780 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



The homogeneity of the average batholith in its visible portion is worthy 

 of special note. The stress continually being laid on evidences of differentiation 

 (basified contact-belts, segregations, etc.,) is in danger of obscuring this princi- 

 pal fact. The homogeneity of one of these large masses, when viewed in true 

 scale, is comparable to that in aqueous salt-solutions in laboratory vessels. The 

 production of this even distribution of oxides must involve vast periods of time 

 and vast stores of heat to keep the magma fluid for the distribution. 



In general a batholith is markedly different from its country-rocks in 

 chemical composition. 



A long list of other chemical relations, which need to be explained by any 

 theory of batholithic intrusion, might here be drawn up, but, to save repetition, 

 their discussion will be transferred to the following theoretical sections on 

 magmatic assimilation and differentiation. 



THEORIES OP BATHOLITHIC INTRUSION. 



Having briefly reviewed the main facts to be explained, we may now proceed 

 to outline the various theories which have been proposed. 



' Laccolithic ' Hypothesis. 



One school of geologists would extend the laccolithic idea to many, if not 

 most, granitic intrusions. Accordingly, the chambers filled with such igneous 

 masses are interpreted as the products of crustal displacement. The planes of 

 single great faults may, in this way, become the locus of the subterranean 

 eruption of magmas, wedging their way along by hydrostatic or other pressure. 

 The well-known ' failure to match ' of the heaved and thrown sides permits of 

 the existence of potential cavities filled with magma during the strong disloca- 

 tion. Encircling faults leading to the foundering of large blocks of the crust, 

 or to the upward thrust of others, are conceived as affording possible modes of 

 intrusion.* Or, finally, as illustrated in the well-known conclusions of Brogger 

 on the Christiania region, colossal masses of granite have been explained as 

 true, deep-seated laccoliths, parting heavy strata after the manner of the trachyte 

 of the Henry mountains.-}- 



Yet it is clear from a survey of geological literature, that the field evidence 

 for such a view is but negative in the great majority of stocks and batholiths. 

 Most of them are not true laccoliths, since they characteristically occur in 

 regions of great structural complexity, where igneous contacts have none but the 

 most remote sympathy with the structural planes of any one bedded series. 

 Many are much too large and irregular in form to be explained as the result 

 of single faults or single zones of faulting ; and the imagined intersecting faults 

 of the ' bysmalith ' or of the submerged graben-block have been generally sought 

 for in vain about those greatest of all granitic massifs. For the latter no other 



• W. C. Brogger, Die Eruptivgesteine des Kristianiagebietes, Vol. 2, 1895, p. 148 ^ 

 J. P. Iddings, U.S. Geol. Survey, Monograph, 32, Part 2, 1899, p. 16. 

 t W. C. BrSgger, op. cit., p. 152. 



