170 Reviews — Prof. Ruirs Physical Geology of Ireland. 



think the river, which flows into the lake from the South, took its course along the 

 general depression caused by a disturbance of the great Miocene basaltic sheets, and 

 which general depression or bottom of the basin of the volcanic country has been 

 already noticed ; and was pent up into a lake by some of the faults, which cross the 

 area of the lake, and some of which have considerable downthrow to the north, 

 corresponding in position with the greatest depth of the lake. This older lake was, 

 as already noticed, of much greater size than the present one— which has gradually 

 retreated, as its barrier was worn down. The lake therefore is considered to be 

 chiefly a basin of submergence formed by the mechanical action of faults in the 

 strata forming its bed. Although it is said that this origin of Lough Neagh is 

 'wholly exceptional' (p. 198), Lough Allen seems, according to the author's views, 

 to be due to very similar causes. This noble lake forms a great reservoir for the 

 head-waters of the Shannon. It fills a wide valley, the banks of which are formed 

 of the Yoredale rocks and the floor of Carboniferous Limestone, and the formation of 

 the lake is attributed to the production of a barrier across its lower end by a large 

 fault, heading £. 20" N., with a downthrow to the north. This fault brings the 

 Carboniferous Limestone on that side down against the Silurian and Old Eed rocks, 

 and has thus had precisely the same efl'ect which an artificially raised barrier would 

 have in converting the river-vaUey above into a lake. This barrier has been con- 

 siderably worn down and lowered since its formation, but it still retains a relative 

 elevation as compared with the floor of the lake, and so, of course, the lake remains. 

 There is no distinct evidence of the age of this faulting, but the author points out 

 that its direction is very similar to the direction of the faiilts which have, as stated, 

 produced Lough Neagli. And he thinks it not improbable that it was really pro- 

 duced about the same epoch— that is, between the Miocene and Pliocene. 



With regard to lakes of Glacial origin, there are said to be two ways in which 

 these forces have acted: 1. by actually scooping out rock-basins, and 2. by de- 

 positing moraines, heaped up across a valley or hollow, and thus pending up the 

 waters of the stream flowing along the depression above. Of both these varieties 

 numerous instances occur in Ireland, though none of any great size. In places they 

 form regular networks of little loughlets and rock-basins. The author also thinks 

 that the wide shallow lakes of the central plain of Ireland owe much of their extent 

 and form to the action of ice, although they are principally due to another cause. 

 After 'giving several well-marked cases of moraine-dammed lakes (Lough Bray, 

 Lough Nahanagan, etc ), he passes to lakes of Chemical origin. To enumerate all 

 the lakes of the great central plains of Ireland would be a useless waste of space. 

 All have one common character, they are all irregular hollows and depressions in the 

 general limestone floor, now fllled with water, and these depressions and hollows are 

 principally due lo the solvent action of the water on the limestone rapidly eating 

 into it, and removing it in solution. Some of these lakes are intimately connected 

 with large underground rivers, the channels of which have been in a similar way 

 dissolved out of the limestone. These give rise to lakes and fountains, the true 

 source of which is often not traceable at first. Similar forces of solution and denu- 

 dation have in many cases produced the deep indentations which abound round the 

 coasts of Ireland, in the limestone districts, and which, subsequently filled with 

 glaciers and scored, and scooped out, passed into the state of " fiords." 



The third part of Prof. Hull's work is devoted to the " Glaciation of Ireland." 

 And here we would first direct attention to the very effective and well-executed little 

 map of the " General Glaciation" which is given. It is an admirable example of 

 how much can be compressed into a small space with perfect legibility and clearness. 

 Starting with the assumption that it has been generally recognized that the surface 

 of the solid rocks of Ireland is widely and extensively glaciated, and that an '"ice- 

 sheet" covered the greater portion of the country, Prof. Hull bears willing and 

 just testimony to the value of the labours of the Eev. Maxwell Close, and proceeds 

 to discuss the essential differences between General glaciation and Local glaciation, 

 one being antecedent to the other, and of much wider extent. Then adopting this 

 division of the subject, he notices each separately, both being said to "have left their 

 traces on the rock-surfaces in the form of polished and moulded bosses, scars, delicate 

 chisellings, or deep groovings often intersecting each other at various angles, and 

 requiring care and some amount of general knowledge of their causes, in order 

 to distinguish to which period of glaciation they belong" (p. 212). Mapping out 

 the lines of scoring at the bottom of the Lower Boulder-clay has determined the 

 general nature of the ice-movement, and has shown, as our author believes, that it 



