740 



SCIENCE 



[N. S. Vol. XXXI. No. 802 



able, half of it may well be spent in the 

 study of elementary crystal optics so that min- 

 erals may be identiiied in slides. The above 

 remarks apply especially to igneous rocks, as 

 there is less variety in the sedimentaries and 

 metamorphics and the loose nomenclature 

 used for them makes them easier to classify. 

 It may be urged that the broader chemical 

 and geological features should be emphasized, 

 that is, petrology rather than petrography 

 should be taught. The writer is in entire ac- 

 cord with this view, but unless the student 

 makes numerous rock analyses, how better can 

 he learn to appreciate the chemical side of 

 petrography than by a study of slides? 



My views on this subject naturally depend 

 somewhat upon my opinion of the recently 

 proposed megascopic or field classification of 

 igneous rocks. One of the serious criticisms 

 applied to the ordinary qualitative classifica- 

 tion is the redefinition of rock names. Yet 

 in this field classification we have such names 

 as syenite and basalt redefined to suit the 

 megascopic determination. Perhaps the dis- 

 tinctions made on a megascopic basis are good 

 ones, but terms that do not conflict with ordi- 

 nary usage are preferable. Such names as 

 leucophyrs are all right, but it seems hardly 

 fair to call an anorthosite a syenite when the 

 plagioclase may be determined at sight, since 

 all its affinities are with the gabbros. It 

 hardly seems reasonable to call a dark-colored 

 porphyritic rock a basalt-porphyry when 

 quartz or orthoclase phenocrysts are visible. 

 Typical andesites can readily be distinguished 

 and it hardly seems necessary to call them fel- 

 site-porphyries. The writer believes that the 

 usually accepted grouping of igneous rocks 

 into granites, rhyolites, syenites, trachytes, 

 diorites, andesites, gabbros, diabases, basalts 

 and peridotites is the best one to follow even 

 in megascopic work. Of course one can not 

 always make the distinctions recognized in 

 this classification, but this is also true of any 

 rock classification. Often one is fortunate if 

 he can distinguish an igneous from a meta- 

 morphic rock in the hand specimen. One of 

 the principal reasons for studying petrography 

 is that the student may be able to read geolog- 



ical literature intelligently. Even though the 

 ordinary classification is purely qualitative 

 and the personal equation large, yet the 

 names for the common rocks given above are 

 fairly definite in their meaning as used in the 

 literature for the last twenty-five years or so. 

 In conclusion the writer would summarize 

 his views as follows : The purpose of the 

 petrography course is to give the student a 

 general idea of rocks, to enable him to make 

 rough determinations of rocks at sight, and 

 to help him in the understanding of geolog- 

 ical literature. With these things in mind the 

 -study of hand-specimens and slides should go 

 hand-in-hand. The student becomes familiar 

 with the common rock types and so can de- 

 termine other rocks by mental comparison 

 with those he has studied in detail. The 

 usual classification (granites, rhyolites, etc.) 

 is suitable for megascopic determinations and 

 is also the one recognized in the literature. 



Austin F. Eogers 

 Laboratoey of Mineealogy 



AND PeTROGEAPHY, 



Stanfoed Univeesity 



SCIENTIFIC B00K8 

 The Mutation Theory. Volume I. "The 

 Origin of Species by Mutation." By Hugo 

 DE Vries. English translation by Professor 

 J. B. Farmee and A. D. Darbishire. Pp. 

 xvi + 582. Four colored plates and 119 

 text-figures. Chicago, The Open Court 

 Publishing Co. 1909. 



The publication of the German work, " Die 

 Mutationstheorie," by Hugo de Vries, marks 

 an epoch, not only in the history of botany, 

 but of all biological science; and the muta- 

 tion-theory itself is, in all probability, the 

 most important contribution to evolutionary 

 thought since the publication of Darwin's 

 " Origin." The importance of de Vries's work 

 lies not only in the elaboration of the theory 

 of saltation as an adequate method of the 

 origination of new forms in the organic world, 

 but (and more especially) in removing the en- 

 tire question forever from the realm of in- 

 effectual debate, and establishing it upon the 

 firm basis of experimentation. 



