588 STEVENSON— FORMATION OF COAL BEDS. [November 3, 



trace of algae, but consisted wholly of fragments from land plants. 

 The same is true of a specimen from the Dollart. After examining 

 samples from all localities of alleged marine peat, Friih felt himself 

 justified in the positive assertion that thus far no marine peat has 

 been discovered. 



The classification of peaty deposits has received much attention 

 from many authors. The literature in America is somewhat limited, 

 as, until very recent years, peat seemed likely to remain indefinitely 

 without economic importance. Among the earliest attempts at clas- 

 sification is that by Shaler,-'-' whose grouping was much in detail. 

 If e divided the forms into marine marshes and freshwater swamps ; 

 the former including grass marshes and mangrove marshes, grow- 

 ing above tide, as well as mud banks and eel-grass marshes, growing 

 below mean tide ; the latter including river, lake and vipland swamps, 

 each with two subdivisions. The grass marshes are along the coast 

 where salt water bathes the roots of the plants, while freshwater 

 swamps are above tide. Davis, ^°" in discussing the freshwater de- 

 posits of Michigan, employed the terms bog, marsh and swamp; a 

 bog is an area of wet, porous land, whose soil is mostly decayed or 

 decaying vegetable n:atter, loosely consolidated and containing so 

 nuicli water as to tremble when one walks on it ; the vegetation 

 varies, but usually consists of mosses, sedges or grasses, or a combi- 

 nation of them along with shrubs and even small trees; a marsh 

 does not shake readily when one walks over it, though it may be very 

 soft and wet ; the vegetation is mostly grass-like, though shrubs may 

 be present in thickets ; a swamp soil is firm, but wet, even to flood- 

 ing at times, and bears trees and shrubby plants as the most impor- 

 tant part of the vegetation. This grouping is not absolute, for the 

 types ma}' all be found in a single basin, the passage from one to 

 the other being very gradual. 



In luirope, where peat is of great economic importance, many 

 students have expended great ingenuity in efforts to classify the 



'■"' N. S. Slialcr, " General Account of Freshwater ^Morasses of tlie United 

 States," Tenth Ann. Rep. U. S. Geol. Survey, 1890, pp. 261 et scq. 



'"" C. A. Davis, "Peat." Ann. Rep. Geol. Survey of ^Ntich.. 1907, pp. 108, 



loa 



186 



