The glacial phenomena of Arvon are of very great importance, since it is in this region that the ice of the sea-basin comes into contact with the glaciers of the mountain-land. The country which had been mapped by me at the time of writing this summary just enters the mouths of the great valleys of the Ogwen and the Peris (“Llanberis,” when repeated often, comes to be a little cumbrous) at Bethesda and Bryn-yr-efail. From north-east to south-west it extends from Llanllechid to Carnarvon. Over most of the platform country, the striae, with the onset-and-lee features, point to the south-west. But at the mouths of the two great mountain valleys they point northwest or north-north-west. Knowing, moreover, as we do ( G. of A. p. 712 &c and fig. 326) that Anglesey was invaded for some three miles by ice from the mountain-land, we shall not be surprised to find that, at some places in Arvon, the one series is crossed by the other. This transgression is far longer in the case of the Ogwen than in the case of the Peris glacier. In the Peris case, it has not been traced for more than a mile beyond Cwm-y-glo. In the Ogwen case it is found as far north as Llandegai, the northward striae being the later. Along the foot of the mountain-land, between the Peris and the Ogwen, no great valley opens out, and here, accordingly, no crossings have been found: the south-westerly trend is not interfered with. Glacial deposits