The Elements of Geology

hree hundred miles from land. Soundings show that offshore deposits are laid in belts parallel to the coast, the coarsest materials lying nearest to the land and the finest fart

by which it is sorted, the coarser being sooner

ORE D

s of the unquiet water,-by the undertow (an outward- running bottom current near the shore), by the ebb and flow of tides, by ocean currents where they approach the land, and by waves and ground swells, whose effects are sometimes felt to a depth of six hundred feet. By all these means the waste is slowly washed to and fro, and as it is thus ground finer and finer and its soluble parts are more an

those with which our science has to do; for it is they which have given shap

o, nearly all the waste of the continent, whether worn from its surface by the weather, by streams, by glaciers, or by the wind, or from its edge by the chafing of the waves, comes at last to its final resting place. The agencies which spread the material of the continental delta grow

s causes them to sink much faster than they would in fresh water,-that they are wafted far before they reach a bottom where they may remain undisturbed. Muds are also

d to other iron compounds by decomposing organic matter in the presence of sea water. Yellow and red muds occur where the amount of

e slowness. A greenish mineral called GLAUCONITE-a silicate of iron and alumina-is then formed. Such deposits, known as GREEN SAND, are now in process of

ock bench each little pool left by the ebbing tide is an aquarium abounding in the lowly forms of marine life. Below low-tide level occur beds of molluscous shells, such as the oyster, with countless numbers of other humble organisms. Their harder parts-the shells of moll

as been made ever since living creatures with hard parts appeared upon the globe. We shall find it sealed away in the stratified rocks of the continents,- parts of ancient sea

e the depth is not so great as to deprive them of needed light, heat, and of sufficient oxygen absorbed by sea water from the air. In such clear and comparatively shallow

sometimes called COQUINA, a local term applied to such beds rece

ompose it to the roe of fish. Corals and shells have been pounded by the waves to calcareous sand, and each

om shore in deeper and quieter water as a limy silt, and hardens into a dense, fine-grained limes

to eighteen hundred feet of water. The rocky bottom consists of limestone now slowly building from the accumulation of the rema

a forms part of the accumulated deposit, either in its original condition, as, for

lime there is in process of making a clayey limestone or a limy sh

between the fragments of shells and corals, the grains of sand and the particles of mud, binding them together into firm rock. Where sediments have accumulated to great thickness the lower portions tend also to consolidate under the weight of the overlying beds. Except in

d ripple marks. One may see this beautiful ribbing imprinted on beach sands uncovered by the outgoing tide, and it is also produced where the water is of consider

next flood tide and sealed away as a lasting record of the manner and place in which the strata were laid. In Figure 150 we have an illustration of a very a

ubaerial agencies. From them we ascertain that sea deposits are stratified. They lie in distinct layers which often differ from one another in thickness, in size of particles, and perhaps in color. They are parted by bedding planes, each of which represents either a change in material or a pause during which deposition ceased and the material of one layer had time to settle and become somewhat consolidated before the m

ce of the tide spreads a film of mud, which dries and hardens in the air during low water before another film is laid upon it by t

thick beds, which soon thin out seaward and give place to deposits of finer stuff. In a general way strata are laid in well-nigh horizontal sheets, for the surface on which they are laid is generally of ve

e angle of repose. In this case each stratum grew by the addition along its edge of successive layers of sediment, precisely as does a sand bar in a river, the sand being pushed continuously ove

is seldom more than six hundred feet in depth at the outer edge, and shallows gradually towards shore. Along the eastern coast of the United States the continental shelf is from fifty to one hundred and more miles in width; on the Pacific coast it is much narrower. So far as it is due to upbuilding, a wide continental shelf, such as that of the Atlantic coast, implies a massive continental delta tho

d by their composition and structure that they were all laid offshore in shallow water. We must infer that, during the vast length of time recorded by the enormous pile, the floor of the sea along the coast was slowly sinking, and that the trough was constantly being fi

t of the sea bottom is brought farther and farther from the shore. The basal conglomerate formed by bowlder and pebble beaches comes to be covered with sheets of sand, and these with layers of mud as the sea becomes deeper an

g denuded by the removal of its mechanical waste. But sediments from the land are spread within a zone but two or three hundred miles in width along the margin of the continents, a line one hundred thousand miles long. As the area of deposition-about twenty-five million square miles-is about one half the area of denudation, the average rate of deposition must be twice the average rate of denudation, i.e. about one foot in twenty-five

f lofty mountains is worn to low plains, are marked each by its own peculiar land forms, and that the forms of the earlier stages are more or less com

e brought to the sea diminishes, until the sluggish streams carry only a fine silt which settles on the ocean floor near to land in wide sheets of mud which harden into shale. At last, in the old age of the region (Fig. 157), its low plains contribute little to the sea except the soluble elements of the rocks, and in the clear waters near the land lime-secreti

he mouths of turbid rivers which drain highlands near the sea, deposits are littl

f the deposits laid offshore. The waste swept in by streams contains much feldspar and other minerals softer and more soluble than quartz, and where the waves have little opportun

e of the land is stored for ages. Again and again they are abandoned and invaded by the sea as from time to time the land slowly emerges and is again submerged. Their deposits are long exposed to the weather, and sorted

OOZES A

inifera, which secrete shells of carbonate of lime. At death these tiny white shells fall through the sea water like snowflakes in the air, and, slowly dissolving, seem to melt quite away before they can reach depths greater than about three miles. Near shore they r

pieces of pumice drifted by ocean currents far from the volcanoes from which they were hurled. The red clay builds up with such inconceivable slowness that the teeth of sharks and the hard ear bones of whales may be dredged in large numbers from the deep ocean be

succession of summer and winter, or of night and day. A mantle of deep and quiet water protects them from the agents of erosion which continually attack, furrow, and destroy the surface of the land. While t

er deposits, akin to those now making on continental shelves. Deep- sea deposits are absent from the rocks of the land, and we may therefore infer that the deep sea has never held sway where

-BUILDIN

r colors of yellow, orange, green, and red. In structure each tiny polyp is little more than a fleshy sac whose mouth is surrounded with petal-like tentacles, or feelers. From the sea water the polyps secrete calcium carbonate and build it up into the stony framework which supports their colonies. Boring mollusks, worms, and sponges perforate and honeycomb this framework even while its surface is covered with myriads of living polyps. It is

pile to leeward coral blocks torn from their foundation, filling the interstices wi

stimated that off Florida a reef could be built up to the surf

solution by infiltrating waters. On the island beaches coral sand is forming oolitic limestone, and the white coral mud with which the sea is milky for miles about the reef in times of storm settl

ff the north-eastern coast, is twelve hundred and fifty miles long, and has a width of from ten to ninety mile

n one hundred and fifty feet in depth, with a winter temperature not lower than 68 degrees F. An important c

three classes,-fringing reef

de along the northeastern coast of Cuba. The outer margin, indicated by the line of white surf, where the corals are in vigorous growth, rises from about forty feet of

reefs. The seaward face rises abruptly from water too deep for coral growth. Low islands are cast up by the waves upon the reef, and inlets give place for the ebb and flow of the tides

e sea, and is covered with a forest of trees such as the cocoanut, whose seeds can be drifted to it uninjured from long distances. The white beach of coral sand leads down to the growing reef, on whose outer margin the surf is constantly breaki

about them is not greater than that at which coral can grow; but barrier reefs and atol

was surrounded by a fringing reef. As the island slowly sinks, the reef builds up with equal pace. It rears its seaward face more steep than the island slope, and thus the intervening space between the sinking, narrowing land and the outer margin of the reef constantly widens. In this intervening space the corals are more or less smothered with silt from the outer reef and from the land, and are also deprived in large measure of th

eet, the lower limit of the reef-building species. At the foot of this submarine cliff a talus of fallen blocks t accumulates, and as it reaches the zone of coral growth becomes the foundation on which the reef is steadily extended seaward. As the reef widens, the polyps of the circumference f

gs are drifted widely over the tropic seas by ocean currents, colonize such submarine foundations wherever the conditions are favorable for their growth. As the reef approaches the surface the corals of the inner area are smothered by silt and starved, and their Submarine Volcanic Peak hard parts are dissolved and scoured away; while those of the circumference, w

an Ocean, is a volcanic pile rising eleven hundred feet above sea level and fifteen thousand five hundred feet above the bottom of the sea. The summit is a plateau surrounded by a rim of hills of reef formation, which re

ere occur many thin reef deposits, but none are known of the immense thickness

are commonly interrupted of

of the shore-form the muds and sands of continental deltas. All of these sea deposits consolidate and harden, and the coherent rocks of the land are thus reconstructed on the ocean floor. But the destination is not a final one. The stratified rocks of the land are for the most part ancient deposits of the sea, which have been lifted above se

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GEOLOGICA