8o 



NATURE 



\Dec. 4, 1873 



of water or other metamorphic influences— it was not 

 difficult to make out half-a-dozen distinct varieties of rock 

 from the same mass and even from the same quarry. 

 And so analysis of rocks grew and multiplied, chemists 

 became more and more nice in their discrimination of the 

 veriest fractions of a per cent., petrography seemed in a 

 fair way of being annexed as a dependent province of 

 chemistry, and the petrographers, who ought to have been 

 geologists, and to have set themselves strenuously to find 

 out what had been the history of the rocks as parts of the 

 architecture of the globe, came gradually to accustom 

 themselves to the notion that, after all, it was really true 

 that rocks were merely so many chemical compounds to 

 be analysed and labelled accordingly. 



In the midst of the darkness wherein the poor petro- 

 graphers ticketed their specimens, carefully arranged their 

 cabinets, and elaborated their dreary treatises, there fell 

 among them (not from heaven, but from the hands of a 

 worthy citizen of .Sheffield) a microscope and a few glass 

 slides, with a description of what could be done therewith. 

 Eyes which had seen no fight for so long could not at first 

 make anything of the apparition, but after a few years it 

 began to take shape before them ; and now the micro- 

 scope promises to do as much in comparison for mine- 

 ralogy and petrography as it has done for the biological 

 sciences. 



From town to town this new light has spread, or rather 

 rushed, all over Germany. There is now a sort of neck- 

 and-neck race who will make the most sfices of rocks and 

 minerals. A cutting or rubbing-machine and a micro- 

 scope have become as necessary implements as a hammer 

 or a lens. Every month brings to light some new 

 " mikromineralogische " contribution, insomuch that if 

 the fever lasts we shall ere long be as over-weighted with 

 microscopic analysis as we used] to be with chemical. 

 Both are excellent and necessary, and yet we may be 

 allowed to believe that neither singly nor together do they 

 disclose to us anything like the whole history of the rocks, 

 and that they cannot by themselves yield a sufficiently 

 broad foundation for a truly philosophical classification in 

 petrography. 



The advantages of microscopic analysis applied to 

 rocks are so many and obvious that we cannot be sur- 

 prised that they should have been so widely recognised 

 and put in practice. This method of investigation 

 throws a direct light upon the construction of rocks in a 

 way which chemical analysis can never do. Moreover, 

 it is'easily adopted. Anybody can make microscopic sec- 

 tions, and with due care and experience may become a 

 skilful analyst. And then this mode of research is so 

 cheap. Even if the observer does not care to give the 

 trouble and time necessary for the construction of his 

 own sections, he can get them made for him at small 

 cost. And once in possession of them and his microscope, 

 he obtains his results at once. No need to wait for days 

 upon a solution, or to weigh and re-weigh his precipitates. 

 It is plain that as rocks arc composed of aggregates of 

 minerals in many various combinations, the first prelimi- 

 nary step in our investigation of their minute structure 

 should be devoted to the study of the microscopic cha- 

 racter of the minerals which compose them. We must 

 know how these minerals are built up in themselves be- 

 fore we can adequately comprehend the manner in which 



they are mingled together to form rocks. Besides, in a 

 crystalline rock, such, for example, as basalt, the compo- 

 nent minerals are crystallised on so minute a scale, and 

 often so imperfectly, that their ordinary and characteristic 

 peculiarities may be so veiled that, unless from previous 

 experience, we could not with certainty recognise them. 

 Hence every student who sets himself, microscope in 

 hand, to find out how the materials of the rocky crust ot 

 the earth have been put together, ought unquestionably to 

 begin the search by accustoming his eye to the variations 

 which tlie simple minerals present when viewed in diffe- 

 rent positions under a strong magnifying power. It will 

 not be necessary for him to cut slices of every known 

 mineral. He will have done enough for his immediate 

 purpose if he has sliced in all directions, and examined 

 with polariscope and otherwise the comparatively few 

 simple minerals which are of prime importance, as those 

 of which most of the visible rocks of the globe have been 

 formed. 



A text-book which will guide him in this most interest- 

 ing and important research has never hitherto appeared. 

 Descriptions of the methods to be employed in the prepara- 

 tion of translucent sections have been published both in this 

 country and in Germany. Indeed, it was Nicol, of Edin- 

 burgh, who, besides giving us the prism named after him, 

 invented and made known more than forty years ago this 

 most ingenious method of investigation. Abundant no- 

 tices have also been published during the last dozen years, 

 chiefly in Germany, regarding the microscopic characters 

 of many minerals and rocks, so that a student who had 

 time and opportunity to consult this very scattered litera- 

 ture, might gain amply sufficient knowledge to start him 

 in his research. But none the less has a general text- 

 book been required to save such needless expenditure of 

 time, and to give the student those practical hints which 

 he is not likely to meet with in mere scientific communi- 

 cations on special subjects. It is this want which iVIr. 

 Rosenbusch endeavours to supply in the volume now 

 before us. 



From his preface we gather that living at Freiburg he 

 caught the microscopic fever, and has had it for a number 

 of years. Anxious to communicate the infection as safely 

 and effectually as possible to the younger mineralogists, 

 he has compiled a text-book which ought to serve its 

 purpose well. It is well arranged, neatly printed, ex- 

 cellently illustrated, and cheap. After some introductory 

 pages which skim over the history of his subject, the 

 author proceeds, in the first or general part of his treatise, 

 to give the student directions how to cut and prepare his 

 slices of mineral or rock for microscopic examination. 

 Then, having the slices prepared, he shows how they are 

 to be used, and what maybe looked for in them. With cha- 

 racteristic German completeness he speaks of the general 

 morphological peculiarities of minerals crystallised and 

 amorphous, and shows how singular and varied are the 

 anomalies in internal structure revealed by the microscope 

 even in what seem to be the most regularly built crystals. 

 The optical properties of minerals are discanted upon 

 with a fulness perhaps hardly in keeping with the other 

 parts of the book, but the importance of this branch of 

 the subject, particularly in reference to the analysis by 

 means of polarised light, may well be pleaded in excuse 

 by the author. The third section of the general descrip 



