THE NATURAL HISTORY OF MARL 499 



deepest part of the more extensive lake which formerly occu- 

 pied the area included by the marsh and marl beds. This marsh 

 covering is general on the marl beds of the region and the lake 

 may be said to be a typical marl lake for the locality in which 

 it lies, for there are several others near by which are practically 

 identical in essential points of structure. 



At Marl Lake, however, the filling of the lake has not reached 

 the same stage. There is practically no open marsh, but the 

 lake is shallow for seventy-five or a hundred meters from the 

 shore, then abruptly deepens to an undetermined depth over a 

 relatively small area. The bottom over the shallow area is of 

 pure white marl, and the water is apparently not more than sixty 

 or seventy centimeters deep at the margin of the central hole, 

 while near the shore it is scarcely one-third as deep. In brief, 

 here is a lake in which there is a broad platform of marl sur- 

 rounding a deep hole, which .again is all that remains of the 

 deep water of a lake which is filling with marl. Boring shows 

 that the bed of the lake is nearly as far below the surface under 

 the marl platform, as where the marl has not yet been deposited. 



Upon the shoreward edges of the platform and in small 

 areas farther out upon it, the turf-forming plants are beginning 

 to establish themselves, but as yet they have not made any 

 marked impression, seeming to have a hard struggle to get a 

 foothold. The conditions are then a broad area of shallow 

 water overlying a wide platform of marl, which, if a strong wind 

 should reach it, would be stirred to its depths, and with it the 

 lighter parts of the marl upon which it rests. The marl thus 

 stirred up, in turn, is carried to all parts of the lake by surface 

 and other currents and makes the water turbid. These facts led 

 to an investigation as to the rapidity with which marl, once 

 stirred up, would settle out of perfectly still water, and some 

 interesting results were obtained. The experiments were made 

 as follows: (i) A glass tube 1.58 meters long and 2.5 centi- 

 meters wide was filled with distilled water, into which a quantity 

 of finely divided marl was turned and the tube was shaken to 

 insure a thorough mixing of water and marl. The tube was then 



