loO The American Geologist. March, i897 
bases (iron oxides, cassiterite, etc.), apatite being, perhaps, 
the most conspicuous exception, although the complete list of 
the pegmatite minerals includes also ti^anite and various nio- 
bates, tantalates, and tungstates. The great disparity as re- 
gards abundance makes possible a quite sharp distinction be- 
tween the so-called principal or essential, and the rarer or 
accessory, species. The essential species include quartz; the 
acid feldspars — -'orthoclase, oligoclase, albite, and microcline 
— the last two being especially characteristic; and the niore 
acid micas, including muscovite and lepidolite and, less char- 
acteristically, biotite. The accessory minerals are more nu- 
merous, in some cases more distinctive, and especially remark- 
able for the large proportion of rare species or species that 
are rare or wholly wanting in other rocks. A single mica 
mine in New Hampshire (the Ruggles mine in Grafton) is 
said to have afforded sixty-one minerals, and it is undoubt- 
edly true that the list of known silicate species is considera- 
bly longer than it would be if these natural cabinets, or 
storehouses of fine minerals, had never been studied. 
The quartz of pegmatite, as of ordinary granite, is espec- 
ially characterized by numerous inclusions of water and car- 
bon dioxide. The proportion of carbon dioxide to water has 
been proved to be so high in some cases — for example, nearly 
1 to 1 in the quartz of the Branchville, Connecticut, pegmatite 
— as to indicate that the crystallization took place under 
enormous pressure.* 
Our observations abundantly eontirm Williams' statementf 
that the most typical pegmatites are intimately associated 
with, and pass gradually into, bodies of pure quartz and ordi- 
nary (quartz veins. But we cannot agree with this writer in 
drawing across this undoubted gradation an arbitrary line 
of demarcation as regards origin. On the contrary, we feel 
that the true theory of pegmatite must recognize tliis obvious 
relationship to quartz veins, by making of the latter a possible 
end-product of the same great process of differentiation which 
yields the former. In other words, some quartz veins are on 
the same road that lead to pegmatite, but a little fartiier 
along, at the end of the road. On the west side of Narragan- 
*A. W.Wright, American Journal Science, March, 1881. 
tUnited States Geological Survey. Fifteenth .\nnual Report, pp. 678 
and 679. 
