by soundings. The beds may be uniform over very large surfaces.
These regions are fifty to eighty miles wdde on the eastern border
of North America (fig. 664, p. 441). In the lagoons or bays, argil-
laceous dei:)Osits are the most extensive ; but sand and pebbles may
be distributed among them, especially off the mouths of streams.
As stated on page 612, the material of the bottom of the submerged plateau,
above referred to, outside of a depth of 90 feet, consists at surface one-half of
Rhizopod shells. Ofi" southern New England, at depths between 300 and 550
feet, from a region southeast of Montauk Point to that southeast of Cape Hen-
lopen, the soundings, according to Bailey, consist chiefly of these shells. At
greater dejiths, beyond the limit of the plateau, Pourtales found almost a pure
floor of Rhizopods. The species are deep-water species, diflfering thus from
those of the New Jersey Cretaceous beds. Pourtales observes, in a recent letter
to Professor Bache (dated May 17, 1862), that along the plateau between the
mouth of the Mississippi and Key West, for two hundred and fifty miles from
the mouth, the bottom consists of clay, with some sand and but few Rhizopods;
but beyond this the soundings brought up either Rhizopod shells alone, or these
mixed with coral sand, Nullipores, and other calcareous organisms.
As microscopic life abounds in harbors where rivers make frequent depositions
of sediment, the presence of a considerable proportion of Rhizopods is consistent
with an annual increase of the plateau from sedimentary depositions. (Bailey,
Smithsonian Contrib. ii. ; Amer. Jour. Sci. [2] xvii. 176, and xxii. 282 ; Pourtales,
Trans. Amer. Assoc, Charleston meeting, 1850, 84; Reports Coast Survey for
1853, p. 83, and 1858, p. 248.)
Ripple^marhs are often made by the waves over the finer beach-
sands, where they are low and partly sheltered, and also over mud-
flats. The flowing water pushes up the sand into a ridgelet, as high
THE OCEAN. 665
as the force can make, and then plunges over the little elevation and
begins another ; and thus the succession is produced. The height
and breadth of the intervening space will depend on the force and
velocity of the flowing water, and the ease with which the sand or
mud is moved. Eipple-marks may be made by the vibration of
waves even at depths of 300 to 500 feet.
The rapid in-flowing tidal or other current over the sands of sand-
bars, and the bottoms of bays, may produce an effect similar in
general character to ripples, although on too large a scale to be re-
cognized as such. The oblique lamination of layers, represented in fig.
61 c, p. 93, is probably a result in this way of a pushing action in
waves or currents.
AVlien a wave dies out on a beach, it sometimes leaves a tracincr
of its sweep on the sand, as a wave-line ; and the returning waters
flowing by any half-buried shell or stone may make rills in the
sand, or rill-viarks (fig. 63, p. 94).
Broken shells, and other marine relics in fragments, are common
in beach-deposits. Below high-tide level, there may be the vertical
borings of sea-worms, of certain Crustaceans (as species of the CaUia-
nassa family), and some Mollusks. In the off-shore shallow waters
occur beds of living Mollusks, and other kinds of animals, as well as
plants, varying according to the depth.
4. Action of the oceanic waters over a suhmerged Continent, and during a gra-
dual submergence or elevation.
1. Marine deposits. — The most obvious effect of the slow submer-
gence of a continent beneath the waters of the ocean would be the
working-over, by the waves and marine currents, of the loose earth,
gravel, and alluvium of the surface, thereby changing them into
marine deposits. The depth to which this alteration would extend
would, for the most part, be much less, probably, than a hundred
feet. Whatever the extent, the ocean, besides exterminating living
species, would obliterate most of the remains of terrestrial life in
the altered deposits, and introduce its own living Mollusks and
other tribes throughout the new continental seas.
2. Features of the surface not altered by an excavation of valleys, hut by a
diminution of its heights and a filling of pre-exisfmg valleys.
It might be supposed, at first thought, that the ocean would wash
through the valleys with great excavating force, and make deep
gorges over the surface. The real effect will be best learned from
the present action on sea-coasts ; for with every foot of submer-
gence the sea-beach would be set a little farther inland, so that
the whole would successively pass through the conditions of a sea-
666 DYNAMICAL GEOLOGY.
shore. On existing sea-shores the action in progress, instead of
tending to excavate valleys, produces just the contrary effect. It is
everywhere wearing oft' exposed headlands, and filling up bays.
The salt waters, in fact, enter but a short distance the river-valleys
of a coast, because they are excluded by the out-flowing stream.
The bottom of the Hudson is below the sea-level for a long distance
beyond the limit to which the pure ocean-water extends : the same
is true of the St. Lawrence, and of many other rivers along the
coast. During a progressing submergence, therefore, the ocean
would have no power of excavating narrow valleys, unless they
happened to be open at both ends, so as to allow the oceanic cur-
rents to sweep through.
As the submergence progressed, there would be, through wave-
aiction, extensive degradation of the ridges and mountains over the
surface, and a distribution of the detritus through the intervening
depressions. In a subsequent emergence of the land, the moun-
tains and ridges Avould be still further degraded, and the valleys
filled by their debris. The laws of sea-coast action would again
come into play, and the wear of all new headlands, and the filling
of baj^s, continue to be the result, as long as the emergence was in
progress.
3. Effects as to ilic formation of marine deposits lolien a continent is viosily
without mountain-ranges and valleys.
If the continent were to a large extent without mountains, the
broad flat surface might then lie slightly above or below tide-level
at once, or nearly simultaneously, so that under a small change of
level the waves could sweep across the whole area. It has been
shown that the Appalachian Mountains were not raised until after
the Carboniferous age, and the greater part of the Rocky Moun-
tains not before the close of the Cretaceous period. The North
American continent was, therefore, in early time, in the condition
here supposed ; and the older formations have a corresponding
extent and character. There were continental oscillations, causing
slight emergences of large areas to alternate with varying sub-
mergences, and through such changes the variations in the forma-
tions were produced, differences of depths causing transitions from
arenaceous to argillaceous or to pebbly and conglomeritic accumu-
lations ; and the differences required for such changes are so small
that the probability of finding the cotemporaneous fragmental depo-
sits of Europe and America, or even of distant parts of one con-
tinent, alike arenaceous, argillaceous, or conglomeritic, is exceed-
ingly small. The details of the history as regards North America
have already been given, and need not be here repeated.
GLACIERS. 667
3. FREEZIXa AND FROZEN WATER.
Water performs part of its geological work in the act of freezing,
and another part when frozen, in the condition of snow and ice.
1. WATER FREEZING.
Rendiyig and disintegration from expansion. — As water in freezing ex-
l^ands on reaching 39 j° F., the freezing-process in the seams of
rocks opens those seams, tears rocks asunder, and tumbles fragments
and masses down precipices ; or in porous strata it crumbles off
the surface, and causes disintegration. Consequently, bluffs in a
cold climate, like the trap hills of Connecticut and the highlands
of the Hudson, have a long talus of broken stone made mainly by
this means, — while in a tropical climate the precipices are generally
free from fragments. This cause of degradation goes on incessantly
in all icy regions where there are melting and freezing, and may have
originated much of the soil and drift of the globe.
2. ICE OF RIVERS AND LAKES.
Ice forming along streams in which there are stones envelops
the stones in shallow water, even to a depth of two or three feet, or
more in the colder climates. Other stones and earth fall on the ice
from the banks. When the floods of spring raise the stream and
break up the ice, both ice, and stones, often float down strearh with
the current, or are drifted up the banks high above their former
level, or are spread over the river-flats.
Ice sometimes forms about stones in the bottom of rivers when
the rest of the water is not frozen, and is then called anchor-ice. In
this condition, it may serve as a float to raise the stones and to
transport them with the aid of the current.
The same modes of transportation are exemplified in lakes as in
rivers, except that there is less current, and the stones are mostly
set back u\) the shore. Large accumulations of stray stones far
above the ordinary level of the lake are in some places thus made.
3. GLACIERS.
1. General features, formation, and movement of Glaciers.
1; Nature of Glaciers. — Glaciers are accumulations of ice descend-
ing by gravity along valleys froin snow-covered elevations. They
are ice-streams, 200 to 5000 feet deep or more, fed by the snows and
668 DYNAMICAL GEOLOGY.
frozen mist of regions above the limits of perpetual frost. They
stretch on 3000 to GOOO feet below the snow-line (limit of perpetual
snow), because they are so thick masses of ice that the heat of the
summer season is not sufficient to melt them. Some of them reach
down between green hills and blooming banks into open culti-
vated valleys. The extremities of the glaciers of the Grindelwald
and Chamouni valleys lie within a few hundred feet of the gardens
and houses of the inhabitants. Each glacier is the source of a
stream, made from the melting ice. The stream begins, high in the
mountains, from the waters that descend through the crevasses to
the ground beneath, and often makes a tunnel in the ice above its
course ; finally it gushes forth from its cavernous crystal recesses a
full torrent, and hurries along over its stony bed down the valley.
2. Glacier regions. — The best known of glacier regions is that of
the Alps. The chain west of the head-waters of the Rhone is
divided into two nearly parallel ranges, a southern and a northern.
The latter includes, besides minor areas, tAvo large glacier districts,
— the Mt. Blanc and the Mt. Rosa or Zermatt district ; and the
former, one of equal extent, though its peaks are less elevated, —
that of the Bernese Oberland. Tliere is another district of glaciers
at the head-waters of the Rhone, and others farther eastward.
Glaciers occur also in the Pyrenees, the mountains of Norway,
Spitzbergen, Iceland, the Caucasus, the Himalayas, the southern
extremity of the Andes, in Greenland, and on Antarctic lands.
One of the Spitzbergen glaciers stretches eleven miles along the
coast, and projects in icy cliffs 100 to 400 feet high. The great
Humboldt glacier of Greenland, north of 70° 20^, has a breadth at
foot, where it enters the sea, of forty-five miles ; and this is but one
among many about that icy land.
3. Many Glaciers from one Glacier district. — The following map (fig.
948) represents the Mt. Blanc glacier region, excepting a small part
at its southwestern extremity. The vale of Chamouni along the
river Arve bounds it on the northwest, and the valley of the river
Doire on the southeast. This mountainous area, though one vast
field of snow, gives origin to numerous glaciers on its different sides,
— each principal valley having its ice-stream. The series of dotted
curves show the courses of the several glaciers. B is Mt. Blanc ;
hs, the Glacier des Bois, or Bois Glacier (so named from a village
near the foot of the glacier) ; m, the Mer de Glace, an uj^per portion
of this glacier. The river Arveiron issues from the extremity of
the glacier, and, after a short course, joins the Arve near the village
of Chamouni. The Geant [g], Talefre [ta], and Lechaud [1] glaciers
are the three largest of the upper glaciers which combine to form
GLACIERS.
669
Fiirs. 9tR-952.
Fig. 948. — Part of the glacier district of Mt. Blanc, the lighter middle portion of the map
16 miles long, out of 22 miles the whole length ; river on the northwest side, the Arve
in the valley of Chamoiini, and that on the southeast side, the Doire; B, Mt. Blanc; G,
Aiguille du Geant; J, the Jardin; T, Aig. du Tour; V, Aig. Terte; a, Argentiere Glacier;
ha, Brenva Gl.; hn, BossonsGl.; hs, Bois Gl.: g, Geant or Taciil Gl. ; I, Lechaud Gl.; to,
Mer de Glace, upper part of the Bois Glacier; mg, Miage Gl. ; ta, Talefre Gl. ; tr. Tour
GL; tt, Trient Gl.
Fig. 949. — Section of the Mer de Glace near to of fig. 94S, or opposite Trelaporte; 950,
section of same near hs of fig. 948, or opposite Montanvert ; 951, View of the Rhone Glacier ;
952, profile of same, c, c, etc. being the transverse crevasses, fading out, and becoming
curved after passing the cascade at am.
670
DYNAMICAL GEOLOGY.
the Mer de Glace. In fig. 949, the bands correspond to different
tributaries of this ghicier, and the broadest one to the right is that
of the Geant Glacier.
4. General appearance. — Fig. 953 is a reduced copy of a sketch in
Agassiz's great work, representing the Glacier of Zermatt, or the
Gorner Glacier, in the Mt. Rosa region. This grand glacier receives
Fig. 953.
Glacier of Zermatt, or the Gorner Glacier.
some of its tributaries from the right, but the larger part beyond
the Riffelhorn, the near summit on the left. The dark bands on
the glacier are lines of stones and earth, called moraines. The lon-
gitudinal lines on fig. 949 rejDresent moraines on the Mer de Glace.
The ice of a glacier is intersected by fractures or crevasses made by
its movement through the irregular valley.
Glaciers descend slopes of all angles. There are cataracts and
cascades among them as well as among rivers. One of the large
tributaries of the Mer de Glace, the Glacier du Geant {g, fig. 948),
descends in an immense ice-cascade from the plateau of the Col
du Geant into the valley below. The Glacier of the Rhone — one
GLACIERS. 671
of the grandest in the Alps — is another ice-cataract. As the glacier
commences its steep descent, it becomes broken across, and thus
great sections of it plunge on in succession, separated partly by
profound transverse chasms. Fig. 951 gives the outline of the lower
jjart of the glacier, am being the cataract, mb its terminal portion
or foot, from the extremity of which the river Rhone issues, and
c, c, c, transverse crevasses of the cascade. The same is shown in
profile in fig. 952, in which c, c, etc. are the transverse crevasses.
Other glaciers in some of the higlier valleys of the Alps reach
the edge of a precipice to descend, perhaps thousands of feet, in a
crashing avalanche, in which the ice is broken to fragments.
5. Formation of Glaciers. — The uppermost portion of a glacier con-
sists of snow and frozen mist, deposited in successive portions, and
usually more or less distinctly stratified. This part is called the
ncvc. At a lower limit, the snow becomes compacted by pressure
into ice, owing to the depth of the accumulations ; and here the
true glacier portion begins. Below the limit of perpetual frost
there is occasional melting in summer, with alternate freezing; and
this process aids in changing the mass, as well as the surface-snow,
to ice. The stratification of the neve is not generally distinct in
the icy glacier.
The following circumstances are essential to, or influence, the formation
of glaciers.
(1.) There must be an elevation, or range of heights, above the
line of perpetual congelation.
(2.) Abundant moisture is as important as for rivers; and hence
one side of a chain of mountains may have glaciers, and not the
opposite.
(3.) A difference of temperature between summer and winter is
requisite ; for otherwise the snows will be melted to the same line
throughout the year, and will not descend much below the line of
perpetual congelation.
The level of perpetual congelation, and the distance to which
glaciers descend, depend on the mean temj^erature and moisture of
the region, and especially the mean temperature of summer as
contrasted with that of winter. The height of the snow-line, or
that of perpetual congelation, is that in which 32° F. is the summer
temperature. Below this runs the year-line of 32° F., along which
32° is the mean annual temperature. Still below this lies the glacier-
limit, — that is, the lowest limit of the glacier. At Mont Blanc, the
show-line is 2000 feet above the 32° year-line, and the glacier-limit is
4500 to 5300 feet below it, or 9000 feet above the sea. In the Pyrenees,
the snow-line is also 2000 feet above that of 32° ; in the Caucasus,
672 DYNAMICAL GEOLOGY.
2500 feet ; in some parts of the Arctic, 3500 feet ; on the south side of
the Himalayas, 2000 feet, and on the north, 2G00 ; while in the equable
climate at the equator in the Andes, the snoiv-Une is 1000 feet below
the year-line of 32°. In Norway, the glacier-limit is 4000 feet below
the line of 32°.
The lower limit of a glacier sometimes varies several miles in the
course of a series of years. A succession of moist years increases
the thickness of the glacier, and thereby its tendency downward;
while dry years have the reverse effect. If the moist years have
also long, hot summers, the descent and lengthening of the glacier
will be further promoted, — since glaciers move most rapidly in
summer. But hot, dry years would shorten it, by diminishing the
ice, and especially at the lower end.
Lowering the mean temperature of a place by cooling the summers would
lower the glacier-limit. Great Britain and Fuegia are in nearly the same lati-
tude; and yet in Fuegia the snow-line is only 3000 feet above the sea. If, by
any means, the climate of Great Britain could be reduced to that of Fuegia, it
would cover the Welsh and Irish mountains with glaciers that would reach
the sea, the snow-line being but 1000 to 2000 feet above it; and the same
cause would place the snow-line in the Alps at 5000 to 6000 feet above the sea,
instead of 9000. This change of temperature involves a removal of tropical
sources of heat, or an increase of arctic soui-ces of cold. The diversion of the
Gulf Stream by the submergence of Darien has been thought of as a means for
the former; but it unfortunately leaves the winds blowing in their old direction,
and these cannot be so easil}^ managed. An increase of arctic lands by such
elevations as have taken place in former times Avould accomplish the latter.
6. The law, rate, and method of flow. — The law of flow is essentially
that of rivers. There is friction along the bottom and sides of the
glacier, and cohesion in the ice adjoining. The flow is, consequently,
most rapid at the surface ; and the axis of greatest velocity varies
from the medial line to one side or the other of it, according to the
bends in the course of the valley. The motion is so slow that there
is no atmospheric friction to retard the surface-movement. The
greater rapidity of the middle portion is shown by the fact that the
transverse ridges made at an ice-cascade, like that of the Rhone,
and the lines of earth and sand in the chasms, become afterwards
arched in front, as shown in fig. 951, in which the crevasses c are at
first transverse, but curve below the cascade. The arch is sometimes
very much elongated, almost to a triangular form, as in the Geant
portion of the Mer cle Glace. This is well illustrated in figs. 949,
950, from Tyndall: tlie right-hand half of the figure corresponding
to the Geant Glacier (the cascade in which is alluded to on p. 070)
has the transverse bands (carrying dirt and stones) elongated
into triangles, while in the other half of the Mer de Glace there
GLACIERS.
673
are no such bands, as the tributaries making it do not descend in
cascade*. (Tyndall.) This difference of velocity between the middle
and sides of a glacier has been proved also by direct experiment.
The rate of movement depends, of course, upon the slope. Accord-
ing to different observations, it varies from five to over fifty inches
a day ; and in some places a glacier may be so embayed as to lie
almost without motion. A rate of eight to ten inches a day is most
common : it is equivalent to 243:J to 304 feet a year, or one mile in
about twenty-two to seventeen years.
Forbes deduced, from his measurements made at two stations on each of the
Bois and Bossons Glaciers, the following results. The first station on the Bois
Glacier was near its upper part, where the rapidity is unusually great, and the
other near its lower extremity.
Bois I. Bois II. Boss. I. Boss. II.
Motion from Nov. '44 to Nov. '45..
Mean daily motion
Mean daily motion in summer,
April to October
Mean daily motion in winter, Oc-
tober to April
847.5 ft.
27.8 in.
37.7 in.
19.1 in.
220.8 ft.
7..3 in.
9.9 in.
4.7 in.
Boss.
I.
657.8 ft.
21.6
in.
28.0
in.
15.8
in.
489.1 ft.
16.1 in.
22.2 in.
10.7 in.
This table shows, further, that the rate of motion is about twice as great in
summer as in winter.
The maximum in July at the upper station on the Bois Glacier was 52.1
inches ; in December, 11.5 inches. The rapidity at the same place is not always
the same in different years. Thus, at one station on the Mer de Glace, Forbes
obtained for the daily motion in 1842-43, 1843-44, 1844-46, the amounts 8.56,
9.47, 10.65 inches. A knapsack lost in the Talefre Glacier {t, in fig. 948) after
ten years was found 4300 feet distant ; the slope here of this high glacier was
14° 55' (Forbes).
The rate (1) at the upper surface, (2) half-way to the bottom, and (3) at the
bottom, was found by Tyndall to be in one case 6 inches, 4.59 inches, and 2.56
inches, in a day ; and the rapidity at the middle above, to be one-half faster
than along the sides.
The power of motion in a glacier depends on —
(1.) The capability it has, to a limited extent, of sliding along its
bed, but only portions at a time.
(2.) A degree of plasticity in ice, in consequence of which the
glacier can adapt itself to any uneven surface ; for ice at a tempera-
ture near 32° F. may be moulded by pressure into almost any shape.
A heavy oblong mass supported at one end may be bent even into
a short arch by its own weight. Kane mentions in his "Arctic Ex-
plorations" the case of one table of ice, eight feet thick and twenty
or more wide, supported only at the ends, which, between the
middle of the months of March and May, became so deeply bent
44
674 DYNAMICAL GEOLOGY.
that the centre was depressed five feet. The temperature in March
was below zero, and during the interval it was at all tini«s many-
degrees below the freezing-point.
(3.) The facility with which ice breaks and mends its fractures
hy r eg elation ; that is, by a freezing together again of the surfaces
that may be in contact. This principle, first brought forward by
Tyndall, is far the most important of the three here mentioned.
Any one may test it by breaking a piece of ice and then putting
the parts together again : in a few seconds they will be firmly
united. A glacier moves on, breaking and mending itself through
its whole course. The multitudes of fractures made on steep
slopes may all disappear below when the motion becomes slow
and the ice feels the pressure from above.
Along the sides of a glacier, esj^ecially when passing prominent
angles in the valley, the crevasses are deep and numerous. The
ordinary direction of these crevasses is obliquely up stream, or at
an angle of forty to fifty degrees with the margin, being at right
angles, nearly, to the lines of greatest tension in the descending
glacier. The crevasses at a bend form especially on the convex
side of the stream, the ice undergoing a stretching on that side and
a compression on the opposite. There are also deep transverse
crevasses, and others of irregular courses, made when a glacier is
forcing its way through narrow passes in a valley'', and when descend-
ing rapid slopes. Afterward, on reaching a border-portion of the
valley, the ice may return to a solid mass with a comparatively