328 



INORGANIC CAUSES OF CHANGE. 



[Ch. XV. 



crust, partly by heaping up new matter in certain localities 

 and partly by depressing one portion, and forcing out another, 

 of the earth's envelope. 



It is difficult, in a scientific arrangement, to give an ac- 

 curate view of the combined effects of so many forces in 

 simultaneous operation ; because, when we consider them 

 separately, we cannot easily estimate either the extent of 

 their efficacy, or the kind of results which they produce. We 

 are in danger, therefore, when we attempt to examine the 

 influence exerted singly by each, of overlooking the modifi- 

 cations which they produce on one another ; and these are so 

 complicated, that sometimes the igneous and aqueous forces 

 co-operate to produce a joint effect, to which neither of them 

 unaided by the other could give rise, — as when repeated 

 earthquakes unite with running water to widen a valley ; or 

 when a thermal spring rises up from a great depth, and con- 

 veys the mineral ingredients with which it is impregnated 

 from the interior of the earth to the surface. Sometimes 

 the organic combine with the inorganic causes ; as when a 





reef, composed of shells and corals, protects one line of coas 

 from the destroying power of tides or currents, and turns 

 them against some other point j or when drift timber, floated 

 into a lake, fills a hollow to which the stream would not have 

 Lad sufficient velocity to convey earthy sediment. 



It is necessary, however, to divide our observations on these 

 various causes, and to classify them systematically, endea- 

 vouring as much as possible to keep in view that the effects 

 in nature are mixed and not simple, as they may appear in 

 an artificial arrangement. 



In treating, in the first place, of the aqueous causes, we 

 may consider them under two divisions ; first, those which 

 are connected with the circulation of water from the land to 

 the sea, under which are included all the phenomena of rain. 



and springs ; secondly, those which arise 

 from the movements of water in lakes, seas, and the ocean, 

 wherein are comprised the phenomena of waves, tides, and 



rivers, glaci 



ers 



currents. 



In turning our attention to the former division, 



we find that the effects of rivers may be subdivided into, first, 

 those of a destroying and transporting, and, secondly, those 



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