only a few inches thick, but generally average 1 to 2 feet thick, and the removal of 1 or 2 additional feet of rock is often necessary before pegmatite of a marketable grade is reached.

The evidence of incomplete prospecting is everywhere apparent in the districts investigated by the bureau, and many valuable deposits have doubtless been overlooked by such practices.

THE FELDSPAR DISTRICTS OF THE NEW ENGLAND AND NORTH APPALACHIAN STATES.

MAINE.

In Maine there are at least three distinct districts where feldsparbearing rocks exist in commercial quantity and several minor centers which are worthy of more thorough prospecting than the scope of this investigation has permitted. The most eastern point at which a commercial grade of feldspar is found in this State is at East Orland, Hancock County, where dikes of a medium-grade microcline pegmatite protrude in many places several feet above the surrounding country. The dikes are generally only a few feet thick and the exposed parts have weathered on their surfaces to a mass of sand (Pl. I, A), in which the quartz grains and the feldspar particles have been separated by the weathering, but below the surface the dikes seem solid and are comparatively free from the injurious impurities of most pegmatites. Inaccessibility is the only drawback to the working of these dikes.

BOOTHLAY DISTRICT.

One of the most important feldspar districts of the United States is the Boothlay district, which extends from Boothlay on the Atlantic coast to Mount Ararat, north of Brunswick, and embraces Sagadahoc County and the eastern part of Cumberland County. The feldspar of this district is a mixture of coarse microcline pegmatite and large masses of graphic granite. The dikes vary greatly as regards impurity. In the northern parts of the district the dike rock contains large lathlike crystals of biotite, the most common impurity in Maine feldspar, but in many deposits along the coast that mineral is largely replaced by black tourmaline. Albite in very limited quantity is finely intergrown with the pegmatite. The feldspar of this district is pale cream in color, the quartz in the pegmatites is clear or of the light shades of the smoky variety. Very little massive sugar quartz is found in this district. Beryl, mostly of the opaque variety which yields no gem material, is not uncommon in the dikes along the coast. The muscovite is ruled and yields little sheet mica.

MOUNT APATITE DISTRICT.

The Mount Apatite district extends west and southwest from Mount Apatite, Androscoggin County. The feldspar occurs almost entirely as isolated pegmatite lenses, which lack uniformity both as regards strike and dip. Albite pegmatite is associated with the microcline pegmatite in some deposits and is also found entirely separate. Lenses of practically pure milk-white albite are also present in a few dikes. These lenses are generally small and not capable of supplying any great part of the present demand for soda feldspar. One of the drawbacks to mining this soda feldspar is the fact that it is chiefly of the lamellar variety cleavelandite, which has a pronounced cleavage and breaks into small particles on handling, thus rendering it especially liable to contamination from surface material and associated impurities in the dike. The potash feldspar is a buffcolored microcline pegmatite of coarse crystallization, intergrown with small amounts of albite. Biotite is present in all the microcline pegmatite deposits and muscovite in sufficient quantity in a few deposits to justify its utilization for small punch mica.

The discovery of gem-bearing pockets in the pegmatite of this district has encouraged the working of many small dikes and much valuable feldspar has been obtained as a result. The chief gem mineral sought is tourmaline, which usually occurs inclosed in pockets of albite.

MOUNT MICA DISTRICT.

The Mount Mica district is situated in Oxford County with Mount Mica as a center. There are two distinct types of feldspar-bearing formations in this district: (a) Sharply defined microcline-pegmatite dikes standing almost vertical and having definite strike; and (b) irregular lenticular deposits of microcline pegmatite and albite containing pockets of gem minerals.

The sharply defined dikes are generally free from any detrimental impurities, except black tourmaline and biotite, and these are rarely associated in the same dike. Garnets are present in a few of these dikes, but rarely in sufficient quantity to seriously injure the product. Muscovite occurs in most of these dikes, and, as a rule, is confined to limited zones, which permits of its easy removal in mining, or is sufficiently coarsely crystalline to permit of its elimination by "cobbing." The feldspar of these deposits is a pale buff to pale cream microcline. The associate quartz is chiefly of the smoky variety, being almost black in some deposits near Mount Rubellite, in the eastern part of this district. Massive sugar-quartz occurs in many of these dikes, and in the neighborhood of Paris is a fine grade of rose quartz. The irregular lenses of feldspathic rock which are distributed through this district can hardly be said to constitute a source of feldspar supply because the feldspar is so hopelessly mixed with other

minerals that no miner would attempt to work the lenses for their feldspar alone. In the search for gems, which are found chiefly in this type of formation, much feldspar has been exposed and from time to time this has been removed and has added considerably to the feldspar production of the district. The feldspar of these deposits is a pale buff microcline and a milk-white albite.

OTHER DEPOSITS.

There are many small dikes of pegmatite in the State of Maine, which are not included in these districts, but they do not differ materially from the feldspars recorded. Throughout the State of Maine. a remarkable uniformity prevails both in the potash and the soda feldspars.

ANALYSES OF MAINE FELDSPARS.

Samples of potash feldspar taken from various deposits, from south to north across the State, show the following analyses:

These potash feldspars have a deformation range of 1,275° C. to about 1,295° C. (cone 7+ to cone 8+). They fuse to a transparent glass practically free from color.

The soda feldspars of the Mount Apatite district show the following compositions:

Analyses of soda feldspars from Mount Apatite district.

These soda feldspars have a deformation range of 1,270° C. to about

1,275° C. (cone 7 to cone 7+).