536 



NA TURE 



[Feb. 12, i ! 



ive can hardly agree with the statement that "clay is 

 identical in composition with felspar" (p. 46), when we 

 remember that the one contains about 47 and the other 

 about 65 per cent, of silica ; it is somewhat a surprise to 

 find the old time-honoured section of a mountain chain, 

 given in Fig. 34, p. So, still surviving ; if one wished to 

 select a section of what a mountain-chain is not like, here 

 we have it. What would Mr. Huggins say to the statement 

 on p. 17, that "the nebula: are now known to be in no 

 respect nebulous" ? On p. 22 we are told that, " when a 

 typical felspar contains potash, it is recognised by frac- 

 turing at right angles to the side (sic) of the prism." This 

 is hardly calculated to convey a notion that potash felspar 

 has two cleavages, and certainly gives no definite idea of 

 the direction of either. 



The treatment of the subject of joints is bewildering ; 

 it reminds me of the advice given to a youthful and diffi- 

 dent teacher, " If you are asked to explain any thing you 

 don't understand, look solemn and talk of polar forces." 

 We have vague hints that the crystallisation of mono- 

 clinic augite and triclinic plagioclase may in some myste- 

 rious and unexplained way have determined the direction 

 of the cracks on the hexagonal jointing of basalt, and that 

 the structure is essentially crystalline (p. 43) ; then, on 

 p. 82, our mind is unsettled by the statement that tension 

 in successively different directions is more probably the 

 true cause of the phenomena of jointing in slaty rocks ; 

 then heat and electricity, it is surmised, may have had 

 something to do with it. Surely in the place of these 

 guesses, or at least in addition to them, it would have 

 been well to mention some of the facts that throw light 

 on the solution of the problem ; that the columnar struc- 

 ture of basalt is closely mimicked by the hexagonal 

 columns of starch and dried clay in the formation of 

 which crystallisation took no share ; and that Daubree 

 has produced, by torsional strain, cracks, the directions of 

 which follow the same law as governs the trend of master- 

 joints in rocks. It is this preference of shadowy surmise 

 to solid substantial fact which constitutes one main fault 

 of the book. The notice of " reversed faults " on p. 77 is 

 incomplete ; Rogers and Heim have taught us that these 

 are the rule in violently contorted districts, and the latter 

 has given a beautiful explanation of how they have been 

 produced. The formation of coal is dismissed in ten 

 lines, at least this is the only reference to coal given in 

 the index, and I have not found anything on the subject 

 elsewhere in the book. When I think what an admirable 

 instance of inductive reasoning the discovery of the terres- 

 trial origin of coal supplies, and how the study of sound 

 inferences like this is one of the best ways of developing 

 the logical faculty in the student, I cannot help regretting 

 that some of the questionable speculations with which the 

 book abounds have not been left out to make room for 

 an account of the way in which this truth was arrived at. 

 I fear very much that the directions given on p. 253 will 

 not help the student much to identify minerals under the 

 microscope. One would gather from them that colour 

 was the one important point to attend to, for this is 

 almost the only thing noticed ; and it is strange that in 

 the case of olivine, where the extreme vividness of the 

 colours is of some little use as a distinctive test, no notice 

 is taken of the fact. Amphibole, it would seem, is to be 

 distinguished from pyroxene by giving brighter colours, 



but the widely different cleavages of the two minerals, 

 and the dichroism of the one and its absence in 

 the other are passed over. Nor is a word said about 

 the dichroism of tourmaline and magnesian mica. Of 

 the many points to be attended to only one is men- 

 tioned, and the most important facts under that head are 

 omitted. 



For such reasons as I have given I cannot help feeling 

 that this work is unsuited for teaching purposes ; indeed, 

 on account of the way in which it mixes up theory and 

 fact, I should say it would be positively dangerous to put 

 it into the hands of a beginner. 



But it will be by no means without its use to those who 

 have made some progress in the study of geology. It is 

 an admirable geological gazetteer. The long lists of 

 localities whe _ e typical examples of the various classes of 

 rocks may be studied and the condensed descriptions 

 of the geological structure and history of the various dis- 

 tricts cited, will be of great value. References to original 

 memoirs are frequently given, but they might be largely 

 increased with great advantage ; it would be scarcely- 

 possible to make them too numerous. There is much, too, 

 in the speculative portion of the book, which, even if it 

 be in places hazy and but slightly supported either by 

 observation or experiment, is still very acceptable. Even 

 the guesses of an acute and original thinker are 

 welcome. 



In the section devoted to palaeontology the puzzles and 

 uncertainties of that branch of geology are stated without 

 reserve, and the lines on which the palaeontologist must 

 work are clearly marked out. The chapter on the " Suc- 

 cession of Life in Classes and Orders of Animals " is too 

 crowded with detail for beginners, but I fancy that the 

 advanced student will often turn to it for reference and 

 thank the author for having furnished him with such a 

 concise index to the subject. The brief indications given 

 under each head will serve as starting points, and as he 

 develops and expands his knowledge by the aid of special 

 treatises and original memoirs, the student will find out 

 for himself where the author is propounding his own 

 peculiar views, and where he is in accord with his fellow- 

 palaeontologists. 



A. H. Green 



OUR BOOK SHELF 



Transit Tables for 1885. By Latimer Clark, M.I. C.E., 

 &c. (London : E. and F. N. Spon, 1885.) 



At a period when the question of universal time is in 

 ever)- one's thoughts, more or less, these tables should 

 possess more than ordinary interest. By the production 

 of a simple, efficient, and inexpensive transit instrument 

 Mr. Clark first demonstrated that transit observations 

 were within the power of others than the professional 

 astronomer or the wealthy amateur, and that by these 

 observations timekeepers could be regulated to the frac- 

 tion of a second. The next step was to simplify the 

 calculations involved in the reduction of these observa- 

 tions, by the yearly publication of tables giving in Green- 

 wich mean time, instead of sidereal time, the transits of 

 the sun and a few of the principal stars conveniently situ- 

 ated for observations for every day in the year. This is 

 chiefly what is accomplished in these tables, now in their 

 fourth year of publication. In addition to the fundamen- 

 tal stars, the transits of five of which are given for every 



