Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/294

Rh 280 (‘r E O L O (i Y [m. m-.'_.:u1(-,r.. the bottom, and the sounding-lin‘, when scarcely out of the [ When this ice melts, the separated mass-.s do not return to water. is covered with sal_ine cr_vstals.‘ These facts furnish an illustration of the circumstances under which the rock- salt deposits in the New I{ed{'-.1ndstone and other geological formations were probably accumulated. The following table shows the proportion of the saline inatcri-ils in the waters of some salt lakes :—- a l D. _. ,7 I i 9'.  :3 "Q u :5  E :3 2  (3  Caspian Sea. “‘°"°‘-) E =  5 " :2 ". ,_ :: ._. V -_-'5. 8 Chloride of Sodium  7'4 '2:)'9  7'1 0'3(‘.‘.; ,, .Ingnc-slum ..... ..| 16-3 1-7 11-3 0-06:;-_3 .. Calcium ......... ..|   3-2 00013 Bicnrb. Mngn. .. Potassium .... .. . I 01 1-7 0-0076 .,' Jlnngnneso ...... .. .. 0'2 .. .. Aluminium  0'1 Bromide f .[aizm-simn .. . 0'4 trace .. Potassium Sulphate of Calcium  ..  0'0-3 0-0490 1! ¥I0tilSSll_lYU  0-04 r 0-M l  0-0171 Bicnrb. Lime. ., . agncsuun :2-2 0-3  0-1239 Water 7::-.3 7:;-s E74-5 99'3S06 9944 99.91 99-05 1000000 11. FRESH '.&TEI1 I.' THE SOLID STATE—ICE. Fresh water under orilinary circumstances, when it reaches a temperature of 32° Fahr., passes into the solid state by crystallizing into ice. In this condition it performs a series of important geological operations before being again melted and relegated to the general mass of liquid terrestrial waters. Five conditions under which ice occurs on the land deserve notice, viz., frost, frozen rivers and lakes, hail, snow, and glaciers. 1. F:-ost.—Water in freezing expands. If it be conﬁned in such away that expansion is impossible, it remains liquid -even at temperatures far below the freezing point ; but the instant that the pressure is removed this chilled water becomes solid ice. There is a constant effort on the part of the water to become solid, and very considerable pressure is needed to counterbalance its expansive power. The lower the temperature the greater this exerted pressure becomes. At a temperature of 30° Fahr. the pressure must amount to 146 atmospheres, or the weight of a column of ice a mile high, or 138 tons on the square foot. Conse- quently when the water freezes at a lower temperature its pressure on the walls of its enclosing cavity must exceed 138 tons on the square foot. Bomb-shells and cannon ﬁlled with water and hermetically sealed have been burst in strong frosts by the expansion of the freezing water within them. It is easy to see, therefore, that we have here a geological agent of great potency. It is true that in nature the enormous pressures which can be obtained arti- ﬁcially occur rarely or not at all, because the spaces into which water penetrates can hardly ever be so securely closed as to permit the water to be cooled down very con- siderably below 32° Fahr. before freezing. Still ice form- ing at even two or three degrees below the freezing point exerts an enormous disruptive force. Soils and rocks are all porous, and usually contain a good deal of moisture. When frost congeals this interstitial water, the particles of the soil or rock are pushed asunder by the expanding ice ; their cohesion is loosened or destroyed, so that when a thaw comes, they seem as if they have been ground down in a mortar. Vater lodges also in the numer- ous joints and crevices of rocks. Freezing there, it exerts great pressure upon the walls between which it lies, pushing them asunder as if a wedge were driven between them. 1 Carpenter, Jouriz. (.'cog..—Soc., vol. xviii., No. 4, quoting from farm Baer's “ Kaspische Studien," in Bull. Acad. Sci. St Petersburg, 55-6. their original position. Their centre of gravity in succes- sive winters becomes more and more displaced, until the sundered masses fall apart. In mountainous districts, where the winters are severe, and in high latitudes, a great deal of waste is thus produced on exposed cliffs and loose blocks of rock. Some measure of its magnitude may be seen in the heaps of angular‘ rubbish which in these regions are so frequently to be met with at the foot of crags and steep slopes. At Spitzbergen and on the coast of Greenland the amount of destruction caused _by frost is enormous. The short and warm summer, melting the snow, ﬁlls the pores and joints of the rocks with water, which when it freezes splits off large blocks of rock from the hills, and sends them to the base of the declivities, where they are further broken up by the same cause. 2. Frozen 1¢’ivu's and Lu/res. In countries where the winter temperature falls considerably below 32° Fahr., the lakes and rivers become solidly frozen over. The amount of geological change effected during the process is probably hardly appreciable. But when the ice breaks up in spring its power as a geological agent becomes apparent. In lakes, such as Lake b'upcrior, the ice in forming encloses beacl1- pebbles and boulders, and when thaw sets in, ﬂoats these off, so as either to drop them in deeper water or to strand them on some other part of the coast. Should a gale arise during the breaking up of the ice, vast piles of the latter, with mingled gravel and boulders, may be driven ashore and pushed up the beach. By this means blocks of stone, even of considerable size, are sometimes forced to a great height inland on some of the Canadian lakes, tearing up the soil on their way, and helping to form. a bank above the water level. It has been observed that during a severe frost ice occasionally forms on the bottoms of rivers where it encloses stones and large boulders. These are borne up to the sur- face in cakes of ground—ice to join the rest of the super- ﬁcial ice—borne detritus. Great damage is frequently done to quays and bridges in Canada by masses of river-ice driven against them on the arrival of spring. Reference has already been made to the increased power of transport and erosion acquired by rivers liable to be frozen over, and especially when their ice is broken up in the higher parts of their courses, before it gives way in the lower. 3. I1uz'l.—Wl1en rain or aqueous vapour is cooled down in the atmosphere to the freezing point of water, it is frozen, and falls to the earth as hail or snow. The forma- tion of hail is not yet well understood. It is chieﬂy in summer and during thunderstorms that hail falls. When the pellets of ice are frozen together so as to reach the ground in lumps as large as a pigeon’s egg, or larger, great damage is often done to cattle, ﬂying birds, and vegetation. Trees have their leaves aml fruit torn off, and farm crops are beaten down. 4. .S'nozI-.—In those parts of the earth’s surface where, either from geographical position or from elevation into the upper cold regions of the atmosphere, the mean annual temperature is below the freezing point, the condensed moisture falls‘ chieﬂy as snow, and remains in great measure unmelted throughout. the year. A line can be traced below which the summer heat suffices to cause the disappearance of the snow, but above which the snow continues to cover the whole or great part of the surface. This line has received the name of the snow—line, or li11e of perpetual snow. It comes down to the sea within the polar circles. Between these limits it rises gradually in level till it reaches its highest elevation in tropical latitudes. In northern Scandi- navia it is less than 3000 feet above the sea. None of the British mountains quite reach it. In the Alps it stands at 8500 feet, on the Andes at 18,000 feet, and on the northern slopes of the Himalayas at 19,000 feet.