5i8 PRINCIPLES OF STRATIGRAPHY 



relative stability of composition, and are not easily decomposed. 

 They may thus form accumulations of greater or less extent, while 

 the plant tissues which they enclose decay wholly. The subfossil 

 Copal is a typical example, while from older rocks (Oligocenic) of 

 North Germany comes the amber. Many minor types of liptobio- 

 liths occur in nature, but they are of more mineralogical than geo- 

 logical value. 



Allochthonous Vegetal Deposits. 



Vegetal material transported from other localities and deposited 

 where it did not grow is found frequently at the present time, both 

 upon the land and in the sea. It also occurred in the past. On the 

 Missouri, Mississippi and other rivers, rafts and whole floating 

 islands of vegetation are known, and logs are found scattered 

 widely through deposits to which they are wholly foreign. "These 

 drifted materials are everywhere distinguishable from plants buried 

 in situ, for they have been deprived of all tender parts ; of the 

 harder woods in Carboniferous times there are few traces except 

 decorticated stems, casts of the interior, indeterminate forms 

 grouped under Knorria, Sternbergia and some other names." 

 ( Stevenson-46 : 642.) 



*Tn all extensive deposits of driftwood the trees are battered, 

 stripped of leaves, bark and often of branches ; they are scattered 

 on the strand or piled in irregular loose heaps, where, exposed to 

 the air, they decay; or, if in more favorable conditions, the inter- 

 stices become filled with sediment, the trees become merely logs in 

 shale or sandstone, even their genus being unrecognizable except by 

 microscopic study of the structure." (Stevenson-46 : 557.) 



The coal deposits of the Commentry basin in France have long 

 been held to be a good example of a fossil allochthonous coal. It is 

 now known, however, that this coal is formed in situ, after all. 

 Stigmaria are found in place, while the trees are arranged in all 

 positions, upright, leaning and prostrate. The resemblance to a 

 forest ravaged by a hurricane is very close. 



Marine Vegetal Deposits. Marine plants, owing to the buoyancy 

 which they possess on account of the numerous air spaces in their 

 tissues, will float on the surface of the water until stranded on the 

 shore or on a mud flat. Their presence in the deep sea deposits 

 is not noted, because they will continue to float until decomposed. 

 On the other hand, in the shallower portions of the littoral district 

 marine plants may accumulate in abundance. Spores of plants are 

 not infrequently found in marine sediments. But by far the most 



