On the Formation of Pegmatite. 39 



mode of formation quite different to that assumed for the 

 wall-rock." 



That these rare minerals, not generally found in the wall- 

 rock, occur in the pegmatite veins, is no reason whatever 

 for believing that the veinstone itself is to be considered as 

 having been deposited from ascending aqueous solutions, 

 even should such an explanation satisfactorily account for 

 the rare minerals themselves which occur in small quantity, 

 and of which by far the greater number have been formed 

 by the agency of special " agents mineYalisateurs." 



Strictly speaking, as the more recent results of petro- 

 graphy have shown, it is true that aqueo-igneous (hydato- 

 pyrogene) magmas such as those from which great masses 

 of plutonic rocks as well as the pegmatite veins have been 

 formed, are to be regarded as silicate solutions ; thus, to a 

 certain extent, vom Eath is correct in his opinion, though 

 not in the sense in which at that time he must have meant 

 it. From ordinary "hot springs" "rising from the 

 depths of earth" the pegmatite veins, as far as their vein- 

 stone as a whole is concerned, have certainly not been 

 formed ; this is proved by many circumstances which for 

 the most part have already been touched upon, and of which 

 a resume will be given below. 



Formerly pegmatite veins were very often looked upon 

 as "contemporary secretions" or " concretions of the sur- 

 rounding eruptive rock." ! Views of this kind have been 

 especially insisted upon in connection with our syenite and 

 nepheline syenite pegmatite veins by many authors. Thus, 

 B. M. Keilhau says : 2 " They form an excellent example of 

 veinlike segregations, which cannot be regarded as having 

 been formed by the filling in of cracks, by any one who con- 

 siders the intimate connection and, with the exception of the 

 size of the grains, the complete agreement in character of the 

 enclosing rock with the enclosed masses, and who also lays 

 due weight upon the fact of their almost horizontal posi- 



1 Compare v. Groddeck I. c, p. 266. 



2 Gaea Norvegica, 1838, 1, 58. 



