Meta/morjphism. 407 



tlie external surface of the earth, that they permeate all matter 

 placed there, and that in so doing they induce chemical action, 

 are among the conclusions which the discoveries of modern 

 science may be said to render certain. Thus the matter origi- 

 nally deposited as mud, and consisting of a heterogeneous 

 assemblage of the minerals most common at the earth's surface, 

 is now placed under the conditions known to be most favourable 

 for enabling all its component parts to exercise their chemical 

 affinities and re-arrange themselves in new forms. 



The first result of this is that like things collect together. 

 In a mixture consisting of clay and limestone mud, the lime- 

 stone or calcareous part collects into bands of nodules at 

 certain distances apart ; the sulphur and iron, if present, com- 

 bine to form pyrites ; the excess of iron, if any, is converted 

 into oxide, and combines with the calcareous nodules, forming 

 poor or rich earthy iron ores, according* to circumstances ; and 

 so on with other elements. 



Meanwhile the whole mass, becoming systematically com- 

 pacted, contracts, and crevices occur — thin, perhaps, and 

 partial at first, but more or less open, and gradually increasing. 

 Crystallization takes place in some of the minerals easily 

 crystallizable, and that curious change takes place called by 

 mineralogists pseudomorphism — the removal of some definite 

 form, and the replacement of the original by some new sub- 

 stance. The remains of organic beings, animal and vegetable, 

 become converted into fossils. 



Throughout the whole mass, whatever else is going on, and 

 in whatever state it may be, water is constantly circulating. 

 Entering the earth at every pore, passing down through all 

 rocks in all conditions, dissolving everything as it goes, leaving 

 behind in every place something new, and modifying every 

 form of matter, this great power, this really universal men- 

 struum, knows no obstacle, and never ceases to do its work. 

 Carried down at first through open fissures in all rocks, it is 

 further conducted through the open and permeable sands in 

 large quantities, and through rocks of closer texture, by the 

 most minute capillary tubes. The actual quantity of water in 

 all rocks varies from five to forty parts in a thousand, even 

 when they are in the driest state to which they can be reduced 

 by exposure to the air. 



And here, again, we are able by actual experiment to 

 understand something of nature's methods in the interior of 

 the earth. It might well be supposed that rocks squeezed 

 under enormous pressure would become more compact and 

 less porous ■ on the contrary, they become more porous, and it 

 is certain that both the dissolving and penetrating power of 

 water are increased in proportion to the pressure. Thus the 



