CHAMBERS'S INFORMATION FOR THE PEOPLE. 



been demonstrated beyond a doubt by a vast 

 accumulation of facts and reasonings on phenom- 

 ena in all parts of the globe. Hence, valleys 

 thus excavated are termed valleys of erosion? from 

 being ground out by the powerful action of these 

 mighty agents. This being proved to be the case 

 even during the human period, we have little 

 difficulty in accounting for the great denudation 

 everywhere seen, and for the immense accumula- 

 tions of sedimentary matter that forms so much oi 

 the solid crust of our globe. 



TRANSPORTING AGENTS. We have also to 

 account for the deposition of strata in one part ol 

 the country, the materials for which have been 

 obtained at great distances ; and for the transport 

 of immense boulders hundreds of miles from their 

 original seats, as exhibited in all parts of the 

 globe. 



1. Aqueous Agency. The most obvious agents 

 of transport are rivers, that bear down from every 

 part of their courses the debris deposited at their 

 mouths. Their power of carrying masses of the 

 heaviest materials is immense, as may be seen 

 after a flood in the smallest streams in our neigh- 

 bourhood. Waves have also a wonderful power 

 in removing and carrying to a distance the blocks 

 on which they daily dash. But the currents that 

 flow through the ocean, which are but mighty 

 ocean rivers, have the greatest power in this 

 respect. By their means, materials of all kinds, 

 organic and inorganic, are conveyed to incredible 

 distances. The Gulf Stream, for instance, conveys 

 substances from the South African coasts to those 

 of Nonvay and the far north. 



2. Ice Agency. But the transporting influ- 

 ence of these currents in bearing rock-masses 

 is greatest when icebergs are carried on their sur- 

 face. These huge frost-mountains have imbedded 

 in their mass huge blocks, which are gradu- 

 ally dropped over wide areas as the ice slowly 

 melts away. The size and number of some of 

 these transported rocks are often almost incred- 

 ible. Every country exhibits such travelled rocks, 

 which are called erratic* boulders ; and our own 

 little island presents no mean examples of such 

 ice-borne masses. 



But ice also acts as a transporter in the form of 

 glaciers those great ice-rivers that fill the upland 

 valleys of the Alps, Himalaya, and other mountain 

 systems. In front of every glacier, along its sides, 

 and on its surface, are great collections of rocky 

 fragments of every size, borne down by the ice- 

 stream, and left as evidences of its existence when 

 the glacier has melted away. Such collections of 

 rocks are called moraines* from their mural or 

 wall-like aspect as seen running across a valley. 

 The distance to which such blocks are borne is 

 astonishing, and depends on the size of the 

 glacier. Evidences of extinct glaciers are seen in 

 most countries, and we may trace their remains 

 in our own island, where now not a particle of 

 glacier ice can exist. 



3. Igneous Agency. It is evident that volcanoes 

 have a great power in throwing out masses of 

 different materials to great distances, and of carry- 

 ing many substances on their mighty lava-streams ; 

 and evidences of their power in this respect in 

 geologic times are everywhere apparent Some- 



1 From Latin e, out, away, and rodo, rorum, to gnaw. 

 J From Latin erro, to wander. 

 * From Latin murui, a wall. 

 32 



times the fine ashes that issue from the crater are 

 borne by the wind to great distances, often fifty or 

 a hundred miles, where they are deposited as a 

 layer of finest dust ; and this may account for the 

 existence of trap-tuffs in places where no volcanic 

 eruption seems to have taken place. 



TRANSFORMING AGENTS. Rocks have also- 

 undergone great changes in structure, character, 

 and hardness from the state in which they were 

 originally deposited. Thus limestone has become 

 marble ; sandstone, quartzite ; coal, anthracite. 

 Such change is known as metamorphism, 1 and. 

 the rocks so changed are called metamorphic. 

 The chief agencies producing such remarkable 

 transformations are these : 



i. Pressure. The effect of pressure is at once 

 apparent when we reflect that by it chiefly the 

 loose materials of rocks have been made to com- 

 bine and form solid strata as sandstone from 

 sand, coal from vegetable remains. But pressure 

 exercises a much greater influence than would at 

 first sight appear. For example, the melting- 

 point of substances is greatly affected by pressure,, 

 some substances melting with much less heat 

 under pressure than without it, others requiring-, 

 more. The metamorphosing power of heat is 

 greatly affected by pressure. Thus chalk, by heat, 

 becomes lime in the open air ; but under pressure 

 fuses and becomes marble. Pressure is one cause,, 

 and may be the chief cause of cleavage in slates : 

 this is one theory, known as the mechanical ;. 

 the other being that of heat, and known as the 

 crystalline. 



2. Chemical Action. This is one of the most' 

 secret, but one of the most general, transforming 

 agencies, and its influence can only be indicated 

 here. By it, substances are held in solution, or 

 precipitated; new substances are formed; rocks 

 become entirely changed through the chemical 

 affinities between their contents, or by being perme- 

 ated by streams and waters bearing other soluble- 

 substances; the different materials in rocks be- 

 come aggregated, and form layers or masses : in 

 short, it manifests itself by countless secret and 

 potent effects. 



3. Heat. The greatest metamorphic agency is- 

 heat, whether general or local. By it rocks are 

 wholly or partially fused, and undergo rearrange- 

 ment of their component crystals, and thus become 

 changed in form and structure ; or they are sub- 

 jected to partial change of all degrees of intensity. 

 The influence of heat is always seen more or less 

 near igneous rocks, and the eye can follow the 

 change from the natural rock, and note the gradual 

 increase of metamorphism as it approaches the 

 igneous seat. By heat the most varied transforma- 

 tions have been effected. Thus limestone and 

 chalk have been changed into marble, schists into 

 lasper and granite, sandstone into quartzite and 

 hornstone, shales into flint and jasper, clay into 

 Lydian stone, &c. But whole systems of rock 

 lave also been affected by heat, which are known 

 as the metamorphic or crystalline systems. The 

 chief metamorphic rocks are gneiss, quartzite, 

 mica-schist, with hornblende and chlorite schists, 

 clay-slate, and metamorphic limestone. These 

 were originally sedimentary rock, with fossils ; but 

 the metamorphism has more or less obliterated 

 the lines of deposit, and more or less destroyed^. 



1 From Greek meta, change, and morphi, form. 



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