474 Reviews — R. A. Daly — Igneous Rocks. 



incorporation of blocks of the country rock sinking in the invading 

 magma. Many of the older petrologists believed that igneous 

 intrusions, such as granite, were formed by the simple fusion in place 

 and recrystallization of ' country rock ' normally sediments. 

 Chemical analysis soon showed this hypothesis to be untenable in 

 such a simple form, but Professor Daly's ground idea is really 

 a modification of this; he postulates an original acid granitic shell, 

 underlain by a basic basaltic shell; by mixtures of these, and by 

 mixtures of basaltic magma and sediments (syntectics) assisted by 

 differentiation, he derives all possible igneous rock-types, in many 

 cases with much plausibility. It is important to note that this 

 theory does not propose to explain fully the igneous history of the 

 earliest pre-Cambrian times ; it only professes to deal with the 

 phenomena of later pre-Cambrian (post-Keewatin) and post-Cambrian 

 time!*. The earlier events seem to have consisted largely in the 

 differentiation by gravity of the fundamental granitic and basaltic 

 shells. The later events are explained in large part as due to the 

 penetration of wedges of the underlying basaltic magma into the 

 granitic shell and into the sediments that subsequently came to 

 overlie it. 



One of the chief lessons to be learnt from a careful reading of this 

 book is the unimportance, in fact even the danger, of an elaborate 

 nomenclature. Tlie names applied to rock varieties are legion; some 

 700 different rocks have been defined and indexed; a very large 

 proportion of these are varieties of the so-called alkaline I'ocks. 

 Hence we obtain a distorted and unnatural idea of the importance 

 of tliis group. When, however, the quantitative method is applied, 

 as is done by Professor Daly, things soon begin to appear in more 

 accurate proportion. It is estimated by the author that in North 

 America the alkaline rocks amount to less than one-tenth of 

 1 per cent of all the igneous rocks, perhaps even less than this, 

 and in Europe certainly to less than 1 per cent. Here, then, the 

 appalling list of rock-species of alkaline type, including pulaskite, 

 kakortokite, naujaite, lujaurite, ouaeliitite, bekinkinite, and many 

 other efforts of a diseased petrological imagination, sink into their 

 true insignificance, and the great fundamental fact emerges that the 

 alkaline rocks as a whole are only incidental products of a planet 

 whose eruptions and intrusions have been overwhelmingly of 

 sub-alkaline quality. Furthermore, from this quantitative treatment, 

 it is clear that basic sub-alkaline rock-types predominate in the 

 extrusive phase of igneous action, acid sub-alkaline types in the 

 intrusive phase, or, in other words, abundant evidence is brought 

 forward to show that the normal rock-types of the world are granite 

 and basalt. Apart from theory, this is a fact of observation resting 

 on statistics accessible to all, and geologists are muuh indebted to 

 Professor Daly for having laid emphasis on such an important, but 

 for long unrecognized, truth. 



In the light of these facts, the great and commonly accepted 

 division of rocU-species into Atlantic and Pacific types loses most 

 of its force, at any rate in so far as concerns their connexion with 

 different types of earth movement, or with actual geographical 



