Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/153

Rh The stalagmitic crust (3) must have been accumulated during a period when conditions were favourable; and these conditions must have varied in different parts of the Cefn cave.

The bed under the stalagmite in the Cefn cave, with its representative in the Pont-newydd cave, is the most difficult to explain. I am unable to say whether it most resembles an accumulation of fine river-shingle or a raised beach. In the caves it does not occur at the height above the present sea-level of the more typical raised beaches. It is not confined to the caves, but may be seen at intervals along the sloping north bank of the Elwy. It may likewise be found in patches along the Elwy valley as far as the Vale of Clwyd, where it would appear to graduato into the middle gravel and sand which underlies the upper clay. However it may have originated, I cannot believe with Professor Hughes that, in the Pont-newydd cave, it was washed in through a swallow-hole, from the Boulder- clay of the neighbourhood, by a freshwater stream. The relative proportion of stones of different kinds is not nearly the same in the pebble-bed and in the Boulder-clay covering the surface of the ground at a higher level. In the pebble-bed nearly all the stones, so far as I could see, are Denbighshire sandstone or grit. In the Boulder-clay a large proportion of the pebbles are limestone. I could only see one felstone specimen in the pebble-bed (though others probably might be found). In the Boulder-clay felstone pebbles and boulders are far from being rare. The pebble-bed is not confined to the cavern or the ground straight in front of it, as it would have been if deposited by a stream flowing out of the cavern; but it may be seen in a recess a short distance east of the mouth of the cavern, and, as I have already remarked, at intervals further east. Several examples of it may be found near the Cefn cave, clinging to the rocky slope. This bed could not have been accumulated before the glacial submergence, as it contains a few erratic pebbles, which must have been transported by ice from regions far beyond the basin of the river Elwy.

Before endeavouring to offer a general explanation of the mode in which the various deposits were introduced into the Cefn and Pont-newydd caves, it may be necessary to state that the drift- deposits of the north-west of England and the borders of North Wales are separable into:—(1) a lower stony Boulder-clay (with glaciated stones), which to the south of the Mersey and in the eastern part of Wales can only be detected at intervals; (2) a middle sand and gravel (without any large boulders or glaciated stones, excepting a few among or near to the mountains), which extends almost continuously over very wide areas, and often attains a thickness of nearly 200 feet; (3) an upper Boulder- or brick-clay (with