miles wide and several thousand feet deep - much more readily than in the case of a synclinal fold. This is the simple explana

Section across Cumberland Plateau and Lookout Mountain, Tenn. (Le Conte).

Diagram showing gently folded Strata and Amount carried away by Erosion.

tion of what otherwise appears to be a rather curious phenomenon, and one which is constantly observed in nearly all regions occupied by gently folded strata.

KINDS OF MINERAL DEPOSITS.

I.

STRATIFIED MINERAL DEPOSITS.

(a.) Non-metallic Minerals.

Coal. This is the simplest of all mineral deposits. Unlike most of the others, it is of sedimentary origin, and therefore it is clear that it must be distinctly stratified, and, moreover, that it cannot be found except in association with stratified rocks, usually strata of sandstone and shale. It is supposed that coal was derived originally from accumulations of decaying wood or other vegetable matter forming what are generally supposed to have been peat bogs or swamps. It is supposed that most of the beds of coal were formed at the mouths of rivers as they emptied into some gulf or brackish arm of the sea, because the fossils in the enclosing rocks are not those which are usually found in saltwater sediments. It is probable that they were formed usually at a place where the water was shallow, and where the areas covered by these peat bogs or original forest swamps were undergoing a gradual subsidence; but after being submerged, owing to the more rapid accumulation of sediments of sand or of mud, these areas were reclaimed and again appeared above the surface of the water, when luxuriant vegetation is supposed to have again sprung up. This explanation is not in all cases absolutely satisfactory, except as to the gradual subsidence of the land and the contemporaneous and more rapid deposition of sediments. It has therefore been suggested that in the case of some seams of coal, a portion, if not all, of the vegetable matter from which the coal has subsequently been formed -- ferns and other vegetable life of that day — was originally washed or drifted from some other locality than that where it was deposited and now lies. In other words, that in the case of some coal beds where it cannot be proven that the vegetable matter grew and rotted in situ, it was deposited in its present position somewhat after the manner of the other sediments which accompany these deposits, that is to say, partially, if not in some cases wholly, through the assistance of moving water. While this is certainly not wholly true with regard to a large proportion of coal deposits, there may be much truth in the theory.

T

Limbs of trees many feet long, now converted into coal, are sometimes found lying horizontally in a coal bed which is not more than four or five feet in thickness, and yet nothing whatever can be found of the trunk or roots of the tree of which this limb formed a part, either in the coal itself or in the underlying or overlying strata. In fact, the line of demarcation between the coal bed and the sandstone or slate (rarely limestone), which overlies or underlies the coal bed, is frequently very abrupt. This phenomenon and that of the thin parting of slate (original mud), which has no roots whatever penetrating it, and which is so often found in a seam of coal, and is often almost as extensive as the coal bed itself, is more easily accounted for on this theory than on that of an original peat swamp. Again, while roots of trees are found in the case of some coal beds, as for example in England and in Nova Scotia, their absence in the underlying sediments, as already implied, is significant of an origin somewhat different from that which is generally attributed to coal beds, and which supposes the vegetable matter to have grown and rotted in the place now occupied by it transformed into coal. In many cases, however, the latter theory of coal accumulations can be proved to be the correct one. It has been suggested that there

Section of Coal Field in Ohio showing also an Interstratified Bed of Iron Ore.

may be truth in both theories, and that both agencies may have contributed to the formation of what we now know as a bed of coal. There is much to recominend this view.

It is quite certain, however, that the land area and, of course, the sea bottom, or sea floor, as it is commonly called, during these coal-making periods were undergoing a gradual subsidence, and in this way the fact of the existence of a great number of coal beds, - sometimes thirty to forty,- one above the other, or younger than the other, with intervening strata of sand or of mud sediments, with an occasional bed of limestone, is explained. Some of these original deposits of vegetable matter or coal marshes, whatever their nature may have been, were evidently of great extent, as is shown by the wonderful continuity and immense area, covering often hundreds and thousands of square miles, of some of the coal seams of the present day. Perhaps as the land underwent a gradual subsidence the coal marsh as gradually crept landward. Other seams are of very limited extent and very irregular in shape, probably conforming, as is thought, to the shape of the original peat marsh or swamp from which the present coal is supposed to have been derived. Or, as suggested above, it is possible in some instances that the irregular contour of the area occupied by present coal beds may be partly due to the irregularity in deposition of the original drifted accumulations of vegetable matter. In this general way we account for the fact that very frequently a coal bed, like a bed of sandstone or of shale, will get thinner and thinner until it ceases to exist; but a short distance further on a bed of coal will be met with in the same geological position; that is, between the same underlying and overlying rocks, - for example, between the same slate (shale) and sandstone strata. As is well known, a bed of coal is apt to be thicker at one place than at another, but we are not concerned immediately with these vagaries of coal deposits. It is sufficient for our purpose to remember that they are distinctly stratified and interbedded layers, extensive over very considerable areas, and varying from a few inches, or a mere band, to sometimes twenty or thirty feet in thickness, or rarely considerably more.

The diagrams will give an idea of typical coal deposits, and will also show how portions of a coal seam and of the accompanying strata have often been carried away by erosion. Some of them will also show how these sedimentary rocks have been folded,

and are not now lying in their original horizontal position. Of course the coal, being held within sedimentary rocks, must conform to all the twists and turns observed in the latter. An excellent example of folding of sedimentary strata can be observed in the anthracite region of Pennsylvania, and for this reason a diagram illustrative of a section of a portion of this region is inserted.

Anthracite Coal Beds in Folded Strata, Schuylkill Co., Pennsylvania

(Ency. Brit.).

Kinds of Coal. It is enough for our purpose to know that the various classes or kinds of coal, commercially considered, are determined principally by the amount of volatile matter which they contain, and that there is less and less of this according to the degree of heat and pressure to which they have been subjected, or usually according to the age of the deposit.

Coal varies from rotting vegetation or, by further alteration, peat, at one end of the series, to graphite, or so-called "black lead," at the other end of the series, according to the degree of metamorphism to which it has been subjected. This is often determined by the age of the coal, but not always, as local influences, such as slow heat, like that produced by unusual compression, or more rarely heat from volcanic sources, may have assisted in altering the character of the coal. The original chemical composition of the coal itself may possibly have contributed somewhat to this result. Generally speaking, however, the oldest coal has been subjected to the greatest amount of change, and the youngest to the least amount. We have coal varying from peat to peaty lignite, or brown coal (always of very modern origin), then to true or black lignite. From the latter we find the lignite grading into bituminous coal, until we reach the true bituminous coal. From the bituminous we find it grading