﻿Correspondence — 3Ir. 8. Gordon McBakin. 39 



Upper contains large striated boulders ; but the Lower, it is admitted, 

 may possibly also contain 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^ (Lower Boulder-clay), and their striee effaced during an 

 inter-glacial period, when the trfinsportation 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 his 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 xny part, candidly I think it is not, and until some upholder of 

 the theory 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 

 have stated are necessary to constitute a geological subdivision, I 

 cannot consider the evidence to be worth much. Looking at the 

 question in a broad asj^ect, it also appears to me that any division 

 foimded as this primarily is on the separation of the Clay by sand 

 and gravel involves, 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 puzzling character as to be interpreted differently 

 by every observer, the supporters of the tripartite division being 

 frequently quite at a loss as to which division the respective beds 

 should be allocated. 



Blundellsands, Liverpool. T. Mellard Ebade. 



A'ov. 9lh, 1876. 



ON THE FORMATION OF GEOUND ICE.^ 



Sir, — Relative to the formation of " Ground Ice," I have to offer 

 the following theory. In order that this phenomenon 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 



' From Mr. Morton's description one would infer this deposit extended all over the 

 dock. 



2 See Dr. Landor's paper on "Ground Ice," Geol. Mag., 1876, Decade II., 

 Vol. III. p. 459. 



