GEOLOGICAL SUEVEY OF THE TERRITORIES. 373 



the print of the details of their structure, their outlines and their nerva- 

 tion, without any trace of their original substance. 



The mode of preservation of the leaves varies like that of the trunks, 

 according to the elements composing the matter in which they are im- 

 bedded, and especially according to the circumstances in which they 

 have been deposited. Immense beds of coal, for example, have been 

 formed from the remains of plants, while the coal itself scarcely shows 

 any distinct prints, except sometimes upon thin lamellae of dry char- 

 coal which separate its layers. Per contra, the prints of fossil plants 

 are abundantly seen in the shale which covers the coal. The reason of 

 this difference is, that when the combustible is heaped by superposition 

 of debris of wood, and no foreign element like clay or sand, is mixed 

 with it, the whole matter, by slow decomposition, is softened into paste, 

 then compressed and hardened, forming a homogeneous mass. In this 

 process all the tissues are destroyed or mixed, and therefore the origi- 

 nal form of vegetables is rarely preserved. But when the growth of 

 the peat or the heaping of the woody materials which have composed a 

 bed of coal have ceased, and when by immersion the surface becomes 

 covered by water, deep enough to stop the active vegetation which 

 originated the coal, some hillocks or islands are left here and there 

 above water, bearing the same kind of plants as those of the coal. Their 

 branches in decaying fall into the water and are imbedded in its muddy 

 deposits, which form the shale, and then preserved by fossilization. 



It is in the same way that the leaves of trees, growing around the 

 swamps at the time of our cretaceous or tertiary formations, have been 

 deposited in clay, and preserved in a fossil state. Sometimes, however, 

 leaves and woody debris have been transported and heaped at some 

 places according to circumstances. In this case they are more or less 

 damaged, rolled up, and mixed together in sandstone or clay, in a more 

 or less indistinguishable mass. We have at our time examples of all 

 these kinds of transportation, deposition, and preservation of leaves. We 

 find them imbedded in the muddy deposits which cover our peat-bogs. 

 The clay of our swamps is full of skeletons of leaves from the trees which 

 surround them, and along our rivers, as, for example, of the Ohio Kiver 

 near Paducah, we find deposits of leaves floated down the river and 

 buried in the bottom clay, in heaps of six to ten feet thick, where they 

 still follow the same process of slow decomposition, whose ultimate term 

 is complete petrification. 



ON THE FOSSIL PLANTS CONSIDERED IN THEIR RELATION TO THE 

 ACTUAL DOMAIN OF MAN. 



As most of the strata composing the crust of the earth have been 

 formed under water, and mostly contain remains of animals, especially 

 of molusks, geology receives from animal paleontology far greater 

 assistance for the determination of the strata and of their relative age 

 than it can obtain from the study of botanical remains. Considered in 

 this point of view, therefore, fossil plants appear of little importance. 

 But when we come to demand from geology some instructions, some 

 light concerning the surface of our earth at different epochs, science 

 can answer nothing if it does not inquire into the data furnished by 

 botanical paleontology. As vegetation is in absolute relation with 



