current, viz., from north to south. Countless blocks were usually to be seen lying between high and low-water mark, but sometimes tracts of the coast were observed to be bare of boulders, and then again at another season thickly strewed with these erratics. "Captain Bayfield saw similar masses carried by ice through the straits of Belle Isle, between Newfoundland and the American continent, which he conceives may have travelled in the course of years from Baffin's Bay, a distance which may be compared in our hemisphere to the drifting of erratics from Lapland and Iceland as far south as Germany, Belgium, and England." Parts of the Baltic, such as the Gulf of Bothnia, where the proportion of salt in the water is only about a fourth of that in the ocean, are often entirely frozen over in the winter to a depth of 5 or 6 feet. Stones are thus frequently frozen into the ice along the shores; and, when summer comes, they are lifted up, and floated away on the ice-rafts (see p. 132). Professor Von Baer states that two huge blocks were transported by packed ice on the south coast of Finland, one of them being carried about a quarter of a mile, and left lying about 18 feet above the level of the sea.1 Dr. Forchhammer relates a remarkable fact which demonstrates how large a number of rock-fragments are annually transported by ice in the Baltic. A diver went down to examine a vessel, which had sunk near Copenhagen thirty-seven years before. He found the deck covered with blocks, from 6 to 8 cubic feet in size, and some of them piled one upon the other. He also affirmed that all other sunken ships in the Sound were covered with similar blocks. Finally, if we consider the extensive areas in the northern hemisphere, over which coast-ice is now acting, and if we regard all the deeply indented coast-lines of Scandinavia, Greenland, and North America, every yard of which is more or less subject to this action, we shall conclude, with Professor Milne, that the coast-ice must, in quantity, be much greater than that of the glaciers, and much more 1 Lyell's "Principles of Geology," tenth edition, vol. i. p. 385. effective in the transport of rock débris. "All the vast icefields which break loose from the frozen regions of the North-and we read of them as 300,000 square miles in extent, and 7 feet in thickness-are in their passage south, driven in upon the land, and help to grind the coast-line, and transport its boulders. The northern field-ice, when it arrives in the latitudes of Newfoundland, is often seen to be covered with boulders, gravel, kelp, and other materials, showing it to have been, at some time or other, in contact with the coast." 1 The distance to which these materials may be transported is indicated by the southward extension of icebergs in the Atlantic. Geol. Mag." Dec. 2, vol. iii. p. 408. WE Chemical and Organic Deposits. 7E have next to consider what becomes of the mineral substances or salts which rivers hold in solution. Since nearly all the rivers of the world flow into the ocean, and nothing but pure water is taken out of it by evaporation, we should naturally expect to find that sea-water contained a large quantity of salts in solution. Everyone knows that this is the case. The average proportion of salts in the ocean is about 34 parts in every hundred parts of water; and the average percentage of constituents, according to Bischof, is given in the following table : Chloride of sodium (common salt). Chloride of potassium Sulphate of lime (gypsum) Sulphate of magnesia Bromide of sodium Total percentage of salts 75.786 9.159 3.657 4.617 5.597 1.184 In the English Channel, the total percentage was found to be 3.551, and in the Mediterranean still higher, viz., 3.769. From the above analysis it will be seen that the principal matters in solution are salts of sodium and magnesium, while the quantity of carbonate of lime, which is so universally dissolved in fresh water, is so small that it does not find a place in the analysis. Sea-water from Carlisle Bay, Barbadoes, is said to contain 10 parts of carbonate of lime in 100,000, and water from between England and Belgium, 5.7 parts in 100,000; and there is reason to believe that it is always present, even in the waters of the Atlantic Ocean (see p. 213). The smallness of the quantity to be found in sea-water, compared with that in almost all rivers, is doubtless owing to its being constantly abstracted in large quantities by marine animals, for the construction of their shells and other hard parts. When we consider the vast number and variety of fish, of Mollusca, Crustacea, Echinodermata, and Actinozoa that inhabit the sea, and especially when we look at the enormous bulk of the coral reefs which are found within the tropics, we shall be in no danger of under-estimating the vast amount of carbonate of lime annually abstracted from the ocean. That it is abstracted more in one part than another, and that yet the ocean maintains a nearly equal average, will not be surprising, when we reflect on the extent of the great oceanic currents, and look upon all the seas and oceans as one vast slowly circulating system of moving water. Another substance occurring in minute proportions in sea-water is silica. Forchhammer found it in all the specimens of sea-water which he analyzed, the greatest proportion being 3 in 100,000 parts of water. The mineral substances above mentioned are extracted from the waters of the sea under certain conditions, and form deposits which are often of very wide extent. This extraction is effected in two ways:-(1), By means of evaporation, causing saturation and precipitation; (2), by means of organisms which take up silica or carbonate of lime to form their shells or skeletons. The subject of the present chapter, therefore, falls naturally into two sec tions 1. Deposits formed by chemical precipitation. 2. Deposits formed by organic agency. I. CHEMICALLY-FORMED DEPOSITS. Whenever portions of sea-water are separated by any means from the main body, so as to form isolated lakes or lagoons, the solution quickly becomes concentrated by evaporation, and some of the salts are precipitated. Gypsum and Rock Salt.-In this process the point of saturation for sulphate of lime is much sooner reached than that for chloride of sodium; the former requiring only 37 per cent. of the water to be removed, and the latter 93 per cent. Gypsum, therefore, must always be deposited before rock-salt, and it is possible for this deposition of gypsum to take place without the point of saturation for rock-salt being attained. This may be the reason why, though the sea contains sixteen times as much salt as it does gypsum, that the latter more frequently occurs as a mineral deposit than the former, though it is not often found in such massive beds. A good instance of the formation of gypsum beds from the concentration of sea-water is described by Professor Dana, as occurring in the dried-up lagoon of a coral island called Jarvis Island in the Pacific Ocean.' The flat surface of the central basin is covered with a deposit of guano, and underlying this is a stratum of sulphate of lime, frequently 2 feet thick, resting upon a bed of coral, sand, and shells. This deposit of gypsum is probably to be explained by the gradual elevation of the island, during which the lagoon waters were partially evaporated, but replenished from time to time by an influx from the sea, so that for a long time the condensation was not sufficient to precipitate chloride of sodium. Eventually, however, the whole was dried up, and salt was deposited, for around the lowest portion of the basin are incrustations of gypsum and common salt, ripple marks, and similar evidences of the gradually disappearing lake. Much of the salt may have been washed out by rain. Similar deposits of gypsum occur on many other elevated lagoons among the Pacific islands. A good instance of the production of rock salt by the evaporation of sea-water is presented by the Bitter Lakes of the Isthmus of Suez. Before the construction of the Suez Canal the surface of these lagoons was far below the level of the Red Sea, and the evaporation of their waters had produced a bank of salt 66,000,000 square metres (16,000 acres) in extent, composed of layers which were 1 Dana's "Coral Reefs," 1875, p. 251. P |