gone still further, and vindicated her claim to be considered the chief handmaid of religion among the sciences. But this would have led me much too far. Thirty years later, and all I have thus far said would have been unnecessary. One generation more and geology will need no defender; both her dignity and her religious tendency will be universally acknowledged. But for this purpose one more generation must first pass away.

Perhaps it may seem to some of you as a startling paradox, but it is nevertheless a fact, that the shortness of human life is one of the most powerful elements of human progress. It would seem as if the human mind grows and develops, the philosophy and opinions which govern the conduct of life continue to be modified and moulded, until about the age of twenty-five or thirty, when the character becomes unchangeable, opinions become prejudices, and the whole mind, as it were, petrified. Further progress would be impossible, but that another generation, with minds still plastic, comes forward, takes up and carries on the work a few steps, and becomes petrified in its turn. There are certainly some noble exceptions to this rule-instances of minds which with their maturity retain the plasticity of youth-but the very rarity of the exception only proves the rule.

You doubtless recollect that the children of Israel wandered forty years in the wilderness before they were fit to enter the promised land. The marks of Egyptian bondage were upon their souls as well as upon their necks. One generation must fall in the wilderness, and a new generation, free from Egyptian prejudices, must arise. We are apt to look upon this as an isolated fact in history, and entirely characteristic of this peculiar people. On the contrary, it is a fact of deepest significance in the philosophy of human progress, and intended for the instruction of us all. To this day it seems to be impossible that any great step should be made in the intellectual progress of our race, except by the sacrifice of at least one generation. We are even now in the midst of such a great change, brought about by the revelations of geology. One more generation dropped in the wilderness and we are fairly in the promised land. Do not misunderstand me, however, as quairelling with this conservative spirit; on the contrary, this brake upon the wheels of the car of progress seems absolutely necessary for its steady motion.

But I find I am again digressing, and therefore hasten to return to my subject.

I have said that the field of geology is the universe of time. It is one of these time-worlds of which I wish to draw a true, though necessarily an outline, picture in the next two or three lectures. I shall not attempt more than an outline, for this would only tire you with a multitude of details, but shall seize, if possible, the most striking features, make a comparison between this and other subsequent time-worlds, particularly our own, and endeavor to find the law which binds the whole into one system.

Among the many time-worlds of which geology tells us I select but one, viz: the COAL PERIOD. Its position is far back in the palæozoic times. Measuring time by space it is in the region of the fixed stars, although one of the brightest in the firmament of time. If I could

transport you in imagination to the surface of Sirius; if I could draw a picture of its physical geography, climate, and, more than all, of its inhabitants, who in this audience would remain unmoved? Shall the interest be less because the separation from us is by time instead of space; because the place is our own earth, and the materials of the picture beneath our very feet?

The coal period is a world distinctly separated from those which precede and those which follow it. As in the geographical distribution of fauna and flora upon the surface of the earth at the present time, we find in some cases contiguous fauna and flora seem to interpenetrate or pass by insensible gradations into one another; the species on the confines of each dying out in number but not in specific character, insensibly replaced but not transmuted. So also in the distribution of fauna and flora in time we find some (as, for instance, those of the tertiary) which pass by insensible gradations into one another, or interlock with the preceding and succeeding, although only by gradual replacement, not by transmutation. But as in geographical distribution we also find many fauna and flora completely isolated by physical barriers, mountain chains, oceans, or deserts, from contiguous fauna and flora, so also in geological distribution we find creations are often distinctly separated from other creations contiguous in time, by physical barriers in the form of convulsions of the earth, and marked by broken, dislocated, and upturned strata. In the history of the earth there seems to have been many such successive creations completely destroyed by convulsions; in other words, the time-worlds are apparently separated by blank spaces, whose dimensions we have no means of estimating. Such a distinct world is the coal period, with its fauna and flora distinctly separated from the old red sandstone which precedes, and still more so from the new red sandstone which succeeds. A distinct world— completely circumscribed in time-having its own poles and equator. Now, in geology, history is recorded upon tablets of stone-stratified rocks. Time is represented by their thickness; remarkable events by their dislocation; the fauna and flora by the contained fossils. Let us, then, examine the strata which represent this period.

They are called the "carboniferous strata," and the period the "carboniferous period," from the remarkable fact that they contain almost all the coal which is found in the world. The deposit of carbonaceous matter is not indeed confined to this period, for it has occurred in every period of the earth's history, as evidenced by the fact that thin seams of coal are found in all the strata. Similar deposits are still going on in peat bogs and deltas of the present day. But the accumulations of carbon in the strata of which we are speaking are so enormous, in comparison to those found elsewhere, that the name carboniferous, as applied to these strata and this period, becomes entirely appropriate. With the single exception of the oolite strata, which belong to the secondary period, and in which coal is profitably mined in Virginia and in England, all known coal mines belong to the carboniferous The knowledge of this simple fact would have saved the useless expenditure of millions of dollars, both in this country and in England. It is worse than useless to expend money and labor in

strata.

following up signs of coal, unless we are sure we are in the region of the carboniferous strata.

The carboniferous strata are subdivided into two very distinct groups, representing, of course, distinct subdivisions of the carboniferous period. These are called lower and upper carboniferous, or the mountain limestone, and the "coal measures." The former are mostly limestone, the latter mostly shales and sandstone; the one mostly of marine origin, the other mostly fresh water; the fossils of the one are mostly marine animals, of the other terrestrial vegetation. I shall confine myself entirely to the latter, or the true "coal measures," as they are called, from the fact that ninety-nine hundredths of all the coal in the world are found in them.

You will observe, then, that I have taken for my subject one-half of the carboniferous period. The carboniferous is itself but one of the four great subdivisions of the palæzoic period, and the palæzoic period, in its turn, only one of the four great epochs, exclusive of the present, into which the history of our earth is divided. You see, then, that the period of which I wish to give you a rapid sketch is less than one-thirtieth part of the recorded history of the earth; yet the average thickness of these strata is about 3,000 or 4,000 feet. In Wales they are 12,000 feet thick, and in Nova Scotia nearly 15,000. If, then, thickness of strata represent length of time, how great must be the lapse of time represented by the coal measures.

Such being the enormous thickness of the coal measures, it necessarily follows that but a very small proportion of the mass consists of coal. The coal strata consist of thick beds of limestone, sandstone, ironstone, and shale, containing thin seams of coal, and this alternation sometimes many times repeated in the same locality; the whole forming a series like the sheets of a ream of paper, arranged in no discoverable rational order, but indiscriminately alternating. The seam of coal will sometimes be covered with a stratum of limestone, sometimes of standstone, and sometimes of shale; although it rarely happens that the sandstone or limestone comes directly in contact with the coal; but is generally separated by a stratum, sometimes very thin, of shale or slate. In fact a stratum of clay or fine mud rock both underlies and overlies each seam. Below it forms the "fire clay," and above the black slate of the miners.

Sh..

Lim.
Sd...

L...
Sh..

8.

S.

Sh

Sh..

L.

S.

L...

Sh..

Sh..

Fig. 1.

I have said that the order is various in different parts of the same alternating series; but in every part of the same coal field the alternation is the same for the same part of the series. In other words, each stratum is generally horizontally extended over the whole coal field in a continuous sheet, so that each seam is accompanied by the same strata above and below. This is a fact of great importance, as it affords the readiest means of determining the identity of individual coal seams.

Coal strata, like all other sedimentary deposits, were at the time of formation horizontal, or nearly so. Sometimes they are found nearly in this their original position, as in many of the coal fields of our own country. More generally this original horizontality has been disturbed by igneous agency, and the

coal strata are found in the form of basins.

Sometimes the strata are

so folded as to give rise to series of basins belonging to the same

original field. Whether, however, the strata retain their original horizontality, or are thrown into basins by igneous agency, seldom or never do we find the whole of the original mass deposited. A large portion has been carried away by aqueous agency. From this cause a large coal field, covering many thousands of square miles, may exist only in the form of isolated mountains or detached basins of coal strata, as in the accompanying figures, where all the mass represented

by the dotted lines has been carried away by denuding agencies. Thus, for instance, nearly the whole of Illinois was originally occupied by a vast coal field, but little disturbed by igneous agency, but by far the larger portion of the coal strata of this immense field was carried away by denuding agencies.

You will observe, then, the striking difference in mode of occurrence between metallic ores and coal. The former are associated with rocks of every age, except, perhaps, the tertiary; the latter almost exclusively confined to those of a particular age. The former exist in the form of veins intersecting the strata, the latter in the form of seams parallel with the strata. The former extend indefinitely downwards, the latter horizontally. The former are the result of igneous agency, the latter of sedimentary deposit. Ignorance of this simple but radical difference has been the cause of much pecuniary loss, and seems not yet entirely eradicated. When, for instance, some years ago it was rumored in the streets of Philadelphia that the bottom of the Mauch Chunk Summit mine was reached, there was an universal panic, and stocks in coal mines went down enormously, not knowing that the continuation of coal seams was to be looked for horizontally rather than vertically.

This simple rule, when taken in connexion with the one previously enunciated, viz: that a coal seam throughout its whole extent is attended both above and below by the same strata, would render the identification of coal seams, and the tracing of them across valleys from hillside to hillside, a matter of little difficulty, were it not for

dislocation of the strata, producing what are called faults, slips, or troubles. In the accompanying figures, for instance, the strata have

been displaced by the elevation of one part of the field more than another. This is not conspicuous on the surface, because all has been cut down to one level by aqueous agencies. The supposed configuration of surface immediately after such unequal elevation is represented by the dotted outline; the strong line represents the present configuration of surface. All between these, therefore, represents the amount of matter carried away by denuding agencies. These faults occur very often in coal fields, and are a source of serious annoyance to the miner.

I have taken here the simplest case of dislocation. The difficulty becomes very much greater when, instead of being horizontal, the strata are highly and variously inclined. In such cases the skill and knowledge of the geologist is often tasked to the utmost.

I have said that while metallic veins extend indefinitely downwards, coal seams for the most part are extended horizontally, or nearly so. Sometimes, however, coal seams may appear, like metallic veins, to extend downwards. This is the case in highly inclined and particularly in vertical strata, as in the accompanying sketch of the anthracite coal field of Pennsylvania. In such cases, however, as well as in

Fig. 9.

every other, it will be observed that the seams are strictly parallel with the strata, that the strata have been elevated to a vertical position by igneous agency, and the included coal seams have been raised with them, still maintaining their relative position.

The thickness of coal seams varies from a few lines to many feet; sometimes they exist as sheets as thin as paper, in others in masses 30 or 40 feet thick. A single seam of pure coal, however, is seldom more than 6 or 8 feet thick. It is true that in France and in the anthracite region of Pennsylvania they are said to occur 60 or 70 feet thick, or even more, but upon close examination such mammoth seams will be found to consist of two or more seams, separated by thin laminæ of slatę; too thin, however, to form a roof, and, therefore, the several seams are wrought together as one.

The number of scams occurring in one locality and separated by interstratified sandstone and shale is sometimes as great as one hundred, and their aggregate thickness one hundred and fifty feet. Enormous as is this mass of carbonaceous matter, it is but a small fraction of the entire mass of the coal strata. The thickest and purest