CHAMBERS'S INFORMATION FOR THE PEOPLE. 



grew, similar to those on the existing flora of 

 our globe. 



Traces of Natural Operations. But the rocks 

 bear traces of more than all this. On them, we 

 can see the very dints of the rain-drops of these 

 bygone ages, and can calculate the direction and 

 force of the showers that impressed them. We 

 can walk over the rippled sands of the old seas, 

 just as we can do over those we played on in 

 childhood. We can also look on the footprints of 



Erimeval birds, as they stalked in the mud of their 

 ike or river homes ; or gaze with astonishment 

 on the great footprints, as large as a man's hand, 

 of the huge reptiles that waddled among the reeds 

 by the great old rivers. We can look into the 

 craters of extinct volcanoes, can follow the flow of 

 the destructive lava, and can gather the ashes that 

 once illuminated the darkened heavens. We can 

 trace the sources of ancient rivers, and dig in the 

 mud brought down from their mountain sources ; 

 can draw maps of the continents and seas as they 

 existed thousands of ages past; can tell where 

 great ocean-currents flowed, bearing huge icebergs, 

 that grated the sea-bottom, and left their indelible 

 traces on the granite and trap of our present hills ; 

 and can shew where mighty glaciers once existed 

 in valleys now famed for their beauty, where the 

 genial sun sheds its warmest rays. In short, 

 every element in nature, whether of air, river, or 

 ocean, has left its deepest traces on the solid crust 

 of our globe. 



AGENCIES IN THE FORMATION OF ROCKS 



Any explanation of the manner in which rocks 

 have been formed must account for all the 

 phenomena, equally of composition, structure, ar- 

 rangement, and contents. We must, for instance, 

 explain how some rocks are stratified, and others 

 not ; how some are horizontal, and others inclined ; 

 and how plants and animals have come to be 

 imbedded in them so far below the surface. Are 

 there, therefore, any agencies engaged in the for- 

 mation of rocks at the present time that produce 

 effects the same in kind with these older masses ? 

 If we find that such exist, we shall have a key 

 by which to interpret the rock-formations of the 

 past. Let us consider, therefore, the Rock-forming 

 Agents. 



Volcanic Agents. The most obvious rock- 

 formers at present in action are volcanoes. From 

 circular openings, called craters^- from their cup- 

 like shape, at the summits of these mountains, 

 there issue forth at certain times great streams of 

 molten lava, boiling water, red-hot fragments of 

 rock, mingled with flames, and smoke, and steam, 

 amidst confused and thundering sounds, and the 

 general convulsion of the surrounding country. 

 These lava-streams, increased by ashes and other 

 substances, are often of great thickness, sufficient 

 to bury cities ; as Vesuvius once did Herculaneum 

 and Pompeii, and Etna did Catania at its base, 

 where the river of lava gradually rose round the 

 walls, finally drowning the city in its burning 

 flood, after it had flowed twenty-four miles ! Suc- 

 cessive accumulations of such outbursts deposit 

 immense masses of rock, in the course of ages, 

 round the centre of eruption; so great, indeed, 

 that the larger portions of such mountains and 



1 Greek crattr, a cap. 



some of those in America are five miles in height 

 are formed of the successive accumulations oF 

 the crater itself. The molten lava assumes various- 

 appearances after it has lost its heat : under water r 

 it remains hard and compact; in the open air, it 

 becomes porous and cindery ; and in certain cases, 

 it assumes a columnar form. All around, lie light 

 pumice-stone, slag-like masses, fine pulverised 

 dust, and huge calcined blocks. Now, the Un- 

 stratified rocks resemble in every feature these 

 volcanic discharges. We meet with the compact 

 lava in our trap and greenstone ; with the cinder, 

 in the lighter porous rocks ; with the ash, in our 

 trap-tuffs ; and with the columnar, in the basalt. 

 In exposed sections, we see the very vent through 

 which these masses burst and overflowed the 

 strata above ; and can trace the boundaries of 

 the ancient molten streams in the cliff's and hills 

 that everywhere vary the surface of the country. 

 We can also see hardening and crystallising 

 changes produced on the surrounding strata 

 wherever the heat of the erupted matter pene- 

 trated. We have therefore found the explanation 

 of one great class of the rock-formations, the 

 Unstratified, in the volcanoes scattered over the 

 globe, that are at this moment depositing masses 

 similar in kind to those that issued from the 

 bowels of the earth in bygone ages. These unstra- 

 tified rocks, therefore, are termed igneous} from 

 being produced by fire; volcanic, from having 

 issued from volcanoes; and eruptive, from being 

 produced by eruptions. 



Aqueous Agents. Rivers, as they flow over 

 their channels, gather accumulations of mud, sand, 

 gravel, and animal and vegetable remains, accord- 

 ing to the size of the stream and the character of 

 the country through which they pass ; and these 

 they deposit at their mouths in seas or lakes. 

 Sometimes the amount of ddbris thus deposited is 

 so great as to form large tracts of land, as at the 

 protruding mouths of the Ganges, Nile, or Mis- 

 sissippi. Even in historic times, the land thus 

 gained is of great extent. For example, at the 

 mouth of the Po, a minor stream, the town Adria, 

 which gave its name to the Adriatic Gulf from its 

 extensive commerce in Roman times, is now nine 

 miles from the sea ! The mass of matter held in 

 solution or borne along by the running water, 

 sinks to the bottom when it reaches the sea, in a. 

 certain order. First, the heavier masses are de- 

 posited, such as boulders and gravel ; then, the 

 sand ; and last, the mud. Mingled with these are 

 various animal and vegetable remains that have 

 been washed into the stream. Thus, every river- 

 mouth presents an ever-growing series of beds of 

 varying thickness and material, superposed the 

 one on the other, and enclosing various remains of 

 animal and vegetable life. These deposits would, 

 in the above order, be converted, by pressure, into 

 conglomerate, sandstone, slate, shale, and coal 

 Thus, again, we have found a beautiful and per- 

 fect explanation of the Stratified rocks as they are 

 presented everywhere, by which their composition, 

 stratification, and contents are fully accounted for. 

 Stratified rocks, therefore, obtain the various 

 names of sedimentary, because formed of the sedi- 

 ment of rivers ; and aqueous? because deposited 

 under water. 



Organic Agents. But animal and vegetable 



1 From Latin ignis, fire. 



* From Latin aqua, water. 



