CHAP. V.]

ELEVATION AND DEPRESSION.

69

To answer this we must begin by endeavouring to ascertain if there are any cases on record, in which land is known to have been raised bodily out of the sea and we may at the same time conveniently extend our enquiry, and collect evidence on all changes of the land's level, including therefore those cases in which land has been bodily depressed for if great movements of this kind take place at all, it may be expected that they will not always be in the same direction, but that they will be sometimes upwards and sometimes downwards.

There is at least one case on record in India, in which a large tract of ground has been depressed within the present century. On the 16th June in the year 1819, a very severe earthquake or shaking of the ground was felt over the greater part of India. The vibration was felt at places as far apart as Khatmandú in Nepál, Calcutta and Pondicherry, but it was of greatest violence in the Province of Kachh in Western India. The village and fort of Sindri on the eastern arm of the Indus were submerged; and a portion of the tract known as the Run was depressed, so that the sea entered an old channel of the Indus, and for some years formed a salt-water lagoon or lake, where the ground had previously been under cultivation. The depression has since been in a great measure filled up by the sediment deposited from the Indus, so that the Run is now covered with water during only a part of the year. At the same time, another tract in the neighbourhood of the Run was elevated into a long low mound, which received the name of the "Allah Band." Changes of this kind have been known to accompany earthquakes not infrequently, and sometimes they are more extensive and more striking than the above. The southern part of South America has been affected in this way several times within the last century. Mr. Darwin tells us that, in the Chonos Archipelago, the island of Lemus was suddenly elevated eight feet, during an earthquake in 1839; that in 1835 a rocky flat off the island of Santa Maria was, at one blow, upheaved above high-water mark, a height of more than eight feet, and

70

RECENT ELEVATION OF INDIAN COAST. [CHAP.

was left covered with gaping and putrifying mussel-shells still attached to the bed on which they lived; also that, in 1822, the city of Valparaiso was suddenly lifted three

feet.

But the depressions and elevations which take place during earthquakes, although more striking on account of the suddenness of the change, are really less important than those movements which progress so slowly that they are insensible to the inhabitants of the district affected; and are known only by reference to rocks and similar landmarks, which in the course of many years are found to have sunk, or to have risen above the position which they formerly occupied; or by beds of recent shells, the wrecks of ships, &c., being found at considerable heights above the sea. Evidence of this kind may be met with around the coasts of India. On the southern bank of the Chilka Lake in Orissa, a bed of mud containing shells of the same kind that now live in the outlet of the lake, (where it communicates with the sea,) occurs at a height of twenty or thirty feet above the highest level of the water. This mud-bed was at one time the bottom of the lake, and must have been elevated thirty or forty feet to its present position, since the animals lived, the shells of which it contains. At many places along the Madras coast, similar beds of shells occur a few feet below the soil, sometimes several miles inland, and at levels of ten or twenty feet above the sea. In the excavation of the coast canals that run to Madras from the Pulicat Lake, and thence to Sadras, beds of this kind are exposed; and in the districts of South Arcot and Tanjore are many similar examples. The northern extremity of Ceylon around Jaffna, with a part of the opposite coast of Travancore, consists entirely of coral rock, containing numerous embedded shells, all of the same kinds as are now living in the sea around the coast and in Katiwár, beds of similar shells were met with by Mr. Theobald, many feet above the sea. In all these cases, an elevation of the coast has taken place, producing more or less new land; and although the people preserve no tradi

v.]

SOUTH AMERICA. SCANDINAVIA.

71

tions of the event, it is clear that it has taken place since the sea was tenanted by the same kinds of animals that now live in it.

But it is in South America that evidence is most abundant of a great elevation of the land in quite recent times. I have already mentioned the small upheavals that have occurred during earthquakes in the present century. But these are only minor episodes in a movement that has raised the whole of Patagonia, Chile, and La Plata to heights varying up to 1,300 feet. At Valparaiso, for instance, shells once living on the sea bottom, are found undecayed on the surface of the ground at this height; and at Lima, the remains of pieces of woven rushes, decayed cotton string, and heads of Indian corn have been found embedded with sea-shells at a height of eighty-two feet, showing that an elevation, to that extent at least, has taken place since the country has been inhabited. Mr. Darwin's observations show, that the whole of the southern part of the South American continent has thus been equably upraised in times geologically recent. In South America, he remarks, "everything has taken place on a grand scale, and all geological phenomena are still in active operation."

On a smaller scale, Europe furnishes examples of a similar kind. The northern part of Scandinavia has long been subject to a slow and imperceptible elevation. Near Uddevalle, shells adhering to the rock, as they once lived, were found by Sir Charles Lyell 100 feet above the sea : and marks cut in the rocks in 1820, at certain places north of Stockholm, were found to be four or five inches higher above the sea level, when re-examined fourteen years afterwards. From all the evidence that has been collected, Sir Charles Lyell concludes that, proceeding from the North Cape southwards, to Stockholm, the rate of elevation diminishes. from several feet to a few inches in a century.

All these observations relate to places on or near coast lines, but it is only in such situations that, as a rule, proofs of elevation or depression are easily obtained. The reason

72

SUBSIDENCE OF LAND.

[CHAP. is, that all heights are measured from the level of the sea, and there are few countries where the heights of the interior have been known long enough and accurately enough, to enable us to say whether they have undergone any change. Only in such cases as that of Patagonia and that of Northern Scandinavia, where both coasts can be shown to have undergone the same kind of movement on a great scale, can we safely infer that the whole mass of the land has participated in the movement.

Hitherto I have spoken chiefly of movements of elevation. But downward movements sometimes occur. They are not in general quite so easy to trace, because when land has once been covered by the sea, it is not open to examination. On the south coast of England and the opposite coast of France, there are many places, however, in which the submerged remains of former forests are exposed when the water falls very low at spring tides; and nearer home, I have already noticed the occurrence of stems of Súndri trees, exposed in digging tanks to depths below the low water level of the sea; and in the case of the boring at Fort William, at 380 feet beneath the surface. The southern part of Sweden is known to be gradually sinking, while the northern part is, as we have seen, rising ; and from evidence drawn from the habits of the coral animal and the shape and character of coral islands such as the Maldives and the Laccadives, it is inferred, that all those parts of the ocean in which these and similar islands occur, have been very slowly subsiding, probably with intervals of rest, for many thousands of years.

What is the cause of these great movements, that slowly, in the course of ages, work such great changes in the form and elevation of the land? Of this we know very little ; little more indeed than that they are in some way connected with the condition of the interior of the earth; and are probably due to the shrinking of the earth, in consequence of a gradual loss of heat. It is now time that we should turn our attention to this subject.

The warmth or temperature of the ground surface is

v.]

INTERNAL HEAT OF THE EARTH.

73

almost entirely dependent on the heat of the sun, and is therefore always greater in the day time than at night. It varies as the temperature of the air varies, but to a greater degree; for when the sun has been shining upon it for some hours it becomes hotter than the air a foot or two above it, while on a clear night it becomes colder than the air. But this is true only of the surface. A few inches below, the difference of the day and night temperatures is very much less; and in the temperate zone, at a depth of from three to five feet, disappears; so that at this depth, the temperature is the same, day and night, at the same time of year. The difference of the summer and winter temperatures affects the ground to a much greater depth, but at eighty or ninety feet in the temperate zone this also disappears. At Paris, a very delicate thermometer in the cellars of the Observatory, ninety and a-half feet below the surface, has marked the same temperature for fifty years. In Calcutta this point of constant temperature is probably much nearer the surface, but no observations have yet been made in India to determine at what depth it lies. Below the level at which the heat of the ground is no longer affected by the difference between summer and winter, the warmth increases the deeper we descend into the earth. The increase is much greater in some places than others; but on an average it is such, that it is calculated, that everywhere at a depth between 10,000 feet and 20,000 feet, the temperature is that at which water boils, or would boil if at the surface.

That the temperature of the interior of the earth is high is further proved by hot springs.

In the Karakpúr hills near Monghyr there are several springs of hot water, gushing out of the earth or from cavities in the rocks. One of these, close to Monghyr, is

A thermometer is an instrument for measuring temperature. It consists of a glass bulb filled with mercury, having a very fine tube attached, which is also partly filled with the fluid metal. With increase of temperature the mercury expands and fills more of the tube, and contracts as the temperature falls.