E. Greenly — Age of Later Anglesey Dykes. 163 



flirection. The 'solitary' and ' gi'egarious ' habits may both be 

 said to be represented. The Plas Newydd dyke is a good example 

 of the solitary, and the group shown in Fig. 1 of the gregarious type. 



Most of them are short, or at any rate are traceable for short 

 distances only, the longest yet mapped being the large dyke that 

 runs from Menai Bridge village to Castellor, about a mile and three- 

 quarters. The widest is the Holland Arms dyke itself, 110 feet, but 

 tlie larger dykes of Menai Bridge, which average about 50 feet, are 

 more typical of the larger dykes of the island. One of these last has 

 been quarried for road-metal in several places, and the abandoned 

 quarries remain as very striking trench-like chasms, one of which 

 is visible from the Suspension Bridge among the houses on the 

 north-east side. It may be observed, moreover, in this connection, 

 that these dykes furnish the principal material for road-metal in 

 the district. 



The discontinuity of the dykes at the surface is not wholly due to 

 concealment by drift, but can in certain cases be shown to be due to 

 failure to reach the existing surface of the ground. Thus, the Menai 

 Bridge dyke can be seen to be interrupted by a space of country rock 

 near the Church, reappearing after a few yards. A small dyke 

 (about 2 feet wide) at Cadnant Creek can be seen to terminate 

 upwards in cross section (Fig. 2), while a large one on the coast 



Fig. 2. — Upward termination of dyke at Cadnant. 



near Careg Onen, sufficiently inclined to be of the nature of a sill 

 (a somewhat rare mode of occurrence) is partly overlain by schist 

 (Fig. 3), and disappears wholly underground before reaching low- 



5SW ^ NNt 



Fig. 3. — Sketch section through dyke near Careg Onen. 



water mark. These characteristics were observed by Henslow, and 

 are illustrated by him in some longitudinal sections. The Castellor 

 dyke actually stops short at the brow of the hill overlooking the 

 village of Menai Bridge, instead of, as might have been expected, 

 being still better exposed in the lower ground. It is curious, though 

 perhaps accidental, that the Cleveland dyke is known to behave in 

 a similar manner (see Geikie, "Ancient Voles. Gt. Brit.," vol. ii, 

 pp. 147-150). Yet, in spite of phenomena like these, dykes cross the 

 summit of Mynydd Llwydiarth at a height of 500 feet above the sea. 



