DALY. — THE NATURE OF VOLCANIC ACTION. Gl 



which is to be expected in large bodies of syntectic magma. This, in 

 brief, seems to be the essence of batholithic intrusion. A batholith is 

 an abyssal injection originally endowed with sufficient thermal energy 

 (size) to enlarge its chamber through incorporating roof and wall rocks. 

 The lower and greater part of its chamber is formed by simple dike- 

 like injection along a widening abys.sal tissure. The upper part, which 

 is generally the only part exposed by erosion, has been opened by the 

 activity of the magma itself The average batholith, at exposed levels, 

 is granitic because granite represents the stable and least dense dilVer- 

 entiate of the average syntectic. 



The integrity of the batholith's roof is evidently threatened in two 

 ways. It is thinned during the process of absorption of the roof-rock 

 by the molten magma. The latter might work its way to the surface 

 through piecemeal stoping, which might continue until a large area 

 of the batholithic roof has disappeared. On the other hand, it is also 

 possible that part or all of the roof should, under special conditions, 

 founder en nuisse in the less dense magma. In either case true vol- 

 canic action is produced. Such wholesale or piecemeal foundering 

 would not fairly be called simple fissure eruption, though it might be 

 accompanied by lava floods emitted from fractures in the roof- rock 

 surrounding the foundered area. The level surface of the lava in the 

 area of foundering would, in form, resemble a plateau (fissure) eruption, 

 but the lava would here be generally liparitic rather than basaltic, as 

 in the great majority of plateau eruptions. Moreover, the liparite 

 would form a continuous mass merging downwards into granite, and 

 thus not a series of superposed distinct flows. According to the topog- 

 raphy, the lava of the foundered area might flood valleys outside that 

 area. If the hydrostatic adjustment were accomplished in stages, 

 it would cause successive, superposed flows in the valleys. (See 

 Figure 2.) 



Needless to say, the field evidences do not favor the idea of founder- 

 ing in the case of many, perhaps most. Paleozoic and later batholiths. 

 These bodies must be regarded as truly plutonic, according to the or- 

 thodox creed. Yet it is expedient to entert^iin the hypothesis in ex- 

 planation of the field relations of some pre-Cambrian batholiths as well 

 as those of a few younger masses. 



In the first })lacc, the evidence «jf local foundering in the past is in 

 special danger uf being obliterated. The glas.sy or scoriaceous phase 

 of the " batholith " will nece.s.sarily be eroded away before the granitic 

 phase can bo exposed, 'i'ho liparitic phase need extend to a depth of 

 no more than a few iiundred meters, where it would rapidly merge into 

 the holocrystalline phase. Therefore, comparatively little time would 



