Al'RIL 23, I9O3 



NA TURE 



579 



graphy is " the descriptive, systematic science, leading 

 to the nomenclature of these objects." This elevation 

 of systematic description to the rank of a science dis- 

 arms a certain amount of criticism. The able authors, 

 who confront us after a long period of careful thought 

 and collaboration, take their stand here as petro- 

 graphers, with all the dignity of a well marshalled 

 mediaeval " battle." Right and left we may read the 

 blazon on their shields; their pages trumpet forth the 

 titles by which they would be known ; they stand for 

 system and for order, for a " hierarchical classifica- 

 tion " (p. 3), against a hitherto careless and indifferent 

 world. 



If the four champions are, for the time, not petro- 

 logists, but petrographers, still more strongly do they 

 stand apart from the geologists. We have usually 

 regarded rocks, and the lessons to be learned from 

 them, as coming within the scope of the geologist. 

 Just as the mineralogist begins with molecular aggre- 

 gates, so the geologist begins with mineral aggregates, 

 and from them seeks to read the history of the world. 

 No such agreeable considerations are to be tolerated 

 in the science of petrography. Our authors, some of 

 whom, at least, have long been welcomed as geologists, 

 have entered the field under sober vows of self-denial. 

 The object of the petrographer (p. 63) is " to secure 

 logical excellence for his system." 



We are truly grateful for Mr. Cross's term " hier- 

 archical." It seems to define the situation, and to add 

 zest to the devious paths of heresy in which most of 

 us at present wander. We even foresee that the petro- 

 logists — not the petrographers — will in the future be 

 divided into two schools, those who desire a classifica- 

 tion and those who would rather be without it. For 

 the first school, there is much salvation in the present 

 treatise; it will, indeed, give them as logical a classi- 

 fication as the imperfect human mind can conveniently 

 grasp. As such, it has been welcomed by Mr. F. D. 

 Adams in the pages of Science (February 27, 1903). 



The reader, in mere fairness, must consider the prin- 

 ciples of any classification independently of its nomen- 

 clature. The first proposition here made is (p. 128) to 

 divide igneous rocks into five " classes," according to 

 the minerals which might have crystallised, under 

 certain conditions, from the magma represented by the 

 chemical analysis of the rock. On the one hand we 

 regard the group of minerals, quartz, felspars, 

 felspathoids, zircon and corundum; on the other the 

 group pyroxenes, olivine, magnetite, haematite, &c, 

 in fact, broadly speaking, the ferromagnesian group 

 (p. 116). The five classes are simply established accord- 

 ing to the numerical predominance of one group or the 

 other. 



Each of the two mineral groups is divisible into two 

 subgroups ; in the first three classes of rocks, the silica- 

 alumina group of minerals is of such importance that 

 five " subclasses " may be based on the relative pro- 

 portion of the two subgroups within this group in 

 each particular rock. Similarly, the two subgroups 

 of minerals of the ferromagnesian group are utilised 

 to establish five subclasses of rocks inside classes iv. 

 and v. (p. 130). 



We may now pass on to "orders." One mineral 

 NO. 1747. VOL. 67] 



subgroup in each subclass of rocks predominates over 

 the other subgroup ; on this basis we obtain orders of 

 rocks, of which there are as many as nine in each of 

 the first three subclasses of each of the first three 

 classes (p. 132). Orders, with equally strict logic, may- 

 be divided into " sections." 



So far, the possible minerals have given a position 

 to the rock. We may, however, consider the " general 

 character of the bases in the minerals of the pre- 

 ponderant group in each class," which enables us to 

 assign a " rang " to the same rock. Rangs are so 

 absorbing that we confess to some annoyance when we 

 come on p. 141 to " grads," which look like another 

 exercise in subclasses, a matter that we have already 

 taken carefully to heart. " Subgrads," a further 

 division, need not be discussed in the present brief 

 review. 



Suddenly it flashes upon us that we have all this 

 time been dealing with a possible but wholly im- 

 aginary object, and not with the rock which we have 

 plucked, after miles of travel, from its parent mountain- 

 side. Let not this thought obtain the mastery; it is 

 a temptation of the evil one, whom we may call Lossen, 

 or Judd, or Rosenbusch — for even the last-named 

 author is now classed with the geologists. The rock, 

 for hierarchical purposes, has both a body and a soul ; 

 the former (p. 147) is its " mode," or actual mineral 

 composition; the latter is its "norm," or standard 

 mineral composition, as obtained by calculation. 

 Some species, of saintly character, have modes co- 

 incident with their norms; where this is not the case, 

 the difference demands investigation. 



Herein clearly lies the great value of precision in 

 petrography, such as our authors introduce. The 

 definite statement of the facts is obviously of first im- 

 portance, before we seek to explain the deviation of 

 mode from norm by experimental or observational 

 geology. In such a statement, geological consider- 

 ations are out of place; geological conditions have con- 

 trolled the mode, but should not influence the name 

 and rank assigned to the resulting product in a system 

 of pure petrography. If we very properly reject geo- 

 logical age as a factor in rock-classification, so it is 

 equally desirable to reject such groupings as " plu- 

 tonic," "dyke-rocks" and "volcanic." Pp. 149 to 

 153 of the present book go far indeed to justify the 

 precision of its system. 



When, however, we come to part ii., on nomen- 

 clature, we prefer to leave the reader to go forward by 

 himself. Every science must have its technical phrase- 

 ology, and Mr. Bather justly objects 1 to the replace- 

 ment of Rhinellus furcatus by " the fork-tailed Nosey." 

 Yet those who multiply technical terms, and especially 

 technical adjectives, forget that even the most special- 

 ised of specialists is not dealing with each term once 

 a day; nor is he forced to describe a natural object as 

 if a railway-whistle had sounded which summoned 

 him for ever from the scene. May not scientific 

 workers take heart, and rain grace, from the cultured 

 descriptions of pictures in an ordinary well conducted 

 catalogue? What are rocks but pictures, recording the 

 most varied incidents in the history of the ground be- 



1 See the Museums Journal, vo'. ii .p. 1^8. 



