D. MACKINTOSH ON HIGH-LEVEL MARINE DRIFTS. 



353 



seen on a sea-beach. Ramsay gives a comprehensive account of the 

 deposits in the last edition of his ' Physical Geography and Geology 

 of Great Britain.' He specifies the indirectly local erratics which, 

 came under his notice, and regards the mountain from what may be 

 called the Snowdon stand-point. Though he had previously observed 

 the drift-sections (in 1852), it would appear that the curved slaty 

 laminae which I have been led to regard as the most interesting of 

 all the Moel-Tryfan phenomena, were not during his visits, or indeed 

 during the visits of any other geologist excepting Darwin, sufficiently 

 striking to arrest attention, In this paper the mountain will be t 

 principally regarded from the N.W. and J\ r . stand-points. 



2. Deposition during Submergence. — It seems to be generally 

 admitted that deposition is in excess during submergence, and denu- 

 dation during emergence ; and when we consider the thickness of and 

 area covered by the Moel-Tryfan deposits, in connexion with the 

 fact that they lie on a sloping surface, and reach up to within a short 

 distance of the rocky crest of the hill, the idea of accumulation during 

 the submergence or sinking of the land appears the most probable, 

 while it is the most consistent with certain facts to be mentioned in 

 the sequel. 



3. Identification of the Local and Erratic Stones. — The most pre- 

 valent pebble, rock-fragment, or boulder in these deposits is a light- 

 coloured felstone, which may have come from the Cambrian conglo- 

 merate of the upper part of the hill, or from adjacent hills. The 

 fragments of talcose or chloritic schists must have come from rocks 

 like those which are now found in the tunnel (near the top of the 

 hill), or from similar rocks in the hills around Moel Tryfan. The 

 basaltic-] ooking diorite or greenstone may have come from bands in 

 the hill or adjacent hills. Many small quartz pebbles in the drift 

 may have come from the Cambrian conglomerate already mentioned*. 

 The numerous fragments of slate may be very nearly in situ, or may 

 likewise have come from neighbouring hills. There are many 

 Eskdale granite pebbles, and a smaller number of granite pebbles 

 from the south of Scotland (chiefly Criffel). Chalk flints are rather 

 numerous (during my last visit they predominated in a particular 

 part of the excavation), and reach a height of about 1350 feet. 

 Eskdale granite on Moel Tryfan is found up to at least 1350 feet 

 above the sea, or 64 feet above the highest range of the rock in situ. 

 Chalk-flints in situ, in Ireland, as Professor Hull has informed me, 

 do not rise higher than about 1000 feet above the sea ; so that on 

 Moel Tryfan their height is 350 feet higher (see VIII. ). On Moel 

 Tryfan I found a pebble of red granite of unknown derivation, but 

 exactly of the same kind as one I saw on the beach of West Cum- 

 berland, and one on the Blackpool beach. 



4. Arrangement of the Gravel and Sand. — Most writers on the 

 Moel-Tryfan deposits have noticed the extent to which the sand and 

 fine gravel are obliquely laminated, and the resemblance they bear 

 to what may be seen on a sea-beach. The sand is often as fine as 



* I have to thank Dr. Hicks for assisting me in tracing the derivation of the 

 local and indirectly local stones. 



