Correspondence—Ur. W. Shone. 93 
of the different bases is fully considered, and to which I have drawn 
attention in my paper. 
Ido not for a moment pretend to have reached the end of my 
- studies of this interesting complex of rocks, and shall await with 
much interest the appearance of the promised paper. As to my 
‘shear-planes’ (not ‘shear-zones’), it is just because they have 
nothing to do with the ‘schist-making’ processes, that they afford 
such strong negative evidence (while they record the action of 
dynamic forces on a grand scale) against the notion of the schist- 
manufacture having been wrought generally in solid crystalline 
masses. The general principles of my work are sufficiently before 
the world for those who care to know them to do so. 
Weuiincton CotteGs, Berks, 16th Dec., 1892. A. Irvine. 
GLACIAL GEOLOGY. 
Str,—I quite agree with Mr. R. M. Deeley in your last number, 
when he writes that he has “read with much interest the papers 
by Mr. Mellard Reade and Mr. Perey Kendall in your July and 
November issues. On the one hand we have the submergence 
theory proved up to the hilt, and on the other, the glacier theory 
sustained with equal show of reason. Does it strike the combatants 
that they may both be right and both be wrong ?” 
It is difficult to conceive a Glacial Period without the usual 
phenomena appertaining to both land and coast ice. Why should 
we, therefore, restrict ourselves to either the one agency or the 
other, when there must have been marine and land moraine drifts 
contemporaneously forming. The Gloppa deposit at Oswestry 
described by Mr. Nicholson in the Q.J.G.S. Vol. xlvii. p. 86 may 
be taken as a typical marine drift with its glaciated lake district 
erratics and Boreal Fauna of recent shells, occurring from 1070 
to 1120 feet above O. D., yet twelve miles to the north-west of the 
Gloppa in the upper valley of the Dee from the neighbourhood of 
Corwen to Bala Lake, which is only 540 feet above O. D.—the drift 
is entirely local and does not contain any fragments of recent 
marine shells, though 500 feet below the Gloppa deposit. 
The plain interpretation of this—to my mind—is that ice filled 
the upper valley of the Dee and the surrounding country to a higher 
level than that to which the marine drift of the Gloppa obtained. 
The Gloppa deposit like the other deposits of high level marine 
drift as Moeh Tryfean, Macclesfield, and Halkyw (Flintshire), are 
situated upon the outskirts of the mountainous areas to which they 
belong. This, I think, would suggest that such mountainous areas 
were covered with a thickness of ice in their central portions, which 
excluded the high level marine drift from the interior mountains 
and valleys. WILLIAM SHONE. 
Urrton Park, Curster, Dec. 16th, 1892. 
