376 



THE EEGOLITH 



period, when plants of tlie lycopod type appeared. Such soils, 

 as soils, have, however, long since disappeared in the never- 

 ending cycle of change, and it is not until we reach the Car- 

 boniferous period that we meet with soils which have been 

 preserved in place and in recognizable form even to the present 

 day. Even here induration and partial metamorphism has 

 rendered them no longer fitted for the support of plant life, 

 but that they once did so serve is amply proven by the occa- 

 sional finding of erect, fossil tree trunks with roots buried in 

 their native soil, as they grew in the marshes and woodlands of 

 the coal period. But as to the time of the beginnings of the 

 formation of such soils as still retain their soil characteristics, 

 we have not in all eases reliable data. Those which are but the 

 unconsolidated sediments of recent geological time, like those 



of the eastern shore of Mary- 

 land, the loess and alluvium 

 of the Mississippi valley, or 

 the swamp and glacial soils of 

 the north and east may, of 

 course, be located with a rea- 

 sonable amount of accuracy. 

 But as for the residual soils, 

 those which result from the 

 breaking down in place of 

 rock masses, we can only say 

 that they must be younger than 

 the rocks from which they 





i^ ti_. 



^^:t^JiS^^--f 



Pig. 38.— Trunk of tree stiU standing 

 in soil of Carboniferous age. a, 

 becl-Tock; hf under clay or ancient 

 soil; c, coal J dj bedded rock; e, 

 fossil tree. 



were derived. The writer has shown that the granite soils of the 

 District of Columbia are post-Cretaceous; in other parts of the 

 Piedmont plateau of Maryland, they may be post-Tertiary. In 

 but few instances, as at Medford in Massachusetts, have we evi- 

 dence of any considerable amount of soil formation by decom- 

 position and disintegration since the close of the glacial period. 

 Obviously the older a residual soil, the greater the amount of de- 

 composition and leaching it will have undergone and the less 

 will it resemble the parent rock. Where horizontally lying 

 strata of varying character have successively undergone decom- 

 position and a loss of their soluble constituents, the resultant 

 soil must periodically vary according to the nature of the rock 

 undergoing decomposition and the inherited characteristics 



