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



we find all the last-mentioned rocks in great plenty in the Till, 

 together with basalt, thanks to this last we get little or no chalk, or 

 flints ; for even in the rare cases where the chalk was cut down to, 

 or was already exposed, the proportion of chalk to other rocks would 

 be very insignificant. 



Now let the land be submerged, and the sea come into play around 

 the chalk escarpments. It would soon form cliffs, and not content 

 with that, would undercut, and eat into the comparatively easy- worn 

 and well-jointed chalk, as it is now doing all along the northern 

 coast, tearing out large blocks, rounding them, and wearing them 

 into pebbles. The overhanging basalt would soon be brought low, 

 and the result would be a gravel composed largely of basalt, chalk, 

 and flints, together with such pebbles as might happen to lie in the 

 Drift — if there were any — previously deposited, overlying the cliffs, 

 and perhaps some others drifted round from distant parts of the 

 then coast. There can be no difficulty on this point, for as the chalk 

 is found at all heights from the present level of the sea up to 1400 

 or 1500 feet, any amount of submergence between these limits would 

 bring it under the influence of the sea. Such a gravel, then, being- 

 swept away over the sea-bottom, would cover the former Drift ; if 

 then the ground emerged at any given point of this area, the " Till" 

 or Boulder-clay would be found to contain most local rocks, and but 

 little chalk ; while the gravel would contain few local rocks, but a 

 great amount of chalk and basalt. 



I find, on referring to Portlock's exhaustive Eeport, that he had 

 somewhat the same idea as to the formation of the shelly clays of 

 the northern part of county Derry, namely, that they were formed 

 in a sea which cut away the chalk and basaltic cliffs, hence the 

 occurrence in them of so many chalk and basalt pebbles. 1 There 

 can be little doubt that these clays belong to the Drift, although 

 Portlock considered them to lie beneath it : for not only are similar 

 clay-beds found everywhere in the gravels of the district, but these 

 particular beds contain the characteristic shell of the Clyde beds, 

 Nucula oblong a, as well as the usual drift shells, Cyprina Islandica, 

 Turritella terebra, and Astarte multicostata (?). 



With relation to these beds the following point occurs. Portlock 

 mentions, and remarks on the fact, of "the delicate Nucula being 

 uninjured, as if deposited on the spot, while the strong Cyprina has 

 been almost destroyed." If then all these beds had been brought 

 into their present position, from 100 to 450 feet above sea-level, 

 by moving ice, why should one genus be shattered, and the other 

 preserved ? They should have all been either preserved without 

 distinction, or broken up indiscriminately. 



The chalk and flints to which this note refers are not only found 

 in the gravels of the North of Ireland, but also — more sparingly — in 

 the midland and southern counties. I have noticed them in the 

 gravels of county Meath frequently, also occasionally in those of 

 counties Carlow 2 and Kilkenny, but more rarely. Still, however, the 



1 Op. cit. p. 737. 



2 In the gravels of Carlow I have found shell fragments ; in one near the town, I 



