the solid asphaltum, and the rigid and brittle
substance known as albertite, with other and allied products. Having
said so much regarding the chemistry of the fuels of the future, we
may now pass to consider their geological record. A somewhat curious
distribution awaits the man of science in this latter respect. Most
readers are aware that the geologists are accustomed to classify
rocks, according to their relative age, into three great groups, known
respectively as the primary, secondary, and tertiary periods. In the
secondary period we do not appear to meet with the fuels of the
future, but as far back as the Devonian or old Red Sandstone period,
and in the still older Silurian rocks, stores of gas and petroleum
abound. In the latest or tertiary period, again, we come upon nearly
all the forms of fuels we have already specified.

The meaning of this geological distribution of the fuels is entirely
fortuitous. Dr. M'Gee tells us that as their formation depended on
local conditions (such as plant growth), and as we have no means of
judging why such local conditions occurred within any given area, so
must we regard the existence of fuel products in particular regions as
beyond explanation. Of one point, however, we are well assured, namely
that the volume of the fuels of the future is developed in an inverse
proportion to their geological age. The proportionate volume, as it
has been expressed, diminishes progressively as the geological scale
is descended. Again, the weight of the fuels varies directly with
their age; for it is in the older formation of any series that we come
upon the oils and tars and asphaltum, while the marsh gas exists in
later and more recently formed deposits. Further geological research
shows us that the American gas fields exist each as an inverted trough
or dome, a conformation due, of course, to the bending and twisting of
the rocks by the great underground heat forces of the world. The
porous part of the dome may be sandstone or limestone, and above this
portion lie shales, which are the opposite of porous in texture. The
dome, further, contains gas above, naphtha in the middle, and
petroleum below, while last of all comes water, which is usually very
salt. In the Indiana field, however, we are told that the oils lie
near the springing or foundation of the arch of the dome, and at its
crown gas exists, and overlies brine.

A very important inquiry, in relation to the statement that upon t