Jan. 17, 1889] 



NA TURE 



26c 



Atlantic on slight provocation is, I fear, rife among us, 

 and ought not to be encouraged. 



The Ordovician and Silurian maps are probably as 

 near approximations as the present state of our knowledge 

 allows — maybe as we shall ever attain to. In the Lower 

 Old Red Sandstone map, the Old Red of South Wales is 

 represented as having been formed in a bay of the Devo- 

 nian sea. This view does get over some difficulties. If 

 we suppose it formed in a fresh-water lake, we must 

 admit that the barrier separating the lake from the Devo- 

 nian sea was narrow, and, as our author remarks, there 

 is no independent evidence for the existence of such a 

 barrier. The absence, as far as we know, of any marine 

 fossils tells the other way, but the district is yet geologic- 

 ally almost a terra incognita, and we must wait before 

 the question can be settled. In the same way it must 

 be confessed that it is hard to see on what grounds the 

 Glengariff Grits are classed as marine ; but here again 

 the "retort courteous" might be, " What reason have you 

 for thinking them fresh-water deposits ? " It is too large 

 a question to go into here. 



The Carboniferous map I turned to with the greatest 

 curiosity, for it so happens that years ago I was rash 

 enough to try my hand at a similar production. The 

 thing has thrust itself upon me many times since, and 

 each time I have seen something in it that dissatisfied 

 me, and it has been touched up and tinkered till now I 

 hardly recognize my own child ; and till I know my own 

 mind, it would be hardly fair, even supposing it possible, 

 to attack another man for differing from me. Really, the 

 two maps have more in common than appears at first 

 sight ; and on some points of fundamental importance— 

 the land-locked character of the Carboniferous sea for in- 

 stance — Mr. Jukes-Browne and I are in complete accord. 

 I should not have ventured on this bit of personal re- 

 miniscence, if it had not been that it seemed to me that 

 it may possibly be typical. Put a number of equally 

 qualified men to construct one of these geographical 

 restorations, and the result will probably be this : there 

 will be some few points on which all must agree ; but the 

 data for settling details will be so vague, that no two 

 maps will be alike m their minor points. Even if this be 

 so, it by no means proves the work to be unprofitable ; 

 but it is as well to bear this in mind when comparing two 

 independent restorations. 



Space will not allow of detailed criticism of the series 

 of maps with which the book is lavishly illustrated ; but 

 all readers will gratefully thank the author for the pains 

 he has taken to render such effectual help to them in 

 following his reasoning. Every chapter bears the mark 

 of patient and conscientious work ; and though in a 

 book of this size no more than an abridged and concise 

 statement of many of its facts can find a place, its sug- 

 gestiveness will insensibly lead the real student to the 

 original papers of which it is an epitome, and open for 

 him a wide field of reading. 



The chapter on the Pleistocene epoch is one on which 

 the author has evidently spent much pains, but it seems 

 to me the least satisfactory in the book. With much 

 that he says I heartily agree. I cannot help feeling that 

 some of our most eminent glacialists have ridden their 

 theories rather hard. That the Scotch Till is a moraine 



profonde seems to me the only hypothesis yet put forward 

 which gives anything like a satisfactory explanation of the 

 origin of that deposit ; but I am not prepared to admit a 

 like origin for all the so-called Boulder-clays : most of 

 those who have studied on the ground in detail the 

 Boulder-clays of the plains of Lancashire and Cheshire 

 have come to the conclusion that they are submarine, and 

 that their boulders have been supplied by floating ice- 

 Some of the objections which Mr. Jukes-Browne urges 

 against the ground-moraine theory, however, do not seem 

 to me serious. He cannot understand how it is that an 

 ice-sheet could groove and polish the rocks and form a 

 ground-moraine at the same time. There are many ways 

 out of the difficulty. Ice-scratching is most conspicuous 

 on high ground and steep slopes, where there is little or no 

 Till. It is true that it is far from uncommon on lower and 

 flatter ground, where it is covered and indeed preserved by a 

 coating of Till. Here it may be the first work of the ice- 

 sheet before much ddbris had been dragged down from the 

 hill country ; but we must also bear in mind the probable 

 character of a ground-moraine : packed closely by the 

 weight of the ice above and frozen hard, it would be very 

 different from the imperfectly consolidated mass we see 

 now : rather it would act as a solid whole, and, as it was 

 dragged along, would be quite capable of effecting a large 

 amount of abrasion. The alternative which is suggested 

 involves the floating of the ice-sheets bodily over wide 

 extents of sea ; but, as far as we know, ice-sheets do not 

 float as a whole when they push their way out to sea, they 

 break up into icebergs. We may picture to ourselves the 

 probable action of an ice-sheet somewhat after this fashion* 

 While descending slopes even moderately steep, it would 

 push before it and drag beneath it any loose debris that it 

 found ready made to its hand or that it had itself torn off 

 the surface. But here its motive power would be sufficient 

 to carry with it all the loose matter ; consequently here 

 no Till would be formed, unless the sheet happened to 

 encounter a gorge in its path. In such a case the stones 

 and dirt would be driven into the hollow till they filled 

 it up, and the ice would then ride over it. When the ice- 

 sheet reached flatter ground, its dragging power would be 

 seriously diminished : it would probably at first heap up 

 the debris into a mound or ridge in front of it : this 

 mound after a time it would override, and flatten and 

 spread out its materials ; by a continuance of the pro- 

 cess a sheet of Till would be spread over the lowlands. 

 Of course here too any valley that lay athwart the path of 

 the ice would be filled up. Thus would be produced 

 exactly the distribution of the Till which occurs : in the 

 hill-country little or none except as filling in valleys ; over 

 the plains a broad sheet, and great thicknesses in the valleys 

 of the low country. So that when our author states that 

 Prof. James Geikie's views might be accepted " if the 

 Boulder-clay was found to fill in lake-like hollows," he is 

 describing very nearly an essential feature in the actual 

 manner of its occurrence. 



Having now discharged the functions of the critic, 

 and pointed out what appear to me some weak points, I 

 will only add that if I seem to have been scant of praise, 

 it is because there was no need. The book recommends 

 itself. 



A. H. Grekn. 



