthy matrix. The material seems to be entirely 

 of local origin, and to be derived in the main from the immediately adjoining rocks 

 above. It sometimes shows a rude bedding, as, for instance, at the east end of Aber- 

 daron Bay, where it consists below of flakes and slivers of rock arranged with their long 

 axes parallel to one another, giving a linear arrangement, and followed above by coarser 

 material in which big angular blocks lie. During the time of the accumulation of this 

 rock rubble the land must have stood higher relatively to the sea than it does at present. 

 The lower ends of the valleys or inlets on the sides of which the rubble is seen are now 

 often choked with the Drift deposits which pass below the level of the modern beach. 



