288 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
Geologically speaking, a petrographic province may belong to any 
period of geologic time, or may conceivably extend over more than 
one period. The region may be small or large, covering hundreds 
or hundreds of thousands of square miles. It may be of any shape, 
forming an elongated band or zone, a highly irregular area, or one 
more or less equidimensional. It may consist of a single, large area 
of connected intrusive or effusive rocks, or of adjacent but isolated 
areas. 
The chemical characters which, being common to the rocks of a 
province, indicate consanguinity are manifold. The rocks may be 
uniformly high in soda or in potash, or in potash and lime, low in 
magnesia and high in iron, generally deficient in silica, and so on. 
Throughout one province the soda may increase relatively to potash 
as silica decreases, while in another the reverse holds good. Or 
again, there may be some combination of such two kinds of charac- 
ters, called respectively absolute and serial. The subject is com- 
‘plicated by the possibility of local differentiation, so that in a region 
of unquestionably related rocks we may meet with some whose 
characters do not conform to the general law of the region, but 
whose presence is to be explained by the extreme differentiation of 
some portion of the general magma. 
Conforming to the chemical features, and in general largely de- 
pendent on them, are the common mineralogical characters. These 
ure very important, not so much in themselves, as because they often 
enable one practically to determine the general relationship without 
the necessity of a long series of analyses. The mineralogical simi- 
larity may be evident in two ways: Either by the general presence of 
certain minerals which are rare or are not usually found in certain 
kinds of rock, as the rare zirconium minerals, or nephelite, or 
leucite, the occurrence of hypersthene in the basalts and andesites, 
or of biotite in peridotites; or by certain peculiarities of color, form, 
or other characters shown among the more usual mineral groups, 
and which are dependent on the introduction of certain chemical 
constituents into the molecule, as bright green, pleochroic augites or 
blue hornblendes, due to their containing much soda, purple augites 
or red-brown hornblendes, due to titanium, and so on. Here again 
the possibilities of difference are numerous, but the mineralogical 
evidence of relationship is often so marked as to be unmistakable to 
the petrographer. 
A good illustration of such petrographic provinces and of their 
distribution is furnished by the United States, which may be briefly 
described, though our knowledge is still incomplete. Stretching along 
and rather close to the Atlantic coast is a zone of small, isolated 
areas of igneous rocks which are chiefly characterized by a high con- 
tent in soda, resulting in the common presence of nephelite-syenites 
