ethereal odor that is rather agreeable; they are of Tertiary age. The
oils of Japan, differing much among themselves, have as, a common
character an odor quite different from the Pennsylvania oils. So the
petroleums of the Caspian, of India, California, etc., occurring at
different geological horizons, exhibit a diversity of physical and
chemical characters which may be fairly supposed to depend upon the
material from which they have been distilled. The oils in the same
region, however, are found to exhibit a series of differences which are
plainly the result of causes operating upon them after their production.
Near the surface, they are thicker and darker; below, and near the
carbonaceous mass from which they have been generated, they are of
lighter gravity and color. We find, in limited quantity, oils which are
nearly white and may be used in lamps without refining--which have been
refined, in fact, in Nature's laboratory. Others, that are reddish
yellow by transmitted light, sometimes green by reflected light, are
called amber oils; these also occur in small quantity, and, as I am led
to believe, have acquired their characteristics by filtration through
masses of sandstone. Whatever the variety of petroleum may be,
if exposed for a long time to the air it undergoes a spontaneous
distillation, in which gases and vapors, existing or formed, escape,
and solid residues are left. The nature of these solids varies with
the petroleums from which they come, some producing asphaltum,
others paraffine, others ozokerite, and so on through a long list of
substances, which have received distinct names as mineral species,
though rarely, if ever, possessing a definite and invariable
composition. The change of petroleum to asphalt may be witnessed at a
great number of localities. In Canada, the black asphaltic oil forms by
its evaporation great sheets of hard or tarry asphalt, called gum
beds, around the oil-springs. In the far West are numerous springs of
petroleum, which are known to the hunters as "_tar springs_," because