168 Bevieivs — Prof. Still's Physical Geology of Ireland. 



and limestone gravel, or by shallow lakes and sluggish rivers. Ex- 

 tensive peat mosses or bogs cover large areas of this central plain.' 



In places over this great plain we find still remaining detached hills of Coal- 

 measure rocks, momiments of the former extension of these over the entire area. 

 The probability of the much greater former extent of the Carboniferous rocks is 

 well shown, and a little sketch-map given, which indicates that only detached parts 

 of Donegal, of Galway, and Wicklow were left uncovered originally by one or other 

 of the deposits of that great period. Such being then the original condition of this 

 area, the effects of marine denudation on such a siu-face are discussed, the existence 

 of such planes, of date even prior to the Carboniferous epoch, pointed out (Slieve 

 Partry), and the grand instances of the general smoothened or planed down surface 

 out of which the bold features of the S.W. of Ireland have since been carved by 

 subaerial action. He then details the features which would result from these suc- 

 cessive operations, and concludes, that at the close of this long period of disturbance 

 and denudation, lying between the Carboniferous epoch on the one hand and the 

 Permian on the other, the surface of Ireland would present the appearance of a plain 

 partly formed of Coal-measures, and partly of older rocks, with slight inclinations 

 in various directions in which the streams and rivers would begin to flow, when the 

 whole had been elevated into land. Out of siich a gently undulating plain the 

 physical features of Ireland were probably sculptured : the rivers collected and 

 carried off the debris; the softer Carboniferous rocks, which offered such favourable 

 conditions for the wasting forces to act upon, began rapidly to disappear ; and the 

 purer limestones below, once reached, were quickly dissolved in the rain-fed waters 

 of the streams, leaving the harder Old Eed and Silurian rocks standing out in bold 

 ridges, their relative elevation due not so much to any forces of upheaval, as to the 

 comparatively greater destruction and removal of the adjoining strata. To estimate 

 the full power of such forces, we must attempt to form some idea of the time during 

 which they were in operation, and here we must recall the fact already noticed, that 

 in Ireland there is no representative (in the portion occupied by the great central 

 plain of which the author is speaking) whatever of the Mesozoic, or older Cainozoic 

 rocks, and that the only satisfactory cause of this absence, when they are so largely 

 and fully represented in the adj oining island, is that they never were deposited in 

 Ireland, or, in other words, that, during the long period represented by their for- 

 mation, part of Ireland was above water, and was subjected to all the destroying 

 forces which can only act energetically on dry land. " If this be so," Prof. Hull 

 eloquently says, " how vast was the lapse of time dming which this portion of the 

 British Islands was being subjected to the wasting influences of rain, rivers, and 

 other subaerial agents of erosion ! During this time the Permian beds, with their 

 varied deposits of sandstone and limestone, stored with marine fossils, were de- 

 posited ; the great salt lakes of the Triassic period were constructed ; successive 

 generations of Saurians, Molluscs, and other marine forms flourished and passed 

 away during the Liassic stage ; great beds of Oolitic limestone, now rising into 

 mountain ridges along both sides of the central axis of the Alps, and composed 

 almost entirely of the shells and skeletons of marine organisms, were laid down over 

 the floor of the Jurassic sea. Then followed in succession subaerial, lacustrine and 

 estuarine conditions, which at length gave place to fresh submersions under the 

 ocean of the Cretaceous period, during which masses of limestone, many hundreds 

 of feet in thickness, were constructed by the ceaseless industry of lowly-organized 

 marine animals, chiefly Foraminifera. To these the varied deposits of the Tertiary 

 age were superadded, and over the south of Europe, along a zone extending from 

 the countries bordering the Mediterranean to the frontiers of China, the Nummulite 



' The wording of the book under review might here give rise to an erroneous 

 idea, which however we do not think the author intends to convey. " He says, " The 

 extensive peat mosses .... are a still more recent covering (of the limestone), and 

 generally occupy the positions of former shallow lakes " (p. 157)- This would imply 

 that they occur resting immediately on, or forming a direct covering to the lime- 

 stone — in reality, we believe they never do. They occupy shallow lake-like depressions 

 in the clays, a thin deposit of calcareous marl with fragments of shells marking the 

 lacustrine deposits at their base, before the growth of vegetation converted the pool 

 into a moss, or bog. We believe it scarcely possible for such a peat moss to be 

 formed upon an open and highly porous base, such as the limestone itself would 

 afford. 



