138 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 
tions of these, together with a considerable number collected by myself, 
and submitted them to microscopic examination, in order to ascertain 
their composition in comparison with the rock in which they are con- 
tained. The results of the examination are given below. It may be well 
to state that all the granites spoken of bear biotite as their character- 
izing accessory and without exception contain together with ortho- 
clase a varying amount of an undetermined triclinic feldspar. These 
nodules are of all sizes from that of a filbert to a foot in diameter, of a 
color varying from light bluish-gray to almost perfectly black, and of a 
texture usually too fine to allow a certain determination of their min- 
eral composition by the unaided eye. They are commonly oval in shape, 
though angular forms are not rare. In definiteness of outline they vary 
considerably. In a few cases the fine black material blends gradually 
into the coarser material of the surrounding rock without sharp lines of 
demarkation. In others again the line of separation is so complete as 
to leave a distinct groove, easily traceable with the point of a knife. 
In the most cases, however, although the separation is as sharp as the 
granular structure of the rock will permit, there is no abrupt line of de- 
markation such as would lead one to suppose them to be foreign bodies 
taken up by the granite»while in a plastic state. They show no tend- 
ency to separate from the matrix, but possess the same amount of tenac- 
ity at their line of contact as elsewhere. These spots are so abundant 
in many of the Maine rocks as to be a serious drawback to their exten- 
sive use for architectural purposes. As they are fine grained and take 
a good polish, they might not at all times be considered as particularly 
objectionable in polished work, but for the fact that owing to the large 
proportion of mica they contain they weather more rapidly than the 
inclosing granite, and thus after a-short time greatly mar the beauty 
of the stone. On bush-hammered and other rough-finished surfaces 
they appear as irregular dark blotches that are very unsightly. 
JONESBORO.—The granite quarried at Jonesboro is a coarse red rock, 
quite poor in mica, of even texture, compact, and hard. Under the 
microscope a part of the biotite is seen to have altered into a greenish 
chloritic product, while a few small apatite and zircon crystals, together 
with scattering grains of magnetite and sphene, are brought to view. 
The feldspars, as is usual in red granites, are found to be quite opaque. 
Dark patches in the rock are not particularly abundant, and I have ex- 
amined but few. These are oval in shape, from one to three inches in 
diameter, of a bluish-gray color, very fine grained, and quite hard. Their 
outlines are usually quite well defined. 
Under the microscope these are seen to be largely feldspathic in com- 
position, the crystals in polarized light often showing a somewhat radial 
arrangement. They are quite impure. The fine, even texture of the 
nodule is sometimes broken by the presence of larger crystals of a 
plagioclastic feldspar, which are of particular interest, since their angles 
