228 EDWARD C. JEFFREY 



not also laid down in open lakes, lagoons, or tranquil estuaries. 

 There seems to be little doubt that this is the case, because not 

 only do they show to a large extent the organization of cannels 

 but they often pass imperceptibly into coals of this type. More- 

 over, an equally cogent argument is supplied by the examination 

 of lacustrine muck of the present age. In the greatest depths of our 

 modern lakes, which are usually of glacial origin, one ordinarily 

 finds quantities of diatomaceous tests, or, if the region be a lime- 

 stone one, a thicker or thinner stratum of a white limy deposit, 

 containing the remains of stoneworts or Characeae. Above the 

 basal deposits occur strata extremely rich in the pollen of conifers 

 and certain catkin-bearing plants, such as the willow, the alder, 

 and the birch. As the water grows shallower by accumulation 

 of organic mud, we begin to find remains of water-lilies and other 

 aquatics mingled with pollen. Then come layers of coarser remains 

 of land plants, twigs, leaves, bits of wood, etc., swept by rain and 

 winds into the lake and always more or less intermingled with a 

 decreasing proportion of spores, until the organic silting of the 

 waters results in the emergence of land, which if not characterizable 

 as dry, still is able to support the growth of land plants. With 

 the use of a peat prober of the type devised by Dr. C. A. Davis, 

 of the United States Bureau of Mines, it is possible to recognize, 

 by probing the depths of the bogs which vegetate on the bosoms 

 of filled lakes, the general order of deposits outlined in the foregoing 

 sentences. Ordinarily the depth of one of these obliterated lakes 

 is not more than ten to twenty feet, so that we have the whole 

 interval from the ice age to the present for the accumulation of the 

 filling material. The estimates of the remoteness of the last glacia- 

 tion vary from 25,000 years to four times that length of time. 

 Even accepting the smallest estimate, the interval required to 

 fill a lake ten feet in depth is great enough to call for a deposit of 

 less than one foot of material in a thousand years, or not more than 

 one-tenth of a foot in a century. This extremely slow rate of 

 deposition relieves us from having to answer a stock objection to 

 the possibility of the lacustrine formation of the raw materials 

 of coal. It has often been pointed out that a rate of erosion neces- 

 sary to fill up a large body of water in a relatively brief interval, 



