114 



KNOWLEDGE 



[Feb. 1, 1886. 



quite unlike then he has to inquire in what respects they 

 diiler. They may be formed of very different materials, 

 or of the same materials, or of the same materials dif- 

 ferently proportioned ; or they may be formed of similar 

 materials similarly proportioned but in different states ; 

 or differences of all these kinds may coexist. It is un- 

 fortunn.te, we think, thit Mr. Geikie has not sufficiently 

 considered this general question, in the light of the most 

 advanced modern inquiries, but has adopted the crude 

 idea, derived in part from misapprehension of Laplace's 

 nebular hypothesis (itself but a crude guess) in part from 

 ignorance of the law.s of physics, that the evolution of our 

 .\ystem has led to the lighter materials finding their place 

 in the outer planets, the heavier in those near the sun. We 

 feel justified in saying that no astronomer now holds this 

 opinion as even probable ; while we can hardly imagine 

 that even those who, not being astronomers, have made 

 more or less incomplete inquiry into the matter from its 

 chemical side, can regard the idea as more than a mere 

 guess. Astronomically, the idea is untenable, it is opposed 

 to the known laws of physics (for instance, the law of 

 gaseous diffusion), while the chemist who considers the 

 relative position of the hydrogen and the oxygen or 

 nitrogen in our earth, will assuredly reject the idea that 

 the lighter elements necessarily tend to the outside even 

 in single orbs — the idea that they do so in systems of orbs 

 formed by processes of aggregation or condensation being 

 still wilder. 



Mr. Geikie's mistake, here, is unfortunate, because it 

 deprives him at the outs?t of :>,11 the light he might 

 hr.ve derived from the consideration that there may be 

 among the planets orbs telling of the influence of various 

 peculiarities of structure on the progress of a planets 

 life-history, while there may be other orbs telling of the 

 fe.-itares characterising (he vnrious stages through which 

 our earth itself has already passed or may have to pass 

 hereafter. For example, we have in our moon an orb 

 which has probably, owing to its smaller mass, had much 

 smaller seas, much rarer air, and also much shorter life- 

 stages than the earth ; while gravity at the surface on 

 which so much of the activity of denuding and resto- 

 r.itive forces depends, has been but one-sixth on the moon 

 what it has been on the earth. So that we are justified 

 in looking on the moon for traces of the effects of those 

 vulcaniau activities which are at work in the earlier 

 history of every planet but the effects of whose work on our 

 own earth has long since been removed by the denuding 

 forces of air and water. On the other hand, we are justified 

 iu expecting to find certain products of denuding action 

 by sea, river, rain, wind, storm, ice, snow, and so forth — 

 r.s mountain ranges themselves for instance — less plenti- 

 ful in the moon than on the earth. Studying the moon 

 then, wo may say that because such and such features are 

 more common on the moon than on the earth they pro- 

 bably belong to the earlier stages of vulcanian activity, 

 and that because such and such features are less common 

 on the moon than on the earth, they may probably be 

 regarded as in the mnin products of denudation. Or 

 slightly altering our point of view, the men who first 

 studied the moon with the telescope, might had they had 

 the genei-.il doctrine of evolution to guide them, have 

 Slid that, becau.se there are hundreds of great craters on 

 the moon and comparatively few on the earth, therefore 

 the more active denudmg forces of the earth must have 

 destroyed her great crater.s, whose wrecks however we 

 may profitably search for : and on the other hand because 

 Eountain ranges are a much more marked feature of the 

 earth than of the moon, the great ranges are probably 

 the work of denudation, and we may profitably examine 



the structure of the Alps, the Himalayas, the Rocky 

 Mountains, and so forth, for the evidence that, lofty 

 though they now are, they have been formed by processes 

 of sedimentary deposition in great trough-like depres- 

 sions beneath former seas. 



So with those planets which like the giants Jupiter 

 and Saturn seem to tell us of earlier stages of planetary 

 life. Those orbs may be profitably studied to throw light 

 upon that past stage of the earth's history when the 

 ! seas once formed a portion of her atmosphere, along with 

 C'Xygen and nitrogen in many times greater amount than 

 I at present, with carbon dioxide (our former friend car- 

 bonic acid gas) in enormous quantities, with sulphurous 

 acid, sulphuretted hydrogen, chlorine, boracic acid, 

 sodium vapour, magnesium vapour, and other components, 

 probably raised to enormous distances from the planet to 

 which they were thereafter to belong by repulsive forces 

 akin to those which act upon the vaporous material of 

 comets' heads.* If we can recognise among the planets 

 some which are younger than our own earth, we may be 

 able by studying such orbs, to see our way to the recog- 

 nition of our earth's state when as yet there were neither 

 oceans nor continents. 



It maj' perhaps bo that Mr. Geikie's failure to recog- 

 nise the obvious bearing of many of the most interesting 

 astronomical researches of our time upon geological 

 problems, has been the reason why he has left almost un- 

 touched some subjects of geological inquiry which have 

 greatest interest for students of astronomy — as for 

 example the history of mountain ranges as studied by 

 some of the leading geologists in Germany and America, 

 the question of the primeval condition of the air and sea, 

 and the inquiry into the probable future effects of 

 changes now in progress upon and within the earth. His 

 study of astronomical evidence on such matters as these 

 is limited to the inquiries — highly important no doubt 

 in themselves — which astronomical physicists have made 

 into the earth's own condition and probable past, inquiries 

 in which we see astronomers dealing with geological 

 problems rather than studying astronomical matters iu 

 such sort as to throw light on geology. 



But if there is some shortcoming in this respect and 

 Mr. Geikie has not been fortunate in his study of astro- 

 nomical evidence, it may safely be said that in no other 

 respect can any fault be found with this splendid treatise. 

 So soon as we enter on the strictly geological matter, we 

 find a combination of the most studious research with 

 admirable literary qualities. The book is a student's 

 treatise of the most thoroughgoing sort, while at the same 

 time it is a work which every one who takes interest in 

 the teachings of science will thoroughly enjoy. The 

 student cannot spare one paragraph of the charmingly 

 written passages in which the results of geological 

 teaching are presented for general reading ; and though 

 the general reader need not be at the pains to study all 

 the details collected for the special iise of the student, 

 yet these are so arranged that he has no trouble in 

 knowing what to omit. 



All the divisions of the work are well done ; but the 

 division relating to what Mr. Geikie calls Stratigraphical 

 Geology, the record of the earth's crust (that great 

 volume of Nature's Bible) as to material life and as to 

 the varied forms of vegetable and animal life which have 

 successively prevailed on the earth, is especially valuable. 

 Complete (so far as on the scale even of so large a book 



* In some such way only can the apparent inconsistency between 

 the observed depths o£ the gaseous and vaporous envelopes of the 

 giant planets (or of the sun) and the laws associating density and 

 pressure of gases and vapours, be removed or explained. 



