Origin of Pegmatite. — Crosby and Fuller. 153 
place to place. The schists are poor in alkalies, being com- 
posed chiefly of silica and alumina; and their absorption by 
the pegmatite magma would obviousl}'^ tend to make the peg- 
matite richer in quartz and muscovite, as the following com- 
parison of the alkaline silicates shows, the formation of a pure 
aluminum silicate being impossible in the presence of alka- 
lies. 
Orthoclase, SiO^ , 64.7; Al^ O.5 , 18.4; alkalies, 16.9. 
Muscovite, SiO." , 46.5; AlJ O.^ , .3:19; alkalies, 10.0. 
This is in harmony with our observation that the mica mines 
of New Hampshire, and so far as we are able to learn of other 
regions, are chiefly in the schist series, and we believe that in 
general the pegmatite is more micaceous in the schists than in 
the granite. It may be that we have here an explanation of 
greisen — that rather anomalous aggregate of quartz and mica. 
Lepidolite has been noted as a specially characteristic feature 
of pegmatite, and the fact that it also occurs in greisen is 
certainly suggestive of the genetic relationship of the two 
rocks. We also venture the suggestion that perhaps pegma- 
tite rich in tourmaline, and tourmaline or schorl rock (tour- 
maline and quartz) may be explained in the same way, 
tourmaline representing a still greater dearth of alkalies — 2 
to 4 per cent. 
Although this close agreement in composition between peg- 
matite and its parent plutdnics certainly holds for the princi- 
pal component minerals, the two types seem to be contrasted 
as regards their rarer or accessory constituents, pegmatite 
being much richer in these than granite. G. vom Rjith, in his 
account of the celebrated pegmatite veins of Elba,* says that 
the ''tourmaline, beryl, lithia, mica, etc., are foreign to normal 
granite," and that "these veins, on account of their wealth in 
rare and peculiar minerals, which for the most part are wanting 
in the w"all-rocks, necessitates the assumption of a special 
mode of formation ([uite dilferent to that assumed for the 
wall-rock." That tiie rare species are a more conspicuous 
feature of pegmatite than of granite is obvious, but that they 
are much more abundant when large volumes of rock are com- 
pared, has certainly not been proved, and is at least doubtful. 
*Die Insel Elba (Googn.-min. Fragraeoteaus ltalien,vni)in Zeitschr. 
d. d. geol. Ges., 1870, 22, 649. 
