The Geolog cal Society of America. — Hovey. 107 
Hampshire, Virginia, North Carolina, the Black Hills region of South 
Dakota, the Cribbensville district in northern New Mexico, western 
Idaho, and additional deposits of promise have been found and de- 
veloped on a small scale in the Appalachian region in Maine. South 
Carolina, Georgia and Alabama; and in California, Wyoming and 
Nevada. 
These deposits of commercial mica are all found in pegmatyte 
"veins" or dikes; and these pegmatyte dikes occur in gneissic and 
granitic rocks which are usually classed as Archaean. The dikes yield- 
mg the best and largest quantities of mica are found in the hornblendic 
and micaceous gneisses and schists in places parallel to, but gen- 
erally breaking across the schistosity of these rocks at varying angles. 
The dikes vary in thickness from a few inches to more than 250 feet. 
<ind can be traced for a distance varying from a few feet in the smaller 
ones to sometimes several miles in the larger ones. In some cases 
they are quite irregular and have arms branching out in almost every 
direction. Many of them are vertical, while others are nearly hori- 
zontal, and still others vary very greatly. 
The pegmatyte consists mainly of quartz and feldspar in equal or 
variable proportions and muscovite mica; in some places the quartz 
and feldspar are somewhat uniformly distributed throughout the 
pegmatyte mass, while in other cases the two are fairly well separated, 
the feldspar sometimes crystallizing out into masses more than a 
ton in weight. In addition to these three common minerals there 
occur in these dikes a large, number of minerals with varying degrees 
of rarity. The dikes in certain localities sometimes contain a con- 
siderable number of these accessory minerals — 20 or more being oc- 
casionally observed in a single dike — and in other regions few or 
none of them are to be found. As a rule crystals or "books" of 
commercial mica need not be looked for in dikes or veins under two 
feet in diameter, but there are cases in which these "books," 2x2^/2 
feet in diameter, have been found in dikes, the width of which was 
scarcely greater than these figures. On the other hand in case of 
some of the largest dikes no mica whatever of commercial value is to 
be found. 
Generally the "books" or crystals of mica are scattered promiscu- 
ously through the matrix of quartz and feldspar and in mining opera- 
tions a large amount of useless material has to be blasted down and 
removed from the mine in order to secure the commercial product; 
in other cases, however, the "books" or crystals of mica occur in the 
outer ]iart of the dike near the wall rock and in these cases the mica 
"lead" can be more easily followed. In some cases the mica con- 
stitutes as much as 10 per cent, of the total mass of the dike, but in 
the majority of cases it will prove to be less than one per cent, of the 
total. Of the mica taken from the dike in ordinary mining opera- 
tions, usually less than 10 per cent, and sometimes less than 2 per 
cent, has a commercial value as sheet mica; the remainder being either 
thrown away as waste material or pulverized for commercial uses. 
