On the Formation of Pegmatite. 45 



not is a matter of superfluous speculation, as we know no- 

 thing about it. 



The above mentioned observations in the Christiania dis- 

 trict, which demonstrate definitely that here the granitic 

 and syenitic, as well as other plutonic rocks, have been 

 formed at relatively small depths below the surface, show 1 

 also that Lehmann's assumption that " it is sufficient to 

 assume for the granite magma no higher temperature than 

 that which prevails at the minimum depth which must be 

 assumed for granite " (1. c, p. 55), must be incorrect, and 

 further that the pressure under which the plutonic rocks 

 solidified is not always so tremendously great as has often 

 been assumed by Lehmann and other authorities. (He 

 assumes, for instance, a pressure of 4,000 atmospheres.) 



The principal requirement for the solidification of deep- 

 seated magmas to noncrystalline plutonic rocks seems, there- 

 fore, to consist in a sufficiently slow cooling of the water- 

 bearing magma, under a pressure of superimposed matter 

 great enough to prevent the water separated out by crystal- 

 lization from freely escaping to the surface, and compelling 

 it by a pressure exerted from above to pass into the wall- 

 rock (contact metamorphism.) 2 The temperature of the 

 deep magma must, however, certainly have been consider- 

 able. The protecting covering of strata, which quickly 

 became impregnated with and warmed by the escaping 

 water- vapor, had, in the Christiania region, a thickness of 

 but a few hundred metres, but was, nevertheless, sufficient 

 to bring about this slow cooling. Of course the rate of 

 cooling is further dependent upon the quantity of the 

 magma solidifying at any one time. 



The peculiar gelatinous consistency of the magma which 

 Lehmann seems inclined to assume, the " viscous silica 

 jelly," etc., of which he speaks, seems also hardly to be 



1 Similar conclusions may also be drawn from other eruptive districts which 

 have recently been described. 



2 That contact metamorphosis is characterised by molecular re-arrangement, 

 and only to a very small extent, and locally in direct contact with the eruptive 

 rock itself, by any addition of material, shows that Lehmann's assumption, that 

 the escaping water could not exist at the level of solidification of the granite in 

 a liquid condition, is highly improbable. 



