NATURE 



[April q, n 



work, especially on volcanic questions, in Zittel's 

 " History of Geology " or Geikie's monumental 

 "Text-book." Ampferer's recent exposition of " Un- 

 terstromung," moreover, as a cause of mountain- 

 crumpling comes very near in many of its details, 

 though not in its foundations, to Reyer's theory of 

 superficial " Stromung " (see Nature, vol. Ixxvi., 1907, 

 p. 423). Though Ampferer quotes very few authori- 

 ties, it seems likely that Reyer's views are still fructi- 

 fying, even if they bear forbidden fruit. At any rate, 

 we may welcome his return to the arena at a time 

 when no champion can assert that his own views on 

 mountain-building have satisfied all known conditions. 



Mr. Reyer himself, in this clearly-written treatise, 

 proposes to leave his statements and arguments un- 

 burdened by a bibliography. The numerous illustra- 

 tive diagrams suggest in most cases drawings made 

 from models, and some of them represent the results of 

 experiments performed in plastic materials. There is 

 a freshness of exposition that brings home to us 

 many facts that are familiar, but which are often 

 liable to be passed over without adequate consider- 

 ation. The exposition itself is, if we may say 

 so, diagrammatic. The author points out very 

 early how delta-deposits slide upon the rocks 

 beneath them, and how a loose valley-side moves 

 out towards the river, which carries away the 

 material, and thus renders the land-tlow less 

 apparent. From p. 52 onward we read of massive 

 extrusions of material from the molten interior of 

 the earth, and we remember the sliding of the deltas 

 when we are asked to picture successive extrusions 

 as lifting the crust above them and allowing of a 

 lateral slip of the ruptured sedimentary layers. The 

 great difference between these postulated extrusions 

 and our common notions of intrusive bodies appears 

 to lie in their long-continued and successive character, 

 whereby a later igneous mass causes a previous one 

 to bulge up and swell out as a coat, through which 

 the later mass may break. It is urged that if this 

 occurs below a deep ocean, the extruded matter may 

 become as cr3'stalline as granite, and subsequent sedi- 

 ments may be laid down on it with a false appear- 

 ance of having been invaded. 



All this should probably be kept in mind, but wc 

 doubt if the average field-observer has been so often 

 misled as Mr. Reyer would wish us to believe. He 

 opposes the idea of the differentiation of a common 

 igneous magma (p. 61), since acid and basic 

 magmas must have separated early in the history 

 of a cooling globe, and massive protrusions are 

 likely to mix them up again, so as to produce 

 the phenomena observed. Such protrusions or extru- 

 sions are held to be sufficient to explain the 

 existence of infolds of sediments between adjacent 

 igneous masses (pp. 69 and 155) ; and an admitted 

 revival of the " crater of elevation " theory appears 

 °" P' 73- There is much in the book that is in 

 pleasant accord with the views consistently maintained 

 in Britain in regard to the relations of various types 

 of igneous rock ; and the principle of successive ex- 

 trusions (or may we say intrusions?) has been adopted, 

 to mention only two examples, by Prof. Sollas for 

 NO. 2006, VOL. 77] 



the Leinster Chain and by Dr. Harker for the gabbros 

 of the Cuillin Hills. 



Mr. Reyer's views cannot fail to make us think 

 more carefully, and we trust that his book will be 

 widely read by those who have to interpret the history 

 of great regions of the crust. We may hereafter be 

 grateful to him for maintaining the theory of gravita- 

 tional gliding (pp. 142, 147, &c.) as opposed to the 

 crumpling of strata by lateral thrust against the 

 influence of gravitation. In this respect he has re- 

 lighted a very ancient candle, which burnt brightly 

 in Scrope's hands in 1825 (" Considerations on Vol- 

 canos," pp. 201-2). It will now', in all probability, 

 never be entirely put out. GREN\n.i.K A. J. Cole. 



HYDRAULICS. 

 IlyJraiiUcs. By F. C. Lea. Pp. xii + 5j6. (London : 

 Edward Arnold, 1908.) Price i8i'. net. 



WH.VTEVER may be the extent to which the 

 science of hydraulics was neglected during 

 the century which elapsed between the theoretical 

 researches of Bernouilli and the practical applications 

 of Armstrong, it cannot be urged that there is any 

 lack of attention to the subject at the present time, if 

 one be permitted to judge from the number of text- 

 books which have appeared within recent years, and 

 even within the last few months. So striking an 

 output must surely be fraught with great potentialities. 



The science of hydraulics has a field which is some- 

 what vague and ill-defined, and is of greater or less 

 extent according to the views of different writers. 

 Originally, the definition of the term only covered 

 a reference to the laws which governed the motion 

 of fluids in pipes and water-courses. Then it was 

 e.xtended to include the use of water for motive 

 purposes, and the means and appliances by which 

 that end was achieved. Other developments have 

 followed, and now it is rare to find two text-books 

 covering even approximately the same ground, or 

 agreeing in respect of either limits or order of treat- 

 ment. We are acquainted with one treatise which 

 includes within its purview the principles of lubrication ; 

 with another which regards what is commonly differ- 

 entiated as the science of pneumatics as a constituent 

 section ; with a third which ignores the motion of 

 bodies in fluids ; and so on. 



The range of the treatise under consideration, 

 though not so extreme as in some cases, is never- 

 theless e.xceedingly comprehensive, and it may be 

 added that the treatment is very complete. In view 

 of the license to which allusion has just been made, we 

 do not presume to offer any criticism on the propriety 

 of presenting the elementary principles of hydrostatics 

 as an introduction to the subject ; we ourselves should 

 hardly have thought of referring to a text-book on 

 hydraulics for them. No doubt it is useful for the 

 student to have his memory refreshed on fundamentals, 

 though he may not unreasonably be supposed to have 

 acquired a competent knowledge of them elsewhere. 

 Still, there they are for consultation, if required, and 

 they form a not inapt introduction. 



