On the Formation of Pegmatite. 67 



main vein mass ; and further, in the case of the formation 

 of pegmatite veins, as compared with that of the corres- 

 ponding normal-grained massive rocks, peculiar conditions 

 of formation, the cooperation of particular " agents mine- 

 ralisateurs," have in a high degree made themselves felt 

 along with direct separation through simple cooling of the 

 magmatic solution. 



As far as the unusual coarse-grainedness which frequent- 

 ly occurs in pegmatitic veins of the most various composi- 

 tions is concerned, this must in some may be connected 

 with what was recognized some time ago, 1 i.e. that the 

 pegmatitic veins generally (though not always) may be 

 looked upon as end products of the series of eruptions with 

 with which they are connected ; both when they occur in 

 the main mass of the allied eruptive rock, and when they 

 occur in the neighborhood — and one of the two is always 

 the case — we may assume that the rock surrounding the 

 veinstone was first heated to a high temperature and that 

 therefore the cooling must have taken place unusually 

 slowly and uniformly; and upon this fact primarily the 

 largeness of the grains may be explained. 2 



That this explanation of the coarse grain and of the im- 

 perfect zonal structure of many pegmatite veins is correct, 

 is rendered probable in the highest degree by the frequent 

 occurrence of pegmatitic structure in those portions of rock 

 bordering on the open drusy cavities of many massive 

 granites. I interpret these as analogous to the formation 

 of the pegmatitic veins themselves, in the following way : 

 First, on account of the contraction due to crystallization 

 of the rock already for the most part solidified, there were 

 formed crystal-free lumina ; 3 the mixture of magma and 



1 See C. F. Naumann's Lehrb. d. Geogn. 2, 230. 



2 In the pegmatite veins at Kure, south of Moss, I have seen feldspar indi- 

 viduals measuring more than 10m. in length. 



3 That these (" miarolitische Drusenraume") are so plentiful in acidic rocks, 

 while they are almost always wanting in basic rocks, may perhaps be connected, 

 with the difference in specific gravity between the glass and the holocrystalline 

 aggregates of the respective rocks. In the case of acidic rocks this difference is 

 very great, in the case of basic rocks often very small ; in the first case therefore 

 the contraction during the cooling of the magma would be greater, and in the 

 latter less. ' * 



