Lakes are fresh or salt; those having outlets are usually fresh; those having none are usually salt. The geological functions of lakes are: 1. To arrest and equalize drainage by, regulating the outflow and preventing or lessening the destructive effects of floods; 2. To filter river water and permit the undisturbed accumulation of new deposits, which, in some modern cases, may cover thousands of square miles of sur face; 3. To furnish an abode for a lacustrine fauna and flora, to receive the remains of the plants and animals washed down from the surrounding country, and to entomb all those organisms in the growing deposits so as to preserve a record of the terrestrial life of the period. Salt lakes are of two classes: 1. Those which owe their saltness to the evaporation and concentration of the fresh water poured into them by their feeders; 2. These which were originally parts of the ocean. Of the first order are Great Salt lake in Utah, and many smaller ones. They were doubtless fresh at first, but ages of evaporation have condensed the salt and made them what they are. The Caspian sea is the most conspicuous specimen of the second class. This was no doubt a part of the Black sea and the Mediterranean, as the formation of the surrounding country shows. The surface of the Caspian is now more than 80 ft,. below that- of the Black sea. Along the shallow pools which border the Caspian a constant deposition of salt is taking place, sometimes forming a layer of rose-colored crystals on the bottom, or gradually becoming dry and covered with drift-sand. This concentration of the water is still more marked in the great bay called the Karaboghaz, which is connected with the middle basin of the Caspian by a channel 150 yardS wide and 5 ft. deep. Through this narrow mouth there flows from the main sea a constant current which Von Baer estimated to carry daily into the Karaboghaz 350,000 tons of salt.
In the form of ice, water performs important geological operations, in the five condi tions of frost in general, frozen lakes and rivers, hail, snow, and glaciers. It is well known that water expands in freezing. At 30° Fahr. the pressure is 146 atmospheres, or the weight of a column of ice a mile high, which is equal to 276,000 lbs. per sq. foot. Cannons and bombshells filled with water have been burst by the expansion of ice. Such an agency must be of great geological importance. Soils and rocks are pushed asunder by the expanding ice, and their cohesion is loosened or destroyed so that when a thaw comes they seem as if they had been ground down in a mortar. In Spitzbergen and on the coast of Greenland amount of destruction caused by frost is enormous. The short and warm summer, vapidly melting the snow, fills the pol-eeitnd joints of the rocks with water, which, when it freezes, splits off large blocks of rock from the hills and sends them down to the valleys, where they are further broken up by similar causes. At the breaking up of ice in the spring many transformations are made. Large rocks are carried from shores, and sometimes from the hollows, to remote points; shores are abraded or heaped with new material, and in many cases disastrous overflows are the result. Hail is infrequent and its consequences are not important. Now and then the pellets are large enough to strip trees of their leaves, and even to kill animals and cattle, but such'results are exceptional. Snow is a more important factor in meteorol ogy or geology. There is a snow-line or elevation at which snow is perpetual over all the earth, varying from 19,000 ft. above sea-level, in the region of the Himalayas, to less than 3,000 ft. in the extreme north or south. Snow is both conservative and destruc tive. As a conservative force, it 'protects the soil from frost, and .thereby protects crops and roots from freezing and winter-killing, On mountain slopes snow may create ava lanches, which in their descent may be very destructive. (For GLACIERS, see ante.) We come to the ocean, which, as a dynamical agent in geology, may be studied from two points:-1, its movements; 2, its geological work. Its movements are tides, cur
rents, and waves. Tides are the oscillations caused by the attraction of the sun and the moon. In the Atlantic ocean the tidal movement is 600 miles an hour. In the open sea this movement is of little consequence, but when the tidal wave enters a nar row or shallow sea the rates of motion and of force are greatly increased, and it is in such places that tides acquire their geological importance. Tides vary in height from nothing to 70 feet. The most remarkable effect of this narrowing and compression is seen in the bay of Fundy, in Nova Scotia, where the flow from the sea raises the water to a height of 70 ft. or more. Other illustrations may be found on the w. coast of Scotland, and on the coast of Norway. In the Pentland firth the current runs 10 m. an hour. Recent researches in ocean temperature have disclosed the remarkable fact that beneath the surface-layer of water affected by the temperature of the latitude there lies a vast mass of cold water, the bottom temperature of every ocean in free com munication with the poles being little above and sometimes actually below the freezing point of fresh water. In the north Atlantic a temperature of 40° Fahr. is reached at an average depth of about 800 fathoms, all beneath that depth being progressively colder. In the equatorial parts of the same ocean the same temperature comes to within 300 fathoms of the surface. In the South Atlantic, off the cape of Good Hope, the mass of cold water below 40' comes likewise to about 300 fathoms from the Surface. This distribution of temperature proves that there must be a transference of solid polar water towards the equator, for in the first place the temperature of the great mass of the ocean is much less than that which is normal to each latitude, and in the second place, it is lower than that of the superficial parts of the earth's crust underneath. On the other hand, the movement of the water from the poles to the equator requires a return movement of compensation from the equator to the poles, and this must take place in the superficial strata of the ocean. Apart, therefore, from those rapid river like streams which traverse the ocean, and to which the name of current is given, there must be a general drift of warm surface-water towards the poles. This is doubtless most noticeable in the north Atlaritic, where, besides the current of the gulf stream, there is a prevalent set of the surface waters towards the n.e. As the distribution of life over the globe is everywhere so dependent upon temperature, it becomes of the highest interest to know that a truly arctic submarine climate exists everywhere iu the deeper parts of the sea. With such uniformity of we may anticipate that the abysmal fauna will be found to possess a corrdsponding sameness of character, and that arctic types may occur on the ocean-bed, even at the equator. But besides this general drift, or set, a leading part in oceanic circulation is taken by the more defined streams termed currents. The tidal wave becomes one of translation only as it passes into shallow water, and is this of but local consequence. But a vast body of water, known as the equatorial current, moves in a general westerly direction round the globe. Owing to the way in which the continents cross its path, this current is subject to considerable deflexions. Thus that portion which crosses the Atlantic from the African side strikes against the coast of South America, and divides, one portion turn ing towards the s., and skirting the shores of Brazil, the other bending north-westward into the gulf of Mexico, and issuing thence as the well-known gulf-stream. This Nita toHal water is comparatively warm and light. At the same time the heavier and colder polar water moves towards the equator, sometimes in surface currents, like those which skirt the eastern and western shores of Greenland, but More generally as a cold under current which creeps over the floor of the ocean as far as the equator.