Revieivs — Prof. SuWs Physical Geology of Ireland. 171 



" belongs to one grand dominant system of central pressure, which, has caused the 

 ice to move outwards in all directions from a central snow-field towards the existing 

 coasts, except where impeded or deflected by local mountain barriers." After point- 

 ing out the reasons for admitting that such markings were caused by ice-borne 

 materials, and the several indications afforded by polished rocks, ' crag and tail ' 

 forms, boulders, and their positions, of which numerous instances are given,' he 

 proceeds to discuss the laws regulating the movements of this great ice-sheet, " of 

 the existence of which there can be no doubt whatever." He points out, and adopl^s 

 entirely the conclusions of Mr. Close, that the movements of this ice-sheet have 

 proceeded in opposite directions seawards, from a line or tract of country, stretching 

 in a belt across the island, occupying the country between Lough Corrib and Lough 

 Mask on the west and Lough Neagh on the east, thus stretching for a distance of 

 more than one hundred miles from "W.S.W. to E.N.E., from which tract, as 

 from an axis of motion, the ice has passed both north and south. The position 

 of this supposed axis is well shown on the little glaciation map to which we referred, 

 This very remarkable conclusion of Mr. Close is particularly noticed as differing 

 from what is already known in any part of the world with which we are acquainted. 

 In every other known case of such distribution of glacial motion from an axis or 

 centre, those centres have been districts of greater elevation than the country adjoin- 

 ing. The Alps, the Pyrenees, the great elevated Scandinavian highlands, the 

 Snowdon range in "Wales, the Cumberland and Westmoreland ranges in England, the 

 Grampians and the Southern highlands in Scotland, are aU well-known instances of 

 this. But in Ireland no part of the axis of movement is more than 400, or at the 

 outside 500 feet, above the sea-level, while it is stated, as a positive and admitted 

 fact, that from this an ice-sheet has been set in motion, which has not only topped 

 and completely passed over such, little minor obstacles, as, for example, the Coal-fields 

 of Castlecomer, etc., which rise to 700 feet, but has overrun hills of more than 

 1000, or even 1500 feet, and has been forced up the flanks of higher mountains to 

 still greater elevations. It is, therefore, only right that the propounders of such a 

 theory should endeavour to explain the actions which took place. Mr. Close 

 suggested that the country had been more elevated over the land now occupied by 

 the counties of Mayo and Roscommon than more to the south and east. But Prof. 

 Hull rejects this as quite an insufiicient cause, inasmuch as the movement has not 

 taken place merely from a central point, or from the western end of the axis alone, 

 but along a line of more than 100 miles. And he has no hesitation in suggesting at 

 once a different explanation. He says (p. 229), "Is it not conceivable that the line 

 of country here described was at the earliest glacial period the region of greatest 

 snowfall — the region of greatest precipitation of snow, as it is now that of greatest 

 precipitation of rain ? " '^ He supposes that the snows were piled up over this belt of 

 country to an ' enormous' depth, and to a less extent over the tracts lying to the south 

 and east, until the vertical pressure of ' thousands of feet ' of snow and ice there 

 accumulated was converted into a lateral pressure, forcing the ice to move in a 

 direction towards the points where it could find an outlet. After expressing his con- 

 fident belief that the vertical pressure of this ' enormous ' pile of ice and snow 

 gave the first initial movement to the ice-sheet, both southwards and westwards, 

 Prof. Hull still confesses that we are obliged to have recourse to other modes of 

 explanation of the facts that this motion has been propagated to upwards of one 

 hundred miles from its source, and that it has caused the ice to move, not only over 

 plains, but up the sides of opposing hills and ridges. And to this end he is disposed 

 to adopt a development of Charpentier's dilatation theory, as explained by Dr. CroU. 

 This, in general, is, that the internal pressure resulting from the solidifying of the 

 fluid particles of water which has filled the interstices of the ice, acts on the mass 

 as an expanding force tending to cause the glacier to widen out laterally in all direc- 

 tions, and the author adds — " to move with a linear motion in the Kne of least 

 resistance," but he does not attempt to show that the direction of motion of the ice- 



1 We do not observe any notice, among the detailed descriptions of the drift deposits 

 of Ireland, of the important fact of the occurrence of pieces of the compact Creta- 

 ceous limestone of the N.E. of Ireland in the clays of the South of Ireland, even so 

 far south as the County of Cork. 



2 The writer adds most naively in a note, " Of this, however, I have no positive 

 proof," no general returns of rainfall being available, but he says, " I have little 

 doubt that it is the case." 



