The differences between these substances really consist in the degree of fixing of the carbon or solid portion of the product, as it were, which exists.  Thus in coal and jet the carbon is of stable character, such as we might expect to result from the slow decomposition of vegetable matter, and the products of this action are not volatile or liable to be suddenly dissociated or broken up.  On the other hand, when we deal with the hydrocarbons as they are called, in the shape of rock gas, naphtha, petroleum, tar, asphaltum, and similar substances, we see how the carbon has become subordinated to the hydrogen part of the compounds, with the result of rendering them more or less unstable in their character.  As Professor M’Gee has shown us, there is in truth a graduated series leading us from the marsh gas and rock gas as the lightest members of this class of compounds onward through the semi-gaseous naphtha to the fluid petroleum, the semi-fluid tar, 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.