NOVEMBEE 13, 1914] 



SCIENCE 



713 



This error vitiates some of the fundamental 

 hypotheses developed in subsequent parts of 

 the book (p. 229). It has been clearly demon- 

 strated that these supposed differences rest 

 on the failure of the qualitative system of 

 Eosenbusch to classify rocks by their actual 

 mineral compositions, or modes, and also upon 

 the fact that the modes of chemically similar 

 magmas may differ because of the different 

 physical conditions which may have controlled 

 the chemical equilibria within solidifying 

 magmas. This well-known principle is lost 

 sight of by Professor Daly. Apparently the 

 author's units for classification have been 

 petrographical names and definitions, and not 

 the rocks themselves, with which he seems to 

 be less familiar. 



Having qualified the Qualitative System of 

 Eosenbusch to suit his conceptions of it, the 

 author proceeds in the third chapter to employ 

 it quantitatively, and undertakes to determine 

 the relative abundance of various groups of 

 names which he has called " clans," assuming 

 that the areal distribution of igneous rocks, as 

 represented on maps made at various times by 

 many geologists and petrographers, will fur- 

 nish a reliable basis for the comparison of 

 actual rock bodies and of the relative amounts 

 of various kinds of igneous rocks! To one 

 familiar with the methods of geological cartog- 

 raphy, with methods of observation and petro- 

 graphical determination of large areas of 

 igneous rocks, and also with the modes of 

 occurrence of rocks in the field, the idea of 

 employing the areal representations of such 

 bodies as a means of estimating the relative 

 quantities of rocks grouped by Professor Daly 

 into " clans " is remarkable both as an evi- 

 dence of the author's respect for the data 

 before him and as an indication of his con- 

 ception of the structural geology involved. 

 Great areas of igneous rocks are commonly so 

 complex that their mapping is not attempted 

 in detail on many maps, and in some regions 

 a thin surface flow of lava may overlie hun- 

 dreds of square miles of other igneous rocks, 

 which is the case in eastern Idaho, for ex- 

 ample, where basalt overlies rhyolites which 

 are not represented on maps of the region. 



The value of his efforts to determine the rela- 

 tive quantities of various kinds of igneous 

 rocks, as well as of the classification he has 

 applied to the rock bodies studied, appears in 

 his conclusion that the rocks of the globe be- 

 long quantitatively to two types, "granite" 

 and " basalt " ; a statement which shows that 

 his petrography goes back to that early period 

 when " granites " and " greenstones " were 

 considered to be the chief groups. 



In shutting his eyes to the great volumes of 

 intermediate rocks which form the chief bulk 

 of igneous magmas Professor Daly exhibits 

 the results of the method which controls his 

 researches, and also to what extent an ob- 

 server may " feel the pressure of the category " 

 (p. 62). It goes without saying that the 

 writer disagrees with Professor Daly as to the 

 value of the observer under such circum- 

 stances. The lack of breadth in his discus- 

 sions of some subjects is shown by his failure 

 to give proper weight to the effects of erosion 

 in revealing the deeper-seated intrusions of 

 older times, as well as in removing older sur- 

 face lavas, which accounts for the apparent 

 differences he finds in the modes of eruption 

 of magmas in different geological periods. 



The chapters on intrusive and extrusive 

 rock bodies are full of excellent diagrams and 

 illustrations of many instances which have 

 been taken from many sources, as the author 

 states, and they contain a great deal of valu- 

 able material. However, with his opinions re- 

 garding batholiths the writer takes many ex- 

 ceptions which have been expressed in the lec- 

 tures on volcanism already referred to. It is 

 to be regretted that in his discussion of these 

 bodies Professor Daly is constantly confound- 

 ing observed facts with hypotheses to the seri- 

 ous confusion of the reader. His suggestion 

 as to the origin of the rhyolite plateaux of the 

 Yellowstone National Park, which is expressed 

 diagrammatically in Pig. 71, shows the limits 

 to which he is willing to be led by his specu- 

 lations, and becomes a reduciio ad ahsurdum 

 for his batholithic hypothesis, of which he 

 says it is a logical outcome, when one con- 

 siders the geological structure of the region 

 and the character of the rhyolite lava flows 



