PLEASE SEND ALL CORRESPONDENCE RESPECTING IN ALL CASES WRITE PLAINLY YOUR NAME AND POST-OFFICE ADDRESS IN FULL. BY A COMPLIANCE WITH THE ABOVE DIRECTIONS NOTE.-UNLESS REPRINT COPIES ARE ORDERED J. B. LIPPINCOTT COMPANY PUBLISHERS, 715 AND 717 MARKET STREET, BY E. W. CLAYPOLE, AKRON, OHIO. PENNSYLVANIA consists almost entirely of massive palæ ozoic deposits of sand and shale with a few limestones. These beds united reach a thickness greater than do those of the same age in any other known part of the world. Reasonably accurate measurements have given them a total depth in some places of thirty thousand to forty thousand feet. Westwardly they thin off, and on the Ohio line scarcely reach one-fourth part of this enormous depth. All geologists are agreed that this immense mass of sediment was deposited on the slowly subsiding bottom of the eastern part of an ocean covering the whole interior of North America,—the North American palæozoic ocean, as it may be called for convenience. The extent and form of this ocean varied considerably at different times during its long existence, but taken as a whole it was uninterrupted sea. It is further a geological truth, so evident as to be now axiomatic, that all this vast mass of paleozoic deposit was obtained by erosion from some contemporaneous palæozoic land. Deposition implies erosion to a precisely equal amount. For every ton of sand or mud deposited on the sea-bottom an equivalent ton has been removed from some land-surface. Sediment is not created. It is only removed. Hence all these huge Appalachian beds from which the mountains have been since carved imply the destruction of exactly equal masses of some more ancient land elsewhere. It is further admitted, at least, I think, by all Pennsylvania geologists, that this ancient land, whose destruction supplied the materials for these immense deposits, existed to the east and southeast of the Appalachian region. It is sufficient to mention the increasing thickness of the strata towards the southeast and the greater coarseness of their material in the same direction, to convince almost every one that the palæozoic land must be sought there. Having laid down these premises, I propose first to notice the immense mass of the Appalachian deposit. Pennsylvania contains at present about forty thousand square miles of surface. Of this about nine-tenths are covered with the palæozoic rocks; that is, about thirty-six thousand square miles. Since their elevation above water erosion has destroyed vast quantities, but, as said above, there is positive evidence that when they lay in horizontal sheets beneath the ancient ocean they measured from eight miles at the east to three miles at the west in depth. It cannot, therefore, be excessive to assume this western measurement as an average over the whole area. If this be done, we find the paleozoic deposits of Pennsylvania represented by a mass three miles thick and thirty-six thousand square miles in area. This estimate is far below the truth for the following reasons: I. As mentioned above, nearly the minimum thickness is employed over the whole area. 2. Vast quantities of material now lying outside of Pennsylvania, in Ohio and Western New York, are omitted. 3. The corrugation and consequent contraction of the surface during the formation of the Appalachian earth-folds are disregarded. The importance of this last fact must not be overlooked. In a paper by the author, read before the British Association at Montreal in 1884, and printed in the AMERICAN NATURALIST for March, 1885, the great diminution of surface which the region in question underwent during the process of crumpling was pointed out. Palæozoic Pennsylvania was far larger than the existing State. Could the strata be flattened out her area would be much increased. Could we iron out-to borrow a term from the laundry-her anticlines and synclines thousands of square miles would be added to her size. The distance from Pittsburg to Harrisburg would be greater by many miles. The present Keystone State bears to the original palæozoic Pennsylvania somewhat the relation which the skin of a shrivelled apple or potato bears to the same skin when plump and fresh. But of this no account is taken in the estimate given above. Turning now to the consideration of the land from which all this sediment was derived, we find that only about one-tenth part of the State can have been above water in paleozoic days, and that this small portion lay in the southeastern corner. All the rest is covered with paleozoic rocks. This means that the only visible source of the material of which these rocks are composed and which we have found is a mass of at least thirty-six thousand square miles in surface and three miles thick, is an area in Pennsylvania of four thousand square miles. Restore in imagination these massive sheets of rock to their original source, and it will be necessary to pile the material twenty-seven miles high on the palæozoic land of the State in order to obtain a sufficient supply,-in order to build a quarry from which the rivers and sea-waves of that age could cut out the blocks which, when torn up and scattered, have composed the paleozoic rocks of Pennsylvania. A correction may be made in the above figures on account of the probability that some of these sediments came from New Jersey on the northeast and Maryland on the south. Parts of both these States consist of archæan terranes now exposed and of others now covered with later mesozoic red sandstones. But if all the areas in both States are added to the possible archæan surface of Pennsylvania, they will not more than double it, and we shall still be compelled to assign an enormous depth (thirteen miles) to the eroded ancient land before sufficient material is at hand to form the paleozoic strata of Pennsylvania. This, then, is the main factor in the great problem under consideration, the vastness of the mass that has been obtained from the archæan area in the southeast. I wish to take up now only a portion of the subject, one which can be treated in the short space here available. Among these heavy deposits there are, omitting some thin and local beds, four great sandstones, more or less pebbly. They are as follows in descending order: The Pottsville Conglomerate, Carboniferous. |