OUTLINES OF GEOLOGY. 



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Vegetable formations are intimately mixed with decom- 

 posed earthy matter ; while those of mechanical origin 

 contain organic substances, either retaining their structure, 

 or decomposed. 



63. The formations of animal origin are principally to be 

 met with in the form of coral reefs. This action is most 

 intense in the Pacific Ocean, and it has even been supposed 

 that islands have been raised from the depths of the sea by 

 the labours of the coralline animals. More close investiga- 

 tion seems to lead to the conclusion, that the work of the 

 species which are capable of forming continuous banks, is 

 limited to the depth of about 30 feet ; and that a shoal, arising 

 from some other cause, must have previously existed. 



Vegetable alluvial formations are called by Brogniart, 

 Phytogene, and are two in number : — Humus, or vegetable 

 mould, and turf, or peat. 



64. The formation called Humus is almost coextensive 

 with the surface of the land, being found every where, except 

 in the most barren deserts, and on the steepest declivities. 

 It is never of any great thickness, and this is one of the 

 most satisfactory proofs that the present geological epoch is 

 of modern date, compared with the whole duration of the 

 earth's existence. 



65. Turf is composed almost wholly of aquatic vegeta- 

 bles, and is found in basins which appear to have once been 

 lakes, in which the water, without being absolutely stagnant, 

 had but small velocity. In such situations, after the vegeta- 

 bles had ceased to live, decomposition had not commenced, 

 and the dead plants have served as the bed on which others 

 have grown, without undergoing putrefaction. In the beds 

 of turf which belong to alluvium, the cause is still in action, 

 so that if a part be removed, as is done in some countries to 

 convert it into fuel, the space will, after a time, be filled with 



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