352 



THE POPULA& EDUOATOB. 



that the temperature melts the ice. Thus glaciers are, in fact, 

 overflow-channels, by which the accumulation of snow, which is 

 continually increasing above the snow-line, discharges itself. 

 Were it not for the glaciers, the snows would increase without 

 any limit, and the summits of elevated mountain-chains would 

 be an anomaly in nature continual recipients of condensed and 

 frozen aqueous vapour, without any means of giving it off again. 

 Many of the Swiss glaciers are thirty or forty miles long, 

 in scmo places as much as three miles wide, and often 600 feet 

 thick. The cause of their motion was for many years a subject 

 of debate. Many theories were broached only to be refuted. 

 The discovery of what is believed to be the true explanation of the 

 motion is due to Principal J. D. Forbes. He carefully measured 

 the progress of the different points of the glacier, and found 

 that its flow corresponded very closely to that of a river. The 

 motion was greater in the centre than at the sides, and at 

 the surface than at the bottom. It did not vary day or night, 

 and therefore whatever might be the cause of its motion, that 

 cause resided in itself, and did not depend on any external cir- 

 cumstances. All theories had hitherto looked for some motive 

 power either in gravitation, or the expansion of the ice, conse- 

 quent on the heat of the sun, and its clinging to the sides of the 

 valley prevented a return to its former position, and so it crept 

 down to the lower regions. However, Forbes showed that 

 ice is a viscous or plastic body, capable of yielding to great 

 pressure, so that the mass of ice on the incline of the mountain 

 slope flowed downwards. But although ice in one sense is 

 not viscous that is, it cannot bear a strain, and will not allow 

 itself to bo pulled out into threads yet it possesses another 

 remarkable property which compensates for this. When two 

 pieces of ice are pressed together, they freeze to each other. 

 They will even do this in warm water ; so that when the glacier 

 comes to a narrow part in the valley, it does not refuse to 

 pass the projecting point, but the enormous pressure behind 

 forces it through, the ice breaking to accommodate itself to its 

 constrained position, and then joining anew. After the narrow 

 place is passed, the glacier spreads out again just as a river 

 would do, and again occupies the whole of the valley. Most 

 glaciers progress at the rate of 400 or 500 feet a year. The 

 termination always occupies much the same position, though 

 in winter the glacier comes down further into the valley. Yet in 

 summer it is melted off. As the ice-river flows dovrn from the 

 heights, it brings rocks and debris which fell upon it as it 

 tore the flanks of the valley. These lines of rocks are termed 

 in Switzerland moraines. When two valleys meet each other, 

 their glaciers unite ; one of the lateral moraines of each joining 

 together become the medial -moraine of the main glacier. By 

 this means boulders are brought down from the inaccessible 

 heights, and piled up in huge heaps at the termination of the 

 glacier. The rocks of the valley over which the ice passes 

 are all smoothened and scratched, thus indicating the direction 

 in which the glacier flowed. Wo shall find many rocks exhibit 

 this grooving, thus telling of the existence of a glacier many 

 ages after the last vestige of the ice had melted. 



Icebergs. When there are glaciers in the Arctic regions, it is 

 evident that they can never melt, the snow-line being at the 

 sea-level. Therefore the flow continues until the sea is reached, 

 and then as the glacier proceeds over the coast cliffs, enormous 

 blocks of ice fall into the sea and are borne away ; but these 

 bergs carry on them part of the glacier moraine, and by this 

 means the fragments of the rocks of the Northern regions 

 are dropped in warmer seas, where the berg melts. Of this 

 operation we shall find many illustrations in the Pleistocene 

 epoch. 



THE ORGANIC AGENCY. 



It is difficult to realise the prominent part which life has 

 played in the formation of rocks. We do not allude only to the 

 beds of coal which represent the forest growth of vast lapses 

 of time, but to the limestone rocks and many of the siliceous 

 deposits. The reader may be aware that chalk or carbonate of 

 lime is not soluble in water, but it becomes so if carbonic 

 acid gas be present. Now Bischof states that there is so much 

 of this gas in sea-water, that five times more chalk could be 

 contained in it than it at present holds in solution. Hence it is 

 evident that no chalk could ever be precipitated from the sea 

 in the ordinary manner. How, then presuming the present 

 state of the sea to have existed with but slight alteration iu 

 past ages can the deposition of the chalk and limestone rocks 



be accounted for ? It is of little avail to assert that tho 

 cretaceous seas were overladen with chalk. They could only 

 have been so by wearing down some already existing chalk cliffs, 

 so that the difficulty is not solved, but only placed back in an 

 earlier period. The solution is offered by our observing the great 

 accumulation of chalky material which composes the coral reefs. 

 The coral zoophyte has, in common with all shell-fish, the power 

 of separating from the sea-water its carbonate of lime, with 

 which it builds its domain. It is in vain to attempt to con- 

 ceive the number of these little animals on one reef ; and yet 

 there are reefs on the Australian coast 1,000 miles long, and 

 from ten to ninety broad ! Tho " bottom " around these reefs 

 was found to be covered sometimes with broken shells, but in 

 other places with fine mud, which proved, on microscopic ex- 

 amination, to be minute foraminifera. Several similar accumu- 

 lations have been discovered to be in progress in many other 

 parts of the world. From these facts, and from an examination 

 of the chalk itself, which reveals under the microscope many 

 thousand perfect shells in a cubic inch, tho conclusion is drawn 

 that the limestone rocks have been built up by the agency of 

 living creatures. 



Professor Ehrenberg, of Berlin, was the first to turn the 

 attention of the geological world to the accumulation of matter 

 by minute organisms. Ho examined the tripoli, or polishing 

 slate, which occurs near Bilin, in Bohemia, in beds many fathoms 

 thick and many miles in extent, and found it to be wholly com- 

 posed of the siliceous coverings of organic beings. They are 

 so minute that, in a single grain of the tripoli, there are no 

 fewer than 187 millions of individuals ! It is still a disputed 

 point whether these are animal or vegetable organisms. Those 

 who place them in the animal kingdom term them Infusoria, 

 because they are generated in any infusion which is left un- 

 disturbed for some time. Those naturalists who believe in 

 their vegetable origin call them Diatomaccce. From the Berlin 

 Professor wo learn that, in the harbour of Wismar in the Baltic, 

 no less than 17,946 cubic feet of these siliceous organisms 

 arc produced annually, though it takes 100 millions of them 

 to weigh a single grain ! However, their extreme minute- 

 ness is in somo measure compensated by their extraordinary 

 power of production. " A single one of these animalcules can 

 increase to such an extent during one month, that its entire 

 descendants can form a bed of silica twenty-five square miles in 

 extent and a foot and three-quarters thick ! " The mountain- 

 meal of tho Swedes and tho edible clay of the North-American 

 Indians are accumulations of this kind. From these remarks, 

 probably, the reader will gather some idea of the geological 

 organic agent. 



THE CHEMICAL AGENCY. 



The chemical agent is not great in accumulating, but it would 

 be impossible to over-estimate tho work which this power does 

 in altering the earth's surface. 



First, the gases in the atmosphere that is, the oxygen and 

 the carbonic acid are constantly employed in weathering rocks 

 - that is. in attacking the exposed surface, and so affecting it as 

 to render it capable of being acted on by the rain and gradually 

 worn away. The most casual examination of any old building 

 or long-exposed rock will show this. 



Deposits from mineral springs may fairly be considered a 

 chemical result. In all volcanic countries this species of depo- 

 sition is carried on to some extent. In Italy the well-known 

 building stone, travatine, or Tiber stone, is of this kind of 

 deposit. This is the stone of which the Coliseum is built. The 

 Carlstab springs, it is calculated, produce 200,000 cubic feet 

 of calcareous matter every twenty-four hours. 



Stalactites and stalagmites are formed on much the same 

 principle. The water is charged with carbonate of lime, whick 

 is held in solution by the presence of free carbonic acid gas. 

 When this water drops from the roof of a cavern, the gas 

 escapes, and the water being no longer capable of supporting 

 the carbonate of lime, deposits it, forming a stalagmite, an 

 icicle-like pendant from the roof. 



All saline deposits are to be ascribed to chemical agency, 

 such as the beds of sulphate of lime, the layers of common 

 salt, the deposits of nitrate of soda and potash. Under this 

 hea<^ also some geologists would class all such exudations as 

 petroleum or rock-oil. We shall treat of these various mate- 

 rials more particularly when we speak of the formations in 

 which they severally occur. 



