160 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 
The general topography of the country as here given is, I believe, 
very nearly correct, and need not again be referred to, as it is with 
the lithological character of the formation only that this paper has to 
deal. The results here given are based upon microscopic examinations 
of thin sections prepared from material gathered by myself, and, though 
they may be subject to alteration on further study, seem at present to 
be conclusive. 
So far as my present experience goes, the prevailing and almost* only 
indigenous rock of the District is an extremely variable hornblendie, chlo- 
ritie, or micaceous schist, which sometimes becomes somewhat gneiss- 
oid in constitution through the presence of a small amount of a plagio- 
clastic feldspar. The finer varieties of the rock are much used for build- 
ing purposes, and make a fairly durable material. They are light-gray 
or slightly bluish in color, compact, and under the microscope are found 
to consist essentially of quartz and biotite, with but few accessories, 
among which a silvery-white mica, magnetite, and apatite are most 
abundant. The coarser varieties of the rock, however, presents a much 
more complex structure, containing frequently plagioclase (oligoclase ?), 
hornblende, chlorite, apatite, epidote, pyrite, magnetite, garnets, and 
rutile. The plagioclase occurs only in small, pure crystals, showing to 
good advantage the characteristic banding in polarized light. The 
quartz is as usual in irregular grains, and contains but few cavities, the 
moving bubbles so frequently seen in the quartz of granites being almost 
entirely wanting. The mica of all these schists is principally biotite, 
which is frequently more or less altered into a green chlorite. It bears 
numerous inclosures of apatite and magnetite, and occasionally there is 
present quite an amount of infiltrated calcite. This ordinarily appears 
as irregular grains, sometimes showing the characteristic rhombohedral 
cleavage and laminated structure in polarized light, or merely as minute 
silvery-white granules imbedded in the mica. 
Magnetite is quite abundant in some parts of the rock, as is also 
pyrite. The garnets, which are of frequent occurrence, are entirely 
devoid of crystalline outline, and are of light salmon color in thin sec- 
tions. They are sometimes quite pure, or again contain numerous 
inclosures of quartz and magnetite. { 
Apatite is abundant in small colorless prisms, usually showing one or 
more lines of fracture parallel to the base. The hornblende, when pres- 
ent, is usually in very imperfect crystals of a faint bluish or greenish 
color; it contains very many cavities and inclosures. Occasionally it is 
met with in the form of small, flat rhombic prisms, which are often 
broken transversely. Rutile is a common accessory in the hornblendic 
varieties of these schists, occurring as small four or eight sided prisms 
of a bright reddish, brown color. They are usually too small to allow 
*A very small outcrop of impure soapstone occurs just north of the Woodley Road 
and west of Rock Creek. 
a 
