TRAVERTIN AND ARGILLOCALCAREOUS MARL. 433 



finally deposited upon the lands flooded by the Mississippi, serve to 

 constitute the wonderfully fertile borders of that river. 



3. Travertin, or calcareous tufa, is another subject of the many 

 provincial and improper applications of the term marl. It is the 

 deposit made by the precipitation of carbonate of lime from its 

 previous solution in lime-stone water. The rain-water, in falling 

 through the atmosphere, absorbs carbonic acid ; which impregnation 

 enables water to dissolve and hold in solution carbonate of lime, 

 with which the water meets in abundance in lime-stone regions. 

 Thus the springs and streams of lime-stone water are produced. 

 But the carbonic acid absorbed by the water is retained with but 

 little force, and parted with to the atmosphere very easily. This 

 occurs wherever the water, so charged, is in contact with the at- 

 mosphere ; and consequently the more in proportion to the exposure 

 of its surface by the agitation of the water. Hence, at rapids and 

 cascades of lime-stone streams, this precipitation is always found 

 most abundant ; and sometimes in immense quantity. It is prin- 

 cipally of carbonate of lime (about 70 or 75 per cent, in the trials 

 I have made), of cellular and open, though hard consistence, when 

 of newest formation, and not difficult to reduce; and much more 

 loose and soft in other cases. This deposit will be found the cheapest, 

 and also a very rich calcareous manure (though never yet used, to 

 my knowledge), for the neighbouring lands. It is the product only 

 of lime-stone streams, either ancient or existing. 



4. Argillo-calcareous marl, or true marl, has already been de- 

 scribed, as to its texture and constitution. This marl is not pro- 

 perly shelly, though shells may be accidentally intermixed during 

 the deposition. Nor can any coarse or separate sand belong to it, 

 nor any other coarse and heavy matters, which would not remain sus- 

 pended in water flowing with but a moderate current. 



This true marl is formed by the washing away and suspension 

 of calcareous and other earth in the waters of transient land 

 floods of rain-waters, or of rapid rivers and smaller streams. The 

 finer parts only of the different earths can remain long suspended 

 in the flowing waters, after the current ceases to be violent. These 

 finest parts of all, aluminous and silicious as well as calcareous,, are 

 most intimately mixed, and chemically combined, while suspended; 

 and finally are deposited in the form and quality of marl, when 

 reaching a lake, or other comparatively still part of the water. Of 

 course no such marl could have been formed unless the source of 

 original supply of materials existed, and also the manner of abra- 

 sion, transportation, and subsidence ; and no such source of calca- 

 reous earth could be presented except in higher chalk or chalky 

 beds, or otherwise highly calcareous soils and sub-soils. Compact 

 lime-stone alone, no matter how abundant, because of its hardness, 

 could scarcely serve as a source of supplv. It follows, that such 

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