turns. They often communicate exclusively with the river for months, years, or even centuries; and then, a breach being made in the bar of sand, they are for long periods filled with salt water. Lym-Fjord. The Lym-Fjord in Jutland offers an excellent illustration of analogous changes; for, in the course of the last thousand years, the western extremity of this long frith, which is 120 miles in length, including its windings, has been four times fresh and four times salt, a bar of sand between it and the ocean having been often formed and removed. The last irruption of salt water happened in 1824, when the North Sea entered, killing all the freshwater shells, fish, and plants; and from that time to the present, the seaweed Fucus vesiculosus, together with oysters and other marine mollusca, have succeeded the Cyclas, Lymnea, Paludina, and Chara.1 But changes like these in the Lym-Fjord, and those before mentioned as occurring at the mouths of great rivers, will only account for some cases of marine deposits of partial extent and thickness resting on freshwater strata. When we find, as in the south-east of England (Chap. XIX.), a great series of freshwater beds, 1,000 feet thick, resting upon marine formations and again covered by other rocks, such as the cretaceous, more than 1,000 feet thick, and of deep-sea origin, we shall find it necessary to seek for a different explanation of the phenomena. Deep-sea deposits. The surface of the sea is alive with minute organisms, many of which have skeletons or shells of carbonate of lime or of silica. These sink to the bottom after death. The floor of the deep sea is tenanted by living forms which have calcareous and siliceous tests, and the wreckage of the land and of coral islands is carried away and finally deposited on the ocean floor. Volcanic ash also falls in the sea and sinks, and pumice also. The deposits accumulate very slowly on the ocean floor, and nothing is known about their thickness; they may be very thin, for fossils of old strata are often dredged up. In the northern seas, arenaceous deposits, with siliceous organisms, sponges, and foraminifera, and a few calcareous corals and foraminifera, are found. These deposits are somewhat analogous to ancient arenaceous marine deposits. As the depth increases, an oaze called Globigerina oaze is found, with from 35 to 65 per cent. of carbonate of lime in it, and a host of invertebrata with calcareous tests. Then, with increasing depth is a grey oaze; and a red clay occupies vast tracts at profound depths. This clay contains much 4 See Principles, Index, Lym-Fjord.' manganese, silicate of alumina and lime, and is probably of volcanic origin. Towards the antarctic regions diatomaceous or siliceous deposits occur, and at different depths there are Radiolaria. The Globigerina oaze resembles chalk to a certain extent, for it contains Coccospheres, Coccoliths, and Globigerina, but the percentage of carbonate of lime is much less than in the chalk. CHAPTER IV. CONSOLIDATION AND SUBSEQUENT ALTERATIONS OF STRATA AND PETRIFACTION OF FOSSILS. Chemical and mechanical deposits-Cementing together of particles-Hardening by exposure to air-Concretionary nodules-Consolidating effects of pressure-Jointing-Mineralisation of organic remains-Impressions and casts, how formed-Fossil wood-Göppert's experiments-Precipitation of stony matter most rapid where putrefaction is going on-Sources of lime and silex in solution. HAVING spoken in the preceding chapters of the characters of sedimentary formations, both as dependent on the deposition of inorganic matter and the distribution of fossils, I may next treat of the consolidation of stratified rocks, and the petrifaction of embedded organic remains. Chemical and mechanical deposits.—A distinction has been made by geologists between deposits of a mechanical and those of a chemical origin. By the name mechanical are designated beds of mud, sand, or pebbles, produced by the action of running water, also accumulations of stones and scoriæ thrown out by a volcano, which have fallen into their present place by the force of gravitation. But the matter which forms a chemical deposit has not been mechanically suspended in water, but in a state of solution until separated by chemical action. In this manner carbonate of lime is occasionally precipitated upon the bottom of lakes in a solid form, as may be well seen in many parts of Italy, where mineral springs abound, and where the calcareous stone, called travertin, is deposited. In these springs the lime is usually held in solution by an excess of carbonic acid, or by heat if it be a hot spring, until the water, on issuing from the earth, cools or loses part of its acid. The calcareous matter then falls down in a solid state, encrusting shells, fragments of wood and leaves, and binding them together. No similar travertin has been found in the bed of the sea, for, as a general rule, the quantity of lime, according to Bischoff, spread through the waters of the ocean is very small but it is found about coral reefs. Carbonate of lime is not precipitated at the bottom of the sea by chemical action alone, but must be produced by vital agency, as in the case of foraminiferal deposits. Caves often have stalactites suspended from their roof, and stalagmite forming layers on the floor, and these calcareous substances are in process of formation at the present time. Rain water percolating through soils takes up carbonic-acid gas, and this, in penetrating with the drainage through the limestone rock in which the cave is situated, displaces a certain amount of the carbonate of lime, and forms a bicarbonate which is soluble. The water thus charged drops from the roof, and gives off carbonic-acid gas, and a corresponding amount of carbonate of lime is set free in the form of pendants or stalactites. The excess of water which drops on to the floor in some instances precipitates its carbonate of lime in layers of stalagmite. Now, the remarks already made in Chapter II. on the original horizontality of strata are strictly applicable to mechanical deposits, and only partially to those of a mixed nature. Such as are purely chemical may be formed on a very steep slope, or may even encrust the vertical walls of a fissure, and be of equal thickness throughout; but such deposits are of small extent, and for the most part confined to vein stones. Consolidation of strata. It is chiefly in the case of calcareous rocks that solidification takes place at the time of deposition. But there are many deposits in which a cementing process comes into operation long afterwards. We may sometimes observe, where the water of ferruginous or calcareous springs has flowed through a bed of sand or gravel, that iron or carbonate of lime has been deposited in the interstices between the grains or pebbles, so that in certain places the whole has been bound together into a stone, the same set of strata remaining in other parts loose and incoherent. Proofs of a similar cementing action are seen in a rock at Kelloway in Wiltshire. A peculiar band of sandy strata belonging to the group called Oolite by geologists, may be traced through several counties, the sand being for the most part loose and unconsolidated, but becoming stony near Kelloway. In this district there are numerous fossil shells which have decomposed, having for the most part left only their casts. The calcareous matter hence derived has evidently served, at some former period, as a cement to the siliceous grains of sand, and thus a solid sandstone has been produced. If we take fragments of many other argillaceous grits, retaining the casts of shells, and manganese, silicate of alumina and lime, and is probably origin. Towards the antarctic regions diatomaceous deposits occur, and at different depths there are Rad The Globigerina oaze resembles chalk to a certain it contains Coccospheres, Coccoliths, and Globigerin percentage of carbonate of lime is much less than in t CHAPTER IV. CONSOLIDATION AND SUBSEQUENT ALTERATIONS OF STR. Chemical and mechanical deposits-Cementing together of particl HAVING spoken in the preceding chapters of the charact sedimentary formations, both as dependent on the dep of inorganic matter and the distribution of fossils, I may treat of the consolidation of stratified rocks, and the pet tion of embedded organic remains. Chemical and mechanical deposits.-A distinction been made by geologists between deposits of a mechanical those of a chemical origin. By the name mechanical are de nated beds of mud, sand, or pebbles, produced by the action running water, also accumulations of stones and scorice thro out by a volcano, which have fallen into their present place the force of gravitation. But the matter which forms a chen cal deposit has not been mechanically suspended in water, b in a state of solution un this manner carbonate o the bottom of lakes in many parts of Italy, w the calcareous ston springs the lin carbonic acid on issuing calcare for, as a general rule, the quantity of lime, according to Bischoff, spread through the waters of the ocean is very small but it is found about coral reefs. Carbonate of lime is not precipitated at the bottom of the sea by chemical action alone, but must be produced by vital agency, as in the case of foraminiferal deposits. Caves often have stalactites suspended from their roof, and stalagmite forming layers on the floor, and these calcareous substances are in process of formation at the present time. Rain water percolating through soils takes up carbonic-acid gas, and this, in penetrating with the drainage through the limestone rock in which the cave is situated, displaces a certain amount of the carbonate of lime, and forms a bicarbonate which is soluble. The water thus charged drops from the roof, and gives off carbonic-acid gas, and a corresponding amount of carbonate of lime is set free in the form of pendants or stalactites. The excess of water which drops on to the floor in some instances precipitates its carbonate of lime in layers of stalagmite. Now, the remarks already made in Chapter II. on the original horizontality of strata are strictly applicable to mechanical deposits, and only partially to those of a mixed nature. Such as are purely chemical may be formed on a very steep slope, or may even encrust the vertical walls of a fissure, and be of equal thickness throughout; but such deposits are of sinall extent, and for the most part confined to vein stones. Consolidation of strata. It is chiefly in the case of calcareous rocks that solidification takes place at the time of deposition. But there are many deposits in which a cementing process comes into operation long afterwards. We may sometimes observe, where the water of ferruginous or calcareous springs has flowed through a bed of sand or gravel, that iron or carbonate of lime has been deposited in the interstices between the grains or pebbles, so that in certain places the whole has been bound together into a stone, the same set of strata remaining in other parts loose and incoherent. Proofs of a similar cementing action are seen in a rock at Kelloway in Wiltshire. A peculiar band of sandy strata belonging to the group called Oolite by geologists, may be traced through several counties, the sand being for the most part loose and unconsolidated, but becoming stony near Kelloway. In this district there are numerous fossil shells which have decomposed, having for the most part left only their casts. The calcareous matter hence derived has evidently served, at some former period, as a cement to the siliceous grains of sand, and thus a solid sandstone has been produced. If we take fragments of many other argillaceous grits, retaining the casts of shells, and |