vel
Deposits along the Eastern Slopes of the Welsh Mountains; and on the
Existence of Drift-Zones, showing probable Variations in the Rate
of Submergence." By D. Mackintosh, "Quart. Journ. Geol. Soc." Volume
XXXVII., pages 351-69, 1881. [Read April 27th, 1881.]) I failed to find
shells on Moel Tryfan, but was interested by finding ("Philosoph. Mag."
3rd series, Volume XXI., page 184) shattered rocks (513/2. In reviewing
the work by previous writers on the Moel-Tryfan deposits, Mackintosh
refers to Darwin's "very suggestive description of the Moel-Tryfan
deposits...Under the drift he saw that the surface of the slate, TO
A DEPTH OF SEVERAL FEET, HAD BEEN SHATTERED AND CONTORTED IN A VERY
PECULIAR MANNER." The contortion of the slate, which Mackintosh
regarded as "the most interesting of the Moel-Tryfan phenomena," had not
previously been regarded as "sufficiently striking to arrest attention"
by any geologist except Darwin. The Pleistocene gravel and sand
containing marine shells on Moel-Tryfan, about five miles south-east of
Caernarvon, have been the subject of considerable controversy. By some
geologists the drift deposits have been regarded as evidence of a great
submergence in post-Pliocene times, while others have explained their
occurrence at a height of 1300 feet by assuming that the gravel and sand
had been thrust uphill by an advancing ice-sheet. (See H.B. Woodward,
"Geology of England and Wales," Edition II., 1887, pages 491, 492.)
Darwin attributed the shattering and contorting of the slates below the
drift to "icebergs grating over the surface.") and far-distant rounded
boulders, which I attributed to the violent impact of icebergs or
coast-ice. I can offer no opinion on whether the more recent changes of
level in England were or were not accompanied by earthquakes. It does
not seem to me a correct expression (which you use probably from
haste in your note) to speak of elevations or depressions as caused
by earthquakes: I suppose that every one admits that an earthquake
is merely the vibration from the fractured crust when it yields to an
upward or downward force. I must confess that of late years I have often
begun to suspect (especially when I think of the step-like plains of
Patagonia, the heights of which were measured by me) that many of the
changes of level in the land are due to changes of level in the sea.
(513/3. This view is an agreement with the theory recently put forward
by Suess in his "Antlitz d