April 15, 1SS6] 



NA TURE 



555 



beyond which it is to be feared the vision of some 

 geologists never reaches ; but as a history, the history of 

 the earth during ages long gone by. And as the historian 

 is careful to inform his readers of the sources from which 

 he has drawn his information, what manuscripts he has 

 collated, what monuments he has inspected, what in- 

 scriptions he has deciphered, so the book opens with a 

 description of the materials which are available for con- 

 structing a history of the earth. First of all we are 

 reminded that " that history is in progress now as really 

 as it has ever been, and that its events are being recorded 

 in the same way and by the same agents as in the far 

 past;" so that "if we would explore its records in the 

 dark backward and abysm of time, we must first make 

 ourselves familiar with the manner in which these records 

 are being written from day to day before our eyes." We 

 are introduced to this study by an account of the manner 

 in which atmospheric agents are bringing about ceaseless 

 decay over the whole of the surface of the globe. The 

 important part played by the freezing of water receives 

 due notice, but it would seem that sufficient stress is not 

 laid on the magnitude of the force generated during this 

 process. That ice "pushes aside the particles between 

 which it is entangled " would hardly prepare one who 

 does not know it for the fact that ice can burst asunder a 

 cast-iron shell. The figure on p. 72, it may be noticed in 

 passing, does not strike us as very happy. We must also 

 take exception to the reasoning on p. 105 : so very little 

 is known about the formation of the manganese nodules 

 and coatings of the abysmal depths of the ocean, that it 

 is somewhat risky to assume that it was an exceedingly 

 slow process. Perhaps, too, a little over-confidence is 

 shown in treating of " fissure eruptions" ; doubtless some 

 otherwise puzzling facts do receive easy explanation on 

 the hypothesis that such eruptions have taken place, but 

 this is not enough to convert the hypothesis into a 

 certainty. 



We next pass to a description of the more important 

 elements, minerals, rocks, and rock-structures of the 

 earth's crust. It is extremely difficult to decide how 

 much or how little chemistry and mineralogy is desirable 

 in an elementary treatise on geology. It is not safe to 

 refer to special works on these subjects, because the 

 majority of readers would not take the trouble to make 

 the reference, and yet the descriptions which the limits of 

 space will allow of are necessarily so curt and meagre as 

 to be of little practical use ; if therefore we criticise any 

 portions of Chapters X. and XI. it is not because we are 

 not fully alive to the difficulties of dealing in an element- 

 ary fashion with the subjects of which they treat. In the 

 definition of a mineral it should certainly have been 

 stated that minerals, besides having definite chemical 

 composition and definite geometrical form, have also 

 definite physical properties, such as hardness, which are 

 most valuable as means of recognition. 



Again, in the description of a crystal, one of the first 

 points to impress on a beginner is that the dimensions of 

 the faces and edges are of no importance, and that the one 

 thing to note is the constancy of the interfacial angles, 

 and this is a simple truth which any one may be got to 

 understand ; we wish this had been brought out more 

 clearly. The author has followed the time-honoured 



custom of giving a brief summary of the six crystallo- 

 graphic systems. We are sadly afraid that the descrip- 

 tions, though perfectly accurate as far as they go, and the 

 excellent figures by which they are illustrated, take up 

 room which might be more usefully employed, for these 

 few paragraphs will never enable a student to read a 

 crystal unless it be of the simplest character, and they are 

 not full enough to be an introduction to a more detailed 

 study of crystallography. It has always seemed to us 

 that the best plan for an elementary work would be to 

 take one actual crystal — say of orthoclase, with basal and 

 prismatic faces, clinopinacoids, and orthodomes — and, 

 without using any of these technical terms, to explain 

 how the crystallographer arrives at this crystal by grafting, 

 so to speak, certain additional faces on an ideal simple 

 prism ; how the shape of that prism can be defined by 

 reference to certain lines Sand their inclination to one 

 another, which are called axes ; how the position of the 

 additional faces are related to these axes — all of which 

 are geometrical truths of the simplest character ; and 

 then to say how all crystals, however complicated, can in 

 like manner be referred to certain simple forms of which 

 there are six ; and if you want to know how, you must go 

 to a work on crystallography. 



The student will then get a real knowledge of one 

 actual crystal, instead of learning by rote descriptions of 

 ideal forms, not one of which he will meet with in nature. 

 We may next note one or two statements which might be 

 usefully amended in a second edition. It is not uni- 

 versally the case that the least fusible mineral crystallises 

 first in a molten rock, as stated on p. 169; it would be 

 well to mention that the rhombohedral crystals of hajma- 

 tite (p. 172) are usually so very fiat that they look like 

 plates or lamina: : some zeolites (p. 176) contain calcium 

 and barium as well as alkalies ; whether alumina replaces 

 the bases or the silica in hornblende (p. 177) is a question 

 on w-hich chemists do not seem to be agreed ; under the 

 head of calcite (p. 179) the wording would imply that the 

 difference between nailhead and dogtooth spar consists 

 only in a difference in the length of the chief axis, which 

 is not the case. When it is said that gypsum is not 

 " affected by acids " (p. 206), it is probably meant that it 

 does not effervesce with acids. The evidence for looking 

 upon heat as a pseudomorph after chalk, so to speak, is 

 so strong that it might have been alluded to on p. 212. 

 Under the head of quartzite it would have been desirable 

 to notice that the conversion of sandstone into quartzite 

 has in many cases been mainly brought about by the 

 deposition of silica between the original quartz grains. 

 And while we are on the subject of metamorphism it will 

 be well to call attention to a slight inaccuracy of language 

 into which our author, in common with many other geolo- 

 gists, has fallen. The important part played by pressure in 

 developing schistose structure in rocks is now verygenerally 

 recognised, and has been nowhere better illustrated than 

 by the labours of Mr. Geikie and his colleagues on the 

 Geological Survey among the crystalline schists of the 

 Highlands, and a fashion has arisen of speaking of the 

 action as " shearing." Now, that shearing has taken 

 place there can be no doubt, but shearing is not all that 

 has happened : the particles have not only been slid over 

 one another, but they have been rolled out and flattened 



