PAL.EOPHYTOLOGY : FOSSIL PLANTS. 271 



exceeding thirty, and even exceeding fifty feet in thick- 

 ness, as at Johnstone in Scotland, and in the Creuzot 

 in Burgundy. In the forests of the temperate zone at the 

 present period, the carbon contained in the trees which 

 grow upon a given surface would hardly suffice to cover it 

 with an average thickness of seven French lines in a cen- 

 tury. (327) it should also be remarked, that the masses of 

 drift-wood transported by rivers or by marine currents, such 

 as those which are found at the mouth of the Mississippi, and 

 those which have formed the " hills of wood," described by 

 Wrangel, on the shores of the Polar Sea, may give us some 

 idea of the accumulations which must have taken place near 

 projecting points of land in inland waters, and along the 

 island shores of the ancient world, and which have produced 

 our present coal-beds : there can be no doubt, also, that 

 these beds owe a considerable portion of their substance, 

 not to large trunks of trees, but to grasses, to low branch, 

 ing shrubs, and to small cryptogamia. 



The association of palms and coniferse, which we have 

 noticed in the coal measures, continues through all the suc- 

 ceeding formations until far into the tertiary period. In 

 the present day it may almost be said that these families 

 avoid each other's presence. We have become so accus- 

 tomed (although without sufficient ground), to regard 

 coniferse as a northern form, that I well remember expe- 

 riencing a feeling of surprise, when, in ascending from the 

 coast of the Pacific towards ChilpansingQ and the elevated 

 valleys of Mexico, between the Venta de la Moxonera and 

 the Alto de los Caxones, at a height of above 4000 English 

 feet above the level of the sea, I rode for an entire day 

 through a thick forest of Pinus occidentalis, and saw 



amongst these trees, resembling the Wey mouth pine, fan 

 YOL. T u 



