coal of Iowa was accumulated in what is called the Carboniferous age—an age of the world that is immensely distant from the present, infinitely so, indeed, as we count time—yet the processes of Nature were the same then as now. That old world was, in every essential particular, the same world that we know, and was governed by precisely the same laws that control it to-day. What is true of coal is also true of every part of the geological record to this extent: that all the strangely fascinating history recorded in the rocks must be read in the light of what may be seen actually taking place now. It has become a maxim of geology that, if we would know how anything was done in the past, we must study the method in which Nature is doing that very same thing in the present. Following these suggestions a little further, we are led from the study of this particular side of the history of coal, through similar studies, to the grander and more significant
| Fig. 4.—Sigillaria reticulata. | Fig. 5.—Sigillaria Græseri. |
generalization that the laws that govern the world have been the same for all time; that the laws of matter were imposed upon it as long ago as that old, old nebula of which you have heard, and that there has been no occasion to repeal the old or enact any new laws since.
But we must get back to our coal, and pursue the series to which it belongs a little further. The product of our Iowa mines is one of the terms, as we have seen, in that interesting series, but it is not the final term. We may start with bituminous coals, like those of Iowa, at Pittsburg, for example, and working our way across the State of Pennsylvania toward the foot-hills of the Alleghanies, we will cross one of the grandest coal-fields in the world, and at every stage of progress will pass from more to less bituminous coal, until by almost imperceptible gradations we will find ourselves in the region of anthracite. Anthracite, then, is only another term in the coal series; it
