Correspondence — J Ir. S. Gordon McDakin. 39 
Upper contains large striated boulders ; but tbe Lower, it is admitted, 
may possibly also eontain large boulders, as it has not been excavated 
to any considerable depth. The most distinctive feature is that 
they are divided in places by a bed of sand and well-worn gravel. 
Mr. Mackintosh considers that these gravels were washed out of 
a pre-existing glacial clay, of which only hummocky patches 
remain 1 (Lower Boulder-clay), and their striee eflfaced during an 
inter-glacial period, when tke transportation of striated stones had 
ceased. Without discussing in detail the accuracy of Mr. Morton’s 
description, which I submit does not produce a faithful impression of 
what actually exists, but rather records what exists in bis own 
mind on the subject, I ask, is the foregoing evidence full enough, or 
of a nature to justify a careful geologist in accepting an interpreta- 
tion of the Boulder-clay fraught with such tremendous consequences ? 
For my part, candidly I think it is not, and until some upholder of 
the tlieory shows that the Lower Boulder-clay is either a subaerial 
deposit or contains fossils differing from those in the bed above. or 
offers any of the distinctive characteristics and continuity such as I 
liave stated are necessary to constitute a geological subdivision, I 
cannot consider the evidence to be worth much. Looking at the 
question in a broad aspect, it also appears to me that any division 
founded as this primarily is on the Separation of the Clay by sand 
and gravel invölves, if applied over a wide area, a physical absurdity. 
Under what possible conditions could a period intervening between 
the deposition of two beds of clay be represented everywhere only 
by sands and gravels? If these were washed out of the pre-existing 
clay, as Mr. Mackintosh infers, what became of the much greater 
bulk of the clay in which they were imbedded? Where are the 
equivalent deposits of clay which would surely have representatives 
somewhere in the interglacial period ? 
So far as my experience goes, the marine Boulder-clay and sands 
of the lower plains — and none but marine beds have hitherto pre- 
sented themselves to me — are from the base of red sand or rock on 
which they rest to the surface, but one great deposit containing local 
variations of such a puzzlitig character as to be interpreted diflferently 
by every observer, the supporters of the t.ripartite division being 
frequently quite at a loss as to which division the respective beds 
should be allocated. 
Blündellsands, Liverpool. T. Mellard Reade. 
Nov. 9 Ih, 1876. 
ON THE FORMATION OF GROUND ICE. 2 
Sir, — Relative to the formation of “ Ground Ice,” I have to offer 
the following theory. In order that this phenomenou may take 
place the water must be near the freezing-point. Then we have an 
analogous condition of things to that of the atmosphere when hoar 
frost is deposited upon the ground, trees and shrubs on account 
1 From Mr. Morton’s description one would infer tliis deposit extended all over tlie 
dock. 
2 See Dr. Landor’s paper on “Ground Ice,” Geol. Mag., 1S76, Decade II., 
Yol. III. p. 459. 
