298 



A hard pan, of which the cement is no doubt lime, is found to be 

 an almost universal sub-stratum to the rich loam of the prairies. To it 

 the fertility of those soils may, in a great measure, be ascribed, since 

 it serves to reatin the moisture which would be quickly swallowed by 

 the porous sands. 



Possibly this fact may aid in illustrating the origin, as well as the fer- 

 tility, of the prairies. This sub-stratum is not commonly met with in 

 the openings. 



M*RL, OR BOG-LIME, AND TUFA. 



That variety of the mineral which is hei-e designated by the name of 

 marl^ is chiefly a carbonate of lime, or lime combined with carbonic 

 acid. It is frequently argillaceous, and mixed with earthy and carbon- 

 aceous matters. Throughout the counties enumerated, this mineral is 

 found only in connection with the gravels, sands and clays which over- 

 lie the rocks, and may be defined as an alluvial deposit from waters 

 which have percolated soils charged with lime. On reaching the sur- 

 face the water parts with a portion of its carbonic acid, and becomes no 

 longer capable of holding the lime in solution, which is then deposited 

 in the form of a pulverulent, chalky substance, in the beds of lakes, or 

 beneath the peat of marshes. 



As carbonate of lime is a constituent of the covering of moluscous 

 animals, these circumstances are favoi-able to the collection of great num- 

 bers of shells, so that these not unfrequently constitute even the main 

 portion of the bed itself which may then receive the name of " sheU 

 marV 



That form of lime which is called tufa, has a similar origin. It dif- 

 fers in external character, being hard, light and porous, and is that which 

 is familiarly known as ^^ honey -comb lime P This characteristic differ- 

 ence is the result of circumstances, not of composition. Tufa is form- 

 ed in situations which allow access of air, when a strong union of the 

 particles takes place. Marl being always deposited under water, or be- 

 neath the peat of bogs, the surrounding fluid prevents cohesion. This 

 condition is that which is very commonly designated as " bog-limeP 



Thus, according to circumstances, we find a variety of forms assumed 

 by these deposits, from a "tufaceiuis marl," in which the particles have 

 but partially cohered, to a hard " tufa," or travertin rock, appearing as 

 ledges in exposed hill-sides. 



