E. T. Hardman — Subglacial Theory of Gravels. 173 



from some distance ; but in no case Chalk or Chalk-flints to the 

 amount of more than about three or four per cent., even when in the 

 Chalk country, unless where in very close proximity to unprotected 

 Chalk exposures. 



On the other hand, the gravels, as"a rule, contain perceptibly less 

 of the local rock, plenty of travelled or foreign pebbles, and a great 

 abundance of chalk and flints ; of the last two so much that the 

 ground is often white with them ; and this often when at a distance 

 of 100 feet from, or sometimes underlying them, is Boulder-clay, on 

 which a considerable amount of time and trouble should be expended 

 before anything from the Chalk would turn up. 



The district to which I refer is that comprising the south of Deny 

 and Antrim, great part of Tyrone, and the northern part of Armagh. 

 In those places, I have examined a great number of exposures in 

 " Till," together with equally many gravel-pits, and always found 

 the distinction noticed above to obtain. 



Portlock : gives, a list of gravel-pits in the counties Deny, Tyrone, 

 Armagh, and Fermanagh, noticing those of 52 parishes, the gravel 

 of 35 of which contained chalk or flints ; and in 25 of these chalk 

 is not the local rock. They comprise from two to three exposures per 

 parish on an average. I mention this to show how general the 

 occurrence of chalk is in the gravel of this large district. 



Now if all the Drift of these parts had been deposited first by ice, 

 and almost simultaneously re-arranged in places by glacial river-action, 

 should we not expect to find in the re-arranged part the same con- 

 stituents as in the so-called Till ? or, to put it more clearly, must not 

 all the pebbles of the gravels have previously existed in the moraine 

 matter of the glacier, and would they not, therefore, be found in all parts 

 of the Drift in nearly the same relative proportions ? According to the 

 hypothesis, the gravels would be merely the Boulder-clay minus the 

 clay, and I see no reason why the chalk debris should become con- 

 centrated by washing. Were the proportion of chalk, etc., in the 

 "Till" very variable, it might indeed be supposed that these gravels 

 had their origin in some such chalky Drift. But this is not con- 

 sistent with the facts. The "Till" is, as I have mentioned, remark- 

 ably free from such pebbles, and in the extensive district I have 

 examined hardly any difference in this respect could be observed. 



This difference may be easily accounted for on the usual theory of 

 the glacial formation of the " Till," or Boulder-clay, and the later 

 marine deposition of these gravels. The glaciers, or ice-sheet, which 

 gave rise to the former had, in order to reach the chalk, to work 

 through the hard basalt overlying it, and this attempt was in but 

 few instances successful, as will be at once seen on looking at the 

 geological map. Beyond the basaltic sheet the ice passed over 

 schists and other metamorphic rocks, Old Bed Sandstone, Carboni- 

 ferous Limestone, and the older Mesozoic strata. 2 While, therefore, 



1 Geological Report on Londonderry, etc., pp. 747-8. 



2 There is evidence that the general Physical Geology of this district just before 

 the Glacial Epoch was not very different from that which prevails there at the present 

 day. This I have given in a paper read at the late meeting of the British Association 

 at Belfast, " On the Age and Mode of Formation of Lough Neagh." 



