116 H. S. JEVONS, H. I. JENSEN AND C. A. SUSSMILCH. 



depths. Basalt spreads out in sheets almost like water 

 when erupted in large quantity, and at high temperatures 

 and the pressures of great depths must be extremely fluid. 

 Probably there is an actual decrease of viscosity up to a 

 certain depth owing to the retention of gases ; but in any 

 case the ratio of motive force to viscosity almost certainly 

 increases considerably with depth. It appears to me, 

 therefore, quite legitimate to assume that all basic magmas,, 

 and, at some depth, all intermediate magmas, are highly 

 fluid ; and that at great depths acid magmas are fairly fluid. 

 On the other hand, intermediate lavas must be considered 

 to be rather viscous, and acid magmas near and at the 

 surface highly viscous. 



With the constant earth movements which probably 

 agitated our globe before it was cool enough for life to 

 originate, magma and solid rock must frequently have been 

 forced into different positions in the crust — usually higher,, 

 but often, also, relatively lower — that is to say, beneath 

 adjoining portions of the crust. As the result of such 

 movements, there must have been not only the progressive 

 silification of the metallic substratum already mentioned, 

 but also, I think, extensive melting up and assimilation of 

 previously formed igneous rocks, due both to release of 

 pressure, and to the high temperature of a magma. In the 

 Alps and Western Highlands of Scotland, and in numerous 

 other cores of mountain chains, we have in streaked gabbros, 

 diorites, etc., abundant evidence of melting up with incom- 

 plete mixture. When the mixture has been complete there 

 exists no reliable means of proving it. Is it not likely that 

 at greater depths than are anywhere accessible to us a 

 considerable amount of remelting and mixture has taken 

 place? 



My conclusions, briefly stated, are that plutonic magmas 

 are not for the most part the result of differentiation as- 



