R. A. Daly — Mechanics of Igneous Intrusion. 283 



be produced. The time required for the formation of a cham- 

 ber appropriate to a " batholith " or large stock may be great, 

 but doubtless no greater than that posited, for example, by 

 Brogger in the fashioning of the huge laccolithic chambers of 

 the Christiania Region,* and certainly no greater than the time 

 demanded for the opening of a similarly extensive chamber by 

 mere caustic action on the molar contact. Stoping will vary 

 in rapidity with the size of the blocks rifted. The average 

 block from visible contacts is most probably smaller than the 

 average block rifted during the much longer period of high 

 fluidity in the magma. Perhaps the partially sunken blocks 

 resting on the quartz porphyry " laccolith " near Drammen, 

 Norway, may represent possible sizes for such blocks. They 

 are long slabs from 250 to 1500 meters in diameter. f 



A brief statement of this central idea of the stoping hypo- 

 thesis has been given by Lawson in a review of certain of 

 Brogger's writings. So far as known to the present writer, 

 this noteworthy paragraph contains the only clear enunciation 

 of the doctrine to be found in geological literature, and is 

 worthy of quotation in full : 



" The essential features of the assimilation hypothesis were 

 formulated by the reviewer some years ago, before the publica- 

 tion of Michel Levy's views, and urged as a satisfactory explan- 

 ation of the remarkable relations which obtain between the 

 Laurentian granites and gneisses and the upper Archaean or 

 Ontarian metamorphic rocks. These intrusive granites and 

 gneisses occupy vast tracts of the Archaean plateau and there 

 seems to be no escape 'from the view that they bear abatholitic 

 relation to the crust which they invaded from below. Portions 

 of the crust were absorbed, but there are two possibilities as to 

 the method of absorption, viz: 1. by fusion; 2. by sinking 

 into the magma. The numerous blocks of rocks scattered 

 through the granites lends much probability to the latter hav- 

 ing played a part in the process. Such batholites were doubt- 

 less accompanied by laccolitic satellites." J 



It is recognized that the stoping hypothesis, like the older 

 ones which demand the supposition of high magmatic fluidity, 

 must meet a difficulty suggested by the last consideration. If 

 the eruptive body was, throughout its whole history, plutonic, 

 as is generally assumed for the world's great granite massifs, 

 how could the roof of its chamber, when of large span, be sup- 

 ported ? The question is of great importance, but the intended 

 limits of this paper will not permit of its being fully dealt 

 with, even if all the elements of the problem were at hand. 



* Die Eruptivgest. des Kristianiagebietes, ii, 144 (1895). 



t Brogger, op. cit., p. 136. 



% Science, New Series, vol. iii (1896), p. 637. 



