thousand feet of sediment, during the same epoch accumulating in the neighboring sea, represent a transient height of land and rivers flowing from it. Thereby events were voluminously recorded.

The general distribution of the black muds of the Hamilton was the last episode of the series of gentle oscillations of sea-level which had varied the sequence of events during all later Silurian and earlier Devonian time. The monotony of the movement was then interrupted.

THE DEVONIAN HIGHLANDS.

CHARACTER AND VOLUME OF SEDIMENTS.

Succeeding the black Hamilton shales from New York to southern Virginia occurs a great volume of sandy shale and argillaceous sandstone comprising the Jennings and Hampshire formations of Maryland or the Chemung and Catskill of New York. It is divisible into several formations according to the composition, color, texture and included fauna of successive beds, but it is a unit in so far as it represents physical conditions of land and sea which were favorable to rapid erosion and deposition. The strata are thin, rapidly alternating in character, current-bedded, and ripple-marked. They vary in color from buff or gray through shades of green and olive to dark purple or deep red tints. Their constituents are clay, fine and coarse sand, occasional quartz pebbles, and in some strata abundant scales of mica. Oxide of iron is an important constituent, especially of the higher beds to which it gives their preponderantly red color.

The thickness of the mass varies in such manner that the deposit has somewhat the form of the internal cast of a great mussel shell. The thin edge of the cast extends from Lake Erie southward through Ohio and eastern Kentucky to eastern Tennessee, where one end of the mass is. Thence the hinge-line of the mussel-shaped form runs northeast to New York, whence the thin edge rounds westward. The upper surface of this cast may be considered to be flat and the under surface curved to the hollow of the shell, which is deepest along the hinge-line. The dimensions of this form are: Width from Washington county, Maryland, to northwestern Ohio, 300 miles;

68

eroded, are evic

activity The th

2,000 to series abo

feet or les the strata sand and c.

same area c

as it was dep the mass of r Quartz, the I is a comparati Cambrian schis decomposition they are cut. definitely knowi a highland which a base of 80,000 12,000 feet. Ha the deposits resu. not only the quart tions. That the C prolonged storage coastal plain as was posits corresponding earlier formations c period. The clay wa shore where the quai washed, until they, to

WESTWARD

The Pottsville cong shale, is a remarkably

70

The width of terraces which may be built by waves at a constant relation to sea-level is narrowly limited by the character of the material and the strength of the waves; but along a shore washed by strong currents there is a reaction between the embankment and the current as a result of which the deposit is extended. As the ter race is built out it encroaches upon the course of the current we becomes accelerated by being confined. The encroachment continue until the acceleration is sufficient to enable the current scour the face and so prevent further widening. Whatever mate within the transporting power of the current, the waves there deliver across the terrace is carried along shore and deposited a current slackens in deeper water.

This process is continu

long as the supply of sand holds out, and the activity of a may thus result in depositing abreast of any section of the great volume of shore drift derived from an adjacent section result material differences in thickness of deposits along ac detritus from the coastal plain of Appalachia was contin During progressive subsidence of the sea-bottom the downward movement may have been to a certain extent the coastal plain; then the shore and its attendant terrac eastward and the shore currents scoured the surface viously wave-built structure, redistributing the sands

of which these were

sidence, however, the movement was probably not s e composed. During the greater pa coastal plain of Appalachia. The maximum thickness ville formation in eastern Pennsylvania is 1,200 fee face of the Mauch Chunk subsided to that extent du ville epoch. Had the coastal plain been equally de would have transgressed far to the eastward and t of coarse detritus would not have been concentrat thickness in a narrow zone. raised and tilted westward, delivering the coarse an to a shore which did not migrate far east of a cert The Appalachian pla The gathering of a great thickness of gravel and zone in the presence of waves and currents was

APPALACEA

aves; int ag ItWee deposit is ext:

on the core b nized. T Sufficient to her widening

the current

corresponding widening of the deposit on the sea-bottom. To estimate the width which the deposit might attain during a simple episode of submergence, reference may again be made to the submarine slopes on the Atlantic shelf. Off the south shore of Long Island the sandy bottom descends 45 to 60 feet in the first mile from the beach, and for 5 miles out at the rate of only 15 to 20 feet per mile. Off Cape Hatteras, east of the outer Diamond Shoal, the bank which lies 15 to 20 fathoms below the water has a greatest slope of 30 feet in one-quarter of a mile, or 120 feet to the mile. The materials of this bank are pebbles and broken shell. If the slope of the Pottsville formation was constructed under similar conditions,

d along shore the width of the base on a level west of the zone of maximum thick

r. This pro

ness would not have exceeded twenty miles. In fact, the extent of

out, and the the coarse pebbly strata west of the maximum thickness considerably east of any exceeds a hundred miles. d from an actrength of currents be made, it is apparent from the above com

iness of deposits

Unless improbable assumptions as to the

arison that a simple episode of submergence is not adequate to

of the sea- ford the conditions essential to spreading so widely this conglom

Appalachia w

itic formation.

een to a certa The Pottsville formation affords much other evidence of the com

scoured the s istributing the ss.

For example,

ong the variable strata of which its mass is composed, are coal Some of these are of driftwood buried in current-bedded

During the greate, but others are evenly laid marsh or lagoon accumulations, and probably gh thin, they record an episode when some portion of the sands

- maximum thick temporarily emerged from the sea.

The growth of vegetation

lvania is 1,200 h formed the coal took place after the surface had passed from d to that extent arine to land condition, that is, after the line of breaking waves n been equally etreated across it. During such retreat the shore lay along the e eastward and ally uniform slope of the latest stratum of sand and gravel,

= been concentrate Appalachian plaz ng the coarse and

was thus subjected to wave erosion. The efficiency of the depended upon the depth near shore and that of the transportrrents upon the relative steepness of the slope. The deeper

ess of gravel and sand, accordingly, the more efficient the waves in eroding the ar east of a certa ter inshore, the steeper the immediate slope away from the

currents was act

length from northeastern Tennessee to Albany, New York, 700 miles; the greatest thickness in Schuylkill county, Pennsylvania, over 10,000 feet. The area covered by it exceeds 100,000 square miles. If this mass, with approximately the dimensions with which it was deposited in the sea, could be restored upon a sea-level plain of Appalachia, it would constitute a mountain range closely resembling in height, extent and mass the Sierra Nevada of California. Its position, with reference to the ancient coast, would also be similar to that of the modern mountain range with reference to the Pacific. These sediments are the waste of a Devonian highland which grew and suffered erosion during that period.

TOPOGRAPHY OF THE HIGHLANDS.

The preceding comparison may serve to give a crude estimate of the magnitude of the Devonian mountains, but it does not follow that the ancient range resembled the Sierra in topographic character. If the former rose more slowly than the latter it may always have been of gentler aspect and never so elevated. Their nearest genetic likeness lies in the fact that both ranges developed where previously a plain extended.

The topography of the Devonian highlands can be conjectured only from the character of the sediments. Their constituents are all the product of thorough chemical decomposition of siliceous crystalline rocks; they are coarse of grain and unassorted. Particles, light and heavy, coarse and fine, are intimately mingled. The The general disintegration of the original rocks to the texture of sand and clay indicates deep accumulation of soil, as in the Appalachian Mountains of the present day where weathering of the crystalline rocks very generally proceeds faster than the removal of the products of rock-decay. The dark red color of many strata is due to oxidation of iron from deeply decayed ferriferous rocks. The Devonian highlands probably exhibited rounded forms of hill and valley, and thus may be contrasted with those ranges which present savage precipitous profiles. The presence of mica and of heavy minerals not easily decomposed indicates that the sediment was derived directly from