Origin of Pegmatite. — Crosbg and Fuller. 151 
sett bay, opposite Conanicut island, there is, in the cliff's 
known as the Bonnet and Packard rocks, a series of mica 
schists backed by normal granite and traversed by numerous 
very typical veins of pegmatite 5 to 50 feet wide. A mile to 
the eastward, on Dutch island, the veins in the schist are 
quartz with some feldspar, but no mica; while on the west 
shore of Conanicut island, so far as observed, they are 
quartz alone. Van Hise has noted a similar gradation in 
the character of the pegmatite veins intersecting the schists 
which envelop the granite core or batholite of the Black Hills, 
the veins changing gradually from typical pegmatite near the 
granite to ordinary quartz veins at a moderate distance. 16th 
Anniuil Report, United States Geological Survey, Part I, p. 
(588. 
lielations of Composition to the Inclosing Hocks. — Broadly 
speaking, the pegmatites are independent of the wall rock in 
composition. In New Hampshire we have found pegmatites 
of strikingly uniform composition traversing in succession 
and without sensible change a whole series of granites, gneisses, 
and schists of the most diverse character. Brogger and 
William^ have specially noted this indifference of the pegma- 
tites to the character of the inclosing formations. The former* 
cites as examples the acid pegmatites of Hittero, which cut a 
very basic (labradorite and anorthite) rock, and those of 
Rudemyr cutting Silurian schists and limestones, and the lat- 
terf speaks of the acid pegmatites of Maryland as occurring 
in gabbro and peridotite and extending from gneiss into lime- 
stone without sensible change. 
The one important exception to this principle is found in 
the fact which lies at the foundation of the modern theory of 
pegmatite, viz., that in ever}'^ pegmatite district there is one 
normal plutonic rock of essentially similar but slightly less 
acid composition, with which the pegmatite is most intimatelj^' 
associated, into whicii it may of ten be traced, and from which 
it has evidently been derived. That is, the pegmatites are 
contrasted with the inclosing formations in all cases except 
where they traverse the plutonic mass of which they are the 
most highly differentiated phase or end-product. Brogger and 
^Canadian Record of Science, 6, 3G and 37. 
tU. S. Geol. Survey, .\nn. Report, If), G83. 
