BURIAL OF PEAT DEPOSITS 517 



then the inner tissues decayed until only the bark, embedded in up- 

 right position in the mud, remained behind. The bark of fallen 

 trees is generally pressed flat on the decay of the inner tissues by 

 the weight of the superincumbent sediment. In Alaska the Yahtse 

 River, "issuing as a swift current from beneath the glacier, has in- 

 vaded a forest at the east, and has surrounded the trees with sand 

 and gravel to a depth of many feet. Some of the dead trunks, still 

 retaining their branches, project above the mass, but the greater part 

 of them have been broken off and buried in the deposit. Other 

 streams, east from the Yahts^, have invaded forests, as indicated 

 by dead trees standing along their borders. Where the deposit is 

 deepest the trees have already disappeared, and the forest has been 

 replaced with sand flats. The decaying trunks are broken off by 

 the wind, and are buried in prostrate position. This deposit con- 

 solidated would resemble closely a Coal Measure conglomerate." 

 (Russell, cited by Stevenson-46.) 



Modern tree trunks have been found on the coast with their in- 

 terior decayed and filled with sand down into the roots. (Potonie- 

 32.) Sand dunes advancing over a peat bed will put an effective 

 stop to further peat formation and serve as a factor in preserv- 

 ing the peat so formed. It is not improbable that many of the 

 sandstones which succeed the coal beds of our Carbonic series owe 

 their origin to such covering wind-blown sands. 



In advancing over the peat the sand dunes will likewise advance 

 over the forests associated with the peat and bury them. This can 

 be seen in many regions where only the tops of large trees project 

 above the sands, as on the shores of Lake Michigan, etc. 



In rare cases peat deposits may be buried by landslides, by mud 

 flows, etc. In all cases where the deposit, first spread over the 

 swamp or bog, is a fine one, impressions of the last fallen leaves 

 and branches may be preserved in great detail. In this manner are 

 formed the roof shales with their wonderful wealth of plant im- 

 pressions, which makes it possible for us to-day to reconstruct the 

 vanished flora of the Coal Period. The clay or soil in which the 

 plants had their roots is in most cases preserved as a "fire clay," 

 that is, a clay wiiich can be used for pottery which has to with- 

 stand intense heat. It contains little iron, and is nearly free from 

 lime and alkalies, of which the clay was deprived by the growth of 

 the plants. 



LiPTOBIOLITHS. 



These may be dismissed with a few words. The exudations of 

 resins, gums, wax. etc., from resinous trees are characterized by 



