54 



PRINCIPLES OF PALAEONTOLOGY. 



Sedimentary Rocks. Plants, again, even when undoubtedly 

 such as must have grown on land, do not prove that the bed 

 in which they occur was formed on land. Many of the remains 

 of plants known to us are extraneous to the bed in which they 

 are now found, having reached their present site by falling into 

 lakes or rivers, or being carried out to sea by floods or gales of 

 wind. There are, however, many cases in which plants have 

 undoubtedly grown on the very spot where we now find them. 

 Thus it is now generally admitted that the great coal-fields 

 of the Carboniferous age are the result of the growth in situ 

 of the plants which compose coal, and that these grew 



on vast marshy or partially 



submerged tracts of level 

 alluvial land. We have, 

 however, distinct evidence 

 of old land-surfaces, both in 

 the Coal-measures and in 

 other cases (as, for instance, 

 in the well-known "dirt- 

 bed" of the Purbeck series). 

 When, for example, we find 

 the erect stumps of trees 

 standing at right angles 

 to the surrounding strata, 

 we know that the surface 

 through which these send 

 their roots was at one time 

 the surface of the dry land, 

 or, in other words, was an 

 ancient soil (fig. 19). 



In many cases fossils en- 

 able us to come to important 

 conclusions as to the climate 

 of the period in which they 

 lived, but only a few in- 

 stances of this can be here 

 adduced. As fossils in the majority of instances are the re- 

 mains of marine animals, it is mostly the temperature of the 

 sea which can alone be determined in this way; and it is import- 

 ant to remember that, owing to the existence of heated currents, 

 the marine climate of a given area does not necessarily imply a 

 correspondingly warm climate in the neighbouring land. Land- 

 climates can only be determined by the remains of land-ani- 

 mals or land-plants, and these are comparatively rare as fossils. 

 It is also important to remember that all conclusions on this 



Fig. 19. Erect Tree containing Reptilian 

 remains. Coal-measures, Nova Scotia. (After 

 Dawson.) 



