172 E. T. Hardman — Subglacial Theory of Gravels. 



Postscript. — On referring to Mr. James Geikie's " Note on the 

 Occurrence of Erratics," I see that Forbes thought there was evidence 

 in the Swiss glaciers of stones being "actually introduced into the 

 ice by friction at the bottom of the glacier." Of course, if there is 

 positive evidence in Switzerland of this sort of thing, the presence 

 of stones in the Till ceases to be a difficulty. 



V. — Note on Mr. Goodchild's Theory op the Sub-glacial Form- 

 ation of Gravels, etc. 



By Edwabd T. Hakbman, F.C.S., etc. ; 

 Of the Geological Survey of Ireland. 



IN the paper on "Drift" in the Geological Magazine for Novem- 

 ber, 1874/ Mr. Goodchild, in proposing his new theory, appears 

 to invite discussion on it. I therefore beg to contribute my quota 

 towards upholding the marine, as against the exclusively glacial, 

 deposition of certain gravels belonging to the Drift. 



According to the last paragraph of his paper, 2 this theory is, that 

 all the glacial deposits were formed under ice, and with the assistance 

 of sub-glacial streams ; and that neither stratification nor the presence 

 of marine remains proves the former agency of the sea in their form- 

 ation. 



Whether this idea will meet with general acceptance I shall leave 

 to others to determine, on the various merits. It is certainly ingenious, 

 and would no doubt serve to explain the presence of lenticular 

 patches of sand, gravel, and laminated clays, such as are frequently 

 found in the Boulder-clay ; but its application in the case of the 

 masses of sand and gravel that are often spread out over large tracts 

 of country appears to be hardly tenable. I shall, however, confine 

 myself to a point which I noticed in connexion with the Drift of the 

 North of Ireland during my work for the Geological Survey, and 

 which seems to me to have a legitimate bearing on the question. 

 Besides bringing forward a new link in the chain of evidence, it 

 refers to a matter not likely to be observed in any other district. 



As every one is aware, one of the chief features in the geology of 

 the North of Ireland is the existence of the Upper Chalk with 

 Flints, covered by a very thick sheet of Basalt, the two formations 

 being in effect co-extensive ; for there is hardly any part of the area 

 occupied by the Chalk which has not its protecting cap of Basalt. 



The Drift, for the purposes of this note, may be divided litho- 

 logically into Till, 3 and stratified sands and gravels with brick-clays, 

 in some of which fragments of shells have been found. Now the 

 contents of these Drifts differ very widely. The " Till " contains 

 invariably a very large per-centage of the local rock, whatever it 

 may happen to be — Limestone when that rock prevails ; Sandstone 

 and shale near Coal-measure ground i and a plentiful supply of 

 Basalt, both in large blocks and small pebbles, when over or within 

 reasonable reach of it, together with igneous and metamorphic rocks 



1 On Drift, by J. G. Goodchild, F.G.S. * Loc. cit. p. 510. 



3 In a few places patches of Boulder-clay (Upper Boulder-clay) resting on the 

 sands and gravels occur in the district with which I am acquainted. 



