IS B. A. Daly — Mechanics of Igneous Intrusion. 



and most forcibly shows the value of the stoping hypothesis 

 and of its implied principles in explaining the rocks and field- 

 relations in that state.* 



Notwithstanding the 'support given the hypothesis by the 

 work of these and other observers, the main conception has 

 not met with favor from many working geologists. f A num- 

 ber of objections have been raised, most of which were dis- 

 cussed in the first two papers of this series. Within the last 

 five years an unusually large amount of experimental data has 

 been added to the confessedly meager store of known facts 

 concerning the physics of rocks and rock- melts. These labor- 

 atory results, when fairly interpreted, seem to the writer to 

 dispose of most of the objections. Other objections fall away 

 as soon as they are confronted with the indisputable, long- 

 known facts concerning rocks and igneous magmas. A third 

 class of the objections are more stubborn and still remain 

 among the frank difficulties of the stoping hypothesis. It is, 

 however, the writer's belief that these difficulties are small 

 when compared to those adhering to the older theories of 

 batholithic intrusion. 



In this third paper some of the more significant, newer con- 

 tributions of the experimental laboratory to the matter at issue 

 will be noted and discussed. In the light of the whole body 

 of fact as understood by the writer, he will attempt to make 

 clear the reasons why the various criticisms against the stop- 

 ing hypothesis do not seem fatal to its acceptance. Finally, a 

 new statement of certain important corollaries and tests of the 

 hypothesis will be offered. In their discussion a certain 

 amount of speculation seems not only warranted but necessary. 

 It is obvious that the basis of any theory of the igneous rocks 

 must, in part, consist of speculative assumptions ; for every 

 fruitful theory must deal with the earth's invisible interior. 

 Neither petrology nor geology can afford to leave the problem 

 of the earth's interior " to the poets. " The advances of mod- 

 ern chemistry have largely been made possible through con- 

 structive speculation as to the nature of molecule and atom ; yet 

 molecule and atom are as inaccessible as the core of the earth. 

 In the nature of the case we can never hope to arrive at the 

 final explanation of igneous-rock bodies without building and 

 testing hypotheses of materials and processes in and under 

 the earth's " crust. " Not only petrology but, in marked 

 degree, mining geology is awaiting a stable theory of batho- 

 lithic intrusion, since upon it must largely depend sound pet- 

 rogenic and minerogenic theory. 



*E. C. Andrews, Records, Geol. Surv., N. S. Wales, vii, Pt. 4, 1904, and 

 viii. Pt. 1, 1905. 



f'Cf. Science, xxv, p. 620, 1907. 



