432 MR T. J. JEHU ON 



large part of the lake is over 80 feet deep, as much as 42 per cent, of the whole 

 superficial area being enclosed within the 80 feet contour-line. The 60 feet contour- 

 line encloses 60 per cent, of the superficial area, and only 17 per cent, corresponds to 

 depths of water below 20 feet. Its contour-lines lie close to each other along the 

 margins and at the lower end, indicating that the slope is steep in those regions ; but, 

 as traced towards the head of the lake, the lines are seen to be separated by greater 

 distances, indicating that the slope is here more gentle. As a rule, the contour-lines 

 follow the irregularities of the shore-line, but it is worthy of note that they are but 

 slightly affected by the bay on the south-west side. 



Representative sections of Llyn Peris are given (Plate VIII.). The section A-B is 

 along the length of the lake ; the other two are cross-sections ; C-D across the deep area 

 near the lower end, and E-F at a place not far from the head of the lake. A study of 

 the map (Plate IV.) and of these sections will show that the configuration of the lake- 

 bottom is simple. The longitudinal section brings out very clearly the difference in 

 steepness between the slopes at the two ends of the lake. The slopes on both sides 

 are also very steep, as shown by the cross-sections, and for some distance the bottom 

 has a form approaching that of a flat plain, which rises gently towards the upper end 

 of the lake. The slight irregularities are in all probability partly due to the irregular 

 distribution of slate-rubbish on the lake-bottom. The lake consists of a long flat- 

 bottomed trough with steep sides, the flatness of the bottom becoming, however, less 

 marked as we approach the head of the lake. 



6. Llyn Padarn. — This lake lies a little further down the valley, being separated 

 from Llyn Peris by about a quarter of a mile of low alluvial ground. The rocky hills 

 of Yr Allt Wen, Fachwen, and Clegir border the lake on the right or northern side, and 

 the slope down to the margin is steep at the upper end, becomes more gentle towards 

 the middle, and again increases a little lower down. On the left the hills fall back, and 

 the slope down to the lake margin is gentle from the head of the lake for a considerable 

 distance below the town of Llanberis, after which it becomes steeper as far as the lower 

 end of the railway tunnel. Just below the tunnel a depression, occupied by alluvium, 

 leaves the lake margin and winds round a rocky mass at the lower end of the lake to 

 join the main valley again above Cwm-y-Glo. The shore-line is fairly even on the 

 Dinorwic side, but is more irregular on the Llanberis side, and is in parts obliterated 

 by accumulations of slate-rubbish, carried down from the quarries on the left side of 

 the lake. Not far from the lower end, on its right side, a rocky island and some rocky 

 promontories are seen to be beautifully glaciated. In addition to Afon-y-Bala, which 

 flows in at the head of the lake, two small tributaries run in just above Llanberis, and 

 another enters from the Dinorwic side, at a point more than a third of the way down 

 the lake. Llyn Padarn is broad at its head, becomes narrower just below Llanberis, and 

 then broadens out again for a short distance ; after this, it narrows once more, maintain- 

 ing a nearly uniform breadth to a point a little below the railway tunnel, then tapers 

 considerably to the termination at the bridge, under which escapes the river Seiont. 



