The discussion by Professor Gregory in the February GeologicalMagazine of certain ridges of glacial deposits in various widely separated areas seems to call for some comment. I am not familiar with all the examples classified by Professor Gregory any more than he is, but a couple of examples visited by him, and that I have examined many times, may serve as tests of his trustworthiness as a field observer in Glacial Geology and his competence as a commentator upon the observations of others.

The middle portion of the D1 beds seen in the Gratton Dale section consists of about 630 feet of dark dolomitic limestones, which appear to be confined to a definite horizon in the sequence. The impression one at first receives is that here are definitely bedded dolomites probably of contemporaneous origin, and this material has already been referred to as a thick bed of dolomite. Working northwards down the dale, i.e. ascending the sequence, one finds that the vertical passage from normal limestone into dolomitic material above is everywhere obscured. Similarly the upper limit of the dolomitized beds is covered, so that it is impossible to say whether the metasomatism abruptly ceases with a certain bed or dies out gradually in the higher strata. In the affected part of the sequence the dolomitization appears persistent in Gratton Dale, and at a few places only have I found lateral transitions, one or two examples on either side of the dale. The most marked transition is on the western side, towards the upper part of the altered material. Here light-coloured limestones are seen to pass laterally within a few yards into darker dolomitic material, with which it contrasts. Careful examination shows that these limestones on both sides of the Dale are really transitions, and are not interbedded with dolomite, a matter of considerable significance. North-westwards the dolomitization extends for some distance up Long Dale which joins Gratton Dale at right angles.

The examination of the fossil fauna, the chief forms of which are shown in Tables I and II and Fig. 1, shows that the beds commence at a high horizon of the Seminula Zone and extend to the top of the Dibunophyllum Zone. Also, the “life-zones” established by Professor Garwood in his North-Westem Province Paper can be readily recognized in our area and the series includes the sequence from the Nematophyllum minus sub-zone of the Productus corrugatohernisphericus Zone to the Dibunophyllum muirheadi sub-zone of the Dibunophyllum Zone. It can further be established that the limestones of our area do not persist up the horizon of the Botany Beds with Aulina mtiformis S. Smith. The Saccammina Band occurs in our area, but Saccammina occurs also in the limestones immediately above and below. The Girvanella Band of the North-West Province has been recognized and forms a constant horizon throughout the area. It occurs associated with a calcitemudstone and never as a nodular bed. Other bands also appear to be algal. The Spiriferina cf. laminosa Band of Garwood forms a constant horizon between the Seventh and Sixth Limestones.

It is well known that the liquid inclusions commonly found in quartz, fluor-spar, and other minerals resemble in some degree those that occur in crystals of sodium chloride, alum, and many salts. Chloride of sodium in particular shows innumerable cavities of varying size, cubical in form (negative crystals), and generally containing an enclosed bubble, especially if heated gently after the crystals are formed. The smaller cavities show Brownian movement of the enclosed bubble in a very striking manner. (See Fig. 1, which gives a diagrammatic view of a salt crystal as seen through the microscope.)

The area under consideration is a portion of the mountain massif bounded on the east by the Conway River, on the north by the narrow coastal plain between Conway and Bangor, and on the west and south by the popular tourist route through Nant.-Francon pass by Llyn Ogwen and down the Afon Llugwy to Bettws y coed. The investigation includes the horseshoe-shaped ring of mountains, of which the highest is Y Foel Frâs (3,091 feet), surrounding the upper course of the Aber River (Cwm yr Afon Goch). To the north-west this igneous complex is bounded by the precipitatious crags over which the above stream descends by leaps and bounds, forming the well-known Aber Falls. Upon Sheet 78, south-east of the old Geological Survey map the area is coloured as “Massive intrusive felspathic rocks”, surrounding two oval patches indicated as “Greenstones”, whilst a further elongated patch of the latter forms the south-west boundary. The north-east boundary is limited by the Aber-Llanbedr fault. Besides the hog-backed summit of Y Foel Frâs, the area includes the similarly outlined summits of Y Drosgl West (2,483 feet), Yr Arryg (2,875 feet), Llwydmor Bâch (2,256 feet), and the more rugged summits of Bera Mawr (2,587 feet), Bera Bâch and Llwydmor (2,749 feet).