inconceivably great results. Apparently the first distinct formulation of the new presentation in America was made simultaneously by Barrell and Grabau,154 each of whom utilized phenomena of the Appalachian basin in illustration. Barrell's elaborate memoir discusses the subject in all its phases and merits careful study. He exhibits clearly the important part which river plains played in the Appalachian history down to the close of the Carboniferous. Grabau lays stress on the progressive overlap away from the source of supply, which, when associated with other facts, becomes an important element of the argument. Study of the facts presented on the pages of this memoir has forced the writer to a conclusion very different from that hoped for when this investigation was begun. The widespread horizontality of the Coal Measures deposits, coarse and fine alike, recalls conditions observed on the Siberian Steppe and other river regions. The folding of the beds proceeded from a common cause, lateral pressure applied at the east. The violence of plication decreases with notable regularity toward the west, until in western Pennsylvania and in Ohio, along a line of more than 100 miles, the folds become so gentle that they can be traced only by close study. Dips of more than one degree are unusual, while at times and for considerable distances the dip is barely one half of a degree. The same condition exists in a great part of West Virginia. The regular decrease in steepness of the folds leads to the belief that originally the beds were, to all intents, horizontal throughout the basin, the condition being that observed on the great river plains of comparable extent. The rare occurrence of driftwood in the widespread deposits is characteristic not only of the Coal Measures but also of vast river deposits, those of the Amazon, as described by Brown, and of the Ganges as described by Medlicott and Lyell. The long narrow areas of coarse to pebbly sandstone, often with driftwood, recall the filled valleys of the Sierra, described by LeConte, as well as filled deserted bows on the 154 J. Barrell, "Relative Importance of Continental, Littoral and Marine Sedimentation," Journ. of Geol., Vol. XIV., 1906, pp. 337, 338, 539–541; A. W. Grabau, "Types of Sedimentary Overlap," Bull. Geol. Soc. Amer., Vol. 17, 1906, pp. 635, 636. PROC. AMER. PHIL. SOC., I.1. 207 I, PRINTED DEC. 17, 1912. Mississippi and the filled channels so often disclosed when a stream in flood cuts across its "bottom." The distinct evidence of sorting of materials in the red shales, where clay and sand are in dovetailing lenses, as well as in some conglomerates, where hardly enough fine material remains to bind the pebbles, leaves little room for doubt that the work was done by streams moving rapidly in some cases, slowly in others. The pebbles are not flat, such as one may find on a shore, but oval or sub-spherical, river pebbles, and their gradual decrease in size as well as number in certain directions shows that the materials were rehandled many times. The rounded pebbles of coal and carbonaceous shale prove equally with those of quartz and sandstone that the deposits, whence they came, cropped out and were exposed to attack by streams of water. The marine limestones, with one exception, are in definite, long, narrow and comparatively insignificant areas, and pass, at the borders, where those remain for observation, into sandstone, chert or shale, the condition being that of an estuary surrounded by lowland, whose rivers bring a minimum of sediment. The shallowness of the water by which sediment was distributed and the short duration of the flooding are disclosed by wave marks, sun cracks and footprints of animals, occurring at so many horizons, while the moderate depth of the estuaries, in which limestone was formed, is apparent from the shore conditions of the limestone. The testimony of the fauna is confirmatory; that life needed not deep water, for it persisted to the very shore line in Ohio. Unconformability by erosion or by overlap marks the contact of Pennsylvanian with the underlying Mississippian in almost the whole basin, showing that the great part was dry land. The record appears to show that the Appalachian basin, between the Alps-like Appalachia at the east and the low-lying Cincinnatia at the west, was divided longitudinally by the flat-topped and only moderately high Alleghania. The deepest portion of the eastern valley lay close to the foot of Appalachia, whence the surface rose westward almost inperceptibly to the crest of Alleghania. western valley extended as a plateau with its low line crossing eastern Ohio in a south-southwest direction and deepening southwardly. The thickness of deposits in the two valleys is no index to the difference in altitude of the surface; the eastern valley is coincident with the The ancient trough of great subsidence, where deposits, throughout the Paleozoic, attained great thickness and whence they decrease quickly toward the west. The assertion of greater altitude for the western valley is based on absence of all deposits earlier than those of the latest New River in the northern half of the area. Each basin had its longitudinal river. That of the east, rising in the present confines of New York, flowed with low gradient for more than 1,000 miles, receiving many tributaries from the bold Appalachia and many, perhaps, unimportant tributaries from the gentle slope at the west. Flowing at first close to Appalachia, it was pressed constantly westward by alluvial fans and cones, which became confluent and finally were modeled into a vast river plain. The main stream was sluggish and often interrupted; during high floods, the surface was covered broadly by a sheet of water and the débris from different streams was mingled. The river in the western basin received no débris-laden tributaries from east or west, except at the extreme north; it was more rapid than that in the east and pushed its coarse materials far southward. Progressive overlaps show that subsidence prevailed throughout the basin until the later stages, when it was confined to the contracting area of deposit; but it was differential and not constant. There were long intervals of slight or no movement during which rivers, reduced to base-level, distributed mostly fine material along their lower reaches. At the close of the Pottsville, the valleys had been filled and Alleghania had become buried; the whole area of deposit was an irregular marshy plain. But the old drainage systems continued until near the close of the Conemaugh and determined the lines of sea invasion; they disappeared only with changes in the topography, induced by the forces which were eventually to obliterate the basin. During the whole of the Pennsylvanian, a very great part of the basin was near sea-level. After the close of the Pottsville, few portions of the area of deposition seem to have been more than 300 feet above tide and there is no reason to suppose that any portion was at any time much more than 100 feet below tide. The writer has become convinced that one must seek explanation of the phenomena of the Appalachian basin in those of the great river plains of modern times; and the phenomena of the Appalachian basin are those of coal regions elsewhere. AN AUTOCOLLIMATING MOUNTING FOR A CONCAVE GRATING. BY HORACE CLARK RICHARDS. (Read April 20, 1912.) "For most spectroscopic problems Rowland's concave grating is an almost ideal aid," says W. Voigt in a recent article.1 Its focal property enables us to dispense with lenses or mirrors, and so avoid the accompanying aberration, absorption and scattering of the light, and when once it is adjusted it is in focus for all orders of spectra. The usual form of mounting, however, is perhaps not quite so ideal. A large, perfectly dark room is required, the apparatus is heavy and cumbersome, or else lacking in rigidity, and what is still more important in some kinds of work, the position and direction of the emergent light change with each change of wave-length. Moreover it is not readily adapted to astronomical purposes. The theory of the Rowland mounting is well known. If the source is placed at any point of the circumference of a circle constructed on the radius of the grating as a diameter, in the plane perpendicular to the ruling, the spectra will all be brought to a focus at points on the same circle. Of these spectra Rowland selected that which was at the center of curvature of the grating as giving a normal spectrum of constant scale. The necessary conditions were insured by placing the slit at the angle of a rectangular track, along the two arms of which moved the grating and the camera or eyepiece, the two rigidly connected by a rod of the proper length (Fig. 1). It is easily seen that while the source is fixed, the image is displaced in passing through the spectrum. To avoid this objection, Lewis2 interchanged the slit and camera, and Abney fixed the position of the grating and camera and 1 1W. Voigt, Phys. Zeits., 13, 217 (1912). E. P. Lewis, Astrophys. Jour., 2, 1 (1895). W. de W. Abney, Phil. Trans., 177, II., 457 (1886). mounted the slit on an arm pivoted at the center of the line joining them (Fig. 2). These methods however require the source of light to be movable, which is usually undesirable and in some cases impracticable. Wadsworth suggested several arrangements to overcome the difficulty, using auxiliary mirrors and more or less complicated mechanism, but these involve additional adjustments and loss of light. It may be added that the grating has also been used S FIG. I. with parallel light in astronomical work, but the aberration is much greater than with Rowland's mounting.5 The method here discussed is briefly that of autocollimation. That part of the light is used which after being diffracted is returned toward the slit. If therefore the slit is on Rowland's circle, the spectrum will be formed on the same circle and one point of it will coincide with the slit (Fig. 3). The ingoing and outcoming beams may be separated when necessary by the usual reflecting prism, or by slightly tilting the grating. Thus a double slit may be *F. L. O. Wadsworth, Astrophys. Jour., 2, 370 (1895). |