B. A. Daly — Mechanics of Igneous Intrusion. 21 



angular projections of country-rock into a well uncovered 

 batkolith are comparatively rare. Such smoothness of main- 

 contact surfaces is that which is to be expected on the stoping 

 hypothesis. A projection of country-rock would suffer spe- 

 cially intense shattering by the magma, which would thus tend 

 to destroy the projection and smoothen the wall of contact. 

 The case is analogous to the familiar exfoliation on sculptured 

 stone in great city fires : architrave, sill, abacus and plinth 

 lose their corners, ornaments in high relief are rifted off, and 

 flutings are effaced. Bowlders of disintegration through 

 weathering furnish other analogies. 



In detail of form as in the larger field-relations of the typi- 

 cal stock and batkolith, therefore, we seem to have cumulative 

 evidence in favor of the theory of replacement and especially 

 in favor of the hypothesis of mechanical replacement. On 

 the other hand, the more intimate becomes our knowledge of 

 these field-relations, the more improbable the "laccolithic 

 theory " becomes. Neither smooth, flowing contact-surfaces 

 against a heterogeneous terrane, nor a general elliptical ground- 

 plan, nor an invariable downward enlargement are expected 

 to characterize a batkolith if it is simply a huge laccolith. 



These summary statements are founded on the writer's field- 

 experience, and on a tolerably wide study of the geological 

 literature relating to granitic intrusions. The essential idea 

 of replacement rather than displacement is far from new ; it 

 has been a lasting merit in the able work of Barrois, Michel 

 Levy, Lacroix and others, that they have persistently held to 

 this fundamental fact of field-occurrence. Yet there are 

 to-day many working geologists who just as persistently refuse 

 to recognize the fact of the field. The chief reason for this 

 refusal has undoubtedly been that the replacement of the 

 country-rocks has, until recently, been attributed to their pro- 

 gressive solution on the main contacts — in other words, to 

 marginal assimilation. The patent difficulties of this one view 

 have prevented many, perhaps most, geologists from subscrib- 

 ing to the conclusions of their French colleagues. The proved 

 insufficiency of the marginal-assimilation hypothesis has thus 

 discouraged belief in that kind of replacement, but it by no 

 means alters the fact of magmatic replacement. On the other 

 hand, this fact will stand, no matter what theories of intrusion 

 may prevail. 



So far as recorded, the stoping hypothesis is the only one 

 which recognizes the progressive assimilation of country-rocks 

 as the magma rises in the crust, and, at the same time, 

 explains the common lack of chemical sympathy between 

 granites and their respective wall-rocks. By this hypothesis 

 the preparation of the upper and visible part of the mag- 

 matic chamber is largely a mechanical process, working along 



