14 



NA TURE 



S^August I, 1889 



If we compare the several series of tables to which we 

 have referred, it is impossible to help being struck by the 

 numerous and important additions which have been made 

 to our knowledge of this branch of science during the last 

 decade. Doelter, in 1876, recognized only 64 species or 

 well-marked varieties of minerals as occurring as rock- 

 constituents ; Hussak, in 1885, raised this number to 

 107; Michel Levy and Lacroix, in 1888, give 160; and 

 Rosenbusch, in 1889, no less than 162 rock-forming 

 types, although none of the opaque minerals are included 

 in these later works. 



Still more striking is the contrast between the earlier 

 and later works of this class, when we come to examine 

 the methods employed for the discrimination of the several 

 species of minerals. In the two later works, the results 

 obtained by the important methods of mineral isolation, 

 first discovered and pursued in the famous petrographical 

 laboratory of the College de France, under MM. Fouque 

 and Michel Ldvy, and the methods of micro-chemical 

 analysis so well elaborated by Boiricky, Behrens, Streng, 

 and other observers are all made use of. Perhaps the 

 most important and characteristic new feature in the two 

 latest published works will be found in the attempt to 

 substitute exact measurements of the double refraction of 

 minerals, for vague statements as to the intensity of the 

 colours which they give with polarized light. It is perhaps 

 too early to pronounce upon the applicability and practical 

 value of the ingenious contrivances of M. E. Bertrand, 

 and of M. Michel Levy for determining respectively the 

 index of refraction and the double refraction in the thin 

 microscopic sections with which petrographers have to 

 deal. If, however, the micro-refractometer and the com- 

 parateur fulfil their promise, it is not too much to hope 

 that the methods of discriminating minerals in cleavage- 

 flakes and thin sections will be as greatly facilitated by 

 the exact determination of their refraction and double 

 refraction as by the study of their positions of extinction 

 or of the optical pictures which they give with convergent 

 polarized light. 



We have spoken of these tables as being of very great 

 service as an index or summary of the excellent " Physio- 

 graphie " of Prof. Rosenbusch, and, indeed, we think their 

 chief value will be realized when they are thus employed in 

 conjunction with that important systematic work. In the 

 treatise itself, it has been possible to point out many 

 important limitations and qualifications of general state- 

 ments, though these have of course to be frequently omitted 

 in the terse indications necessary for the tabular mode of 

 presentation. 



An illustration of the caution which ought to be exer- 

 cised in applying determinations of the so-called optical 

 " constants " of species as absolute standards for the 

 identification of unknown forms is afforded to us in the 

 work before us. In many basalts, like those of Bohemia, 

 there occur large porphyritic crystals of an undoubted 

 amphibole. It has long been known that this " basaltic 

 hornblende" presents some very anomalous characters, 

 especially in respect to its pleochroism and absorp- 

 tion, as well as to the curious chemical changes which 

 it is found in whole or in part to have undergone. 

 The important series of determinations of Michel Levy 

 and Lacroix, which have been incorporated in these 

 tables by Rosenbusch, show that this form of amphi- 



bole is not less anomalous in its double refraction 

 than in some of its other properties. While the differ- 

 ence between the maximum and minimum indices of 

 refraction in all other forms of hornblende ranges be- 

 tween o'02i and 0*028, that of the basaltic hornblende is 

 I found to be as high as 0*072 ! Whether this remarkable 

 I and anomalous departure from the typical characters of 

 an amphibole can be accounted for by the peculiar and 

 somewhat exceptional conditions under which this par- 

 ticular variety is found to occur, it is not necessary here 

 to discuss. But petrographical students cannot be too 

 frequently reminded that^as the isolation and chemical 

 analyses of rock constituents have so often proved — the 

 minerals in rocks may present important differences from 

 those which crystallize out in veins and cavities, and con- 

 stitute the '• types " of our mineralogical collections. 

 Moreover, even small differences in the proportion of 

 certain chemical ingredients have been shown again and 

 again to exercise a very important influence in modifying 

 the values of the so-called optical " constants." 



The great interest of the determinations of Michel Ldvy 

 and Lacroix, which are adopted in these tables — and 

 indeed constitute their most novel and striking feature — is 

 derived from the fact that the observations on which they 

 are based were made upon actual rock-constituents, and 

 not upon the large crystals, often of exceptional character,, 

 which have hitherto been chiefly employed in investiga- 

 tions of this kind. A reference to the numbers repre- 

 senting the refraction and double-refraction of aegyrine 

 among the pyroxenes, and of the several varieties of the 

 epidotes and scapolites, will show that this remarkable 

 case of basaltic hornblende does not by any means stand 

 alone. We trust that the valuable contributions to our 

 knowledge of the characteristics of the actual rock-form- 

 ing species and varieties of minerals which have been 

 made in M. Fouque's laboratory, will be greatly added to 

 in the future : — for by such researches only can we hope 

 that many of the difficulties and anomalies which still 

 surround the pursuit of petrological science will in the 

 end be removed. 



With the kind co-operation of Prof Rosenbusch, Dr. 

 Hatch has been enabled to bring these tables quite up to 

 date ; and he has fairly earned the thanks of all students 

 of petrography by the careful and thorough manner in 

 which he has performed his task. John W. Judd. 



THE INFLUENCE OF SNOW ON THE SOIL 

 AND ATMOSPHERE. 

 Der Einfltiss einer Schneedecke auf Boden, Klima tind 

 Wetter. Von A. Woeikof Pp. i -11 5. Geographische 

 Abhandliingen, herausgegeben von Prof; Dr. Albrecht 

 Penck in Wien, Band III. Heft 3, pp. 321-435- 

 (Wien und Olmiitz : Eduard Holzel, 1889.) 

 T N a country such as ours, which lies on the borders of 

 a great ocean and a great continent, the protean 

 phases of the weather are so greatly determined for us by 

 causes which operate at a distance from our shores, that 

 the influence of any temporary variation in the condition of 

 the local surface is comparatively unimportant, and, excep 

 locally, perhaps altogether inappreciable. But the furthe 

 we recede from this border-land and penetrate to the interior 

 of the continent, the more influential do such temporary 



