On the Formation of Pegmatite. 6.9 



corresponding way must also be assumed in the case of the 

 formation of the minerals in the pegmatite veins them- 

 selves, is naturally also important for the correct under- 

 standing of many veins not truly pegmatitic, but clearly 

 very closely connected with these. Between crack-fillings, 

 principally magmatic, of a pegmatitic character, and those 

 corresponding only to the later stages of mineral deposit in 

 the pegmatitic druses and veins (e.g. in the class of acid 

 granitic pegmatite veins, as final member, the quartz veins,) 

 all possible gradations are known, as has been correctly 

 emphasized by earlier authors (particularly by Lehmann) ; 

 it must, however, always be borne in mind, that these 

 crack-fillings, although genetically in part related, are how- 

 ever in no sense pegmatitic veins. Pegmatites form only 

 one stage in the series of vein equivalents of a massive 

 plutonic rock ; granitite, granophyre, aplite, pegmatite, are 

 different stages in the magmatic vein formations of the 

 plutonic rocks, the pegmatites as a rule still in the main 

 magmatically solidified veins, therefore formed under some- 

 what altered conditions, and even passing into the crack- 

 fillings which succeed them in point of time and which are 

 not in the main, or are not at all, deposited from true mag- 

 matic solutions. l Although in what has been said above, 

 the coarse-grained structure, as usual, has been very 

 strongly emphasized, it must be remembered that this 

 alone does not condition the pegmatitic nature of the veins, 

 nor is even necessarily present in order to justify the 



1 That many large-grained veins of a pegmatitic structure have been formed 

 principally by pueumatolitic processes, and not mainly by magmatic solidifica- 

 tion, has already been stated above many times ; such are the apatite bearing basic 

 veins, also many occurrences of cassiterite, of tourmaline and topas, «fcc. That 

 also the mmcovite granite pegmatite veins, containing especially beryl, topas, 

 etc., and having as principal minerals microcline, oligoclase, albite, quartz, mus- 

 covite, are, in comparison with the ordinary granite pegmatite veins with which 

 they frequently occur and which among fine-grained veins correspond to the 

 genuine aplites rich in muscovite, perhaps of a somewhat later formation than 

 these which are of a slightly different magma and to a larger extent of pneuma- 

 tolitic formation, is for many reasons probable ; this would also explain very well 

 why they occur along the eruptive boundary of genuine granitite or within the 

 granitite along with genuine granitite pegmatite veins, although massive rocks of 

 corresponding composition are generally wanting in the neighbourhood. To enter 

 into details would lead too far. 



