IOWA ACADEMY OF SCIENCES. 183- 



Til considering drift material deposited in deltas as an 

 origin for coal deposits, the question arises whether suffi- 

 cient stress has been laid on the probability that consid- 

 erable quantities of silt and sand would be deposited with 

 the vegetable matter. The conditions must have been 

 unique under which drift timber sufficient to make twenty 

 feet of lignite could accumulate, and yet so little silt be 

 deposited with it that the ash of the lignite is but one or 

 tw^o per cent higher than the percentage of mineral matter 

 in the wood. A second point that demands consideration 

 is the origin of this vast amount of drift material. 



The Laramie beds are regarded as accumulations in 

 fresh water. The great fresh water lakes of the present 

 do not seem to present conditions which, though operative 

 for a long period of time, would give rise to similar depos- 

 its, for the amount of drift material that becomes water- 

 logged in them and sinks to the bottom far from the shore 

 where it could accumulate without addition of silt, is prob- 

 ably small. Strong currents and winds either carry most 

 of the drift wood out of the lake or crowd it to shore 

 where it is buried in sand. In the smaller lakes of northern 

 Michigan and Minnesota, located in the heart of the timber 

 country, conditions are different. Vegetation is abundant 

 to the water's edge and sand beaches are rare. Any one 

 who has seen certain of them during logging time can 

 readily believe that, if by natural conditions logs were 

 poured into the lakes as they are yearly during the log- 

 ging season and became w^aterlogged there, woody beds 

 equal to those of the Laramie lignites would result. Per- 

 fectly natural conditions as they exist to-day, operating 

 through a very long period of time, would doubtless con- 

 tribute to one of these lakes enough material for a lignite 

 bed, but as the time in which the accumulation takes place 

 is increased, the probability of a large admixture of for- 

 eign matter is increased. It is true that the forest condi- 

 tions that exist around these lakes prevent the carrying in 

 of large quantities of sand and dust by wind, but tributary 

 streams that are active enough to bring down considerable 

 quantities of timber would contribute to the lake a good 



