as compared with that of the present day. 75 



of the stimulus of light, or of their being subjected to continued 

 frosts. 



4. The consequence of the existence of the coal-plants has been 

 the formation of coal ; but how this operation was conducted is a 

 question still unsolved. The under-clay or soil upon which the coal 

 rests, and upon which some of the plants grew, seems in general to 

 have suffered little change thereby, further than what was effected by 

 the intrusion of a vast number of roots throughout its mass. The 

 shales, on the other hand, are composed of inorganic matter, mate- 

 rially altered by the presence of the vegetable matter which they con- 

 tain. The iron-clays, again, present another modification of this 

 mixture of organic and inorganic matter, often occurring in the form 

 of nodules. These nodules seem to be the result of a peculiar action 

 of vegetable matter upon water charged with soil and a salt of iron, 

 the iron-stone nodules of existing peat-bogs appearing altogether 

 analogous to those of the carboniferous period, whether in form or in 

 chemical constituents. 



Here, then, the botanist recognises in one coal-seam a vegetable 

 detritus under three distinct phases, and which has been acted upon 

 in each by very different causes. In the under-clay there are roots 

 only;* these permeate its mass, as those of the water-hly and other 

 aquatic plants do the silt at the bottom of still waters. 



The coal is the detritus either of those plants whose roots are pre- 

 served in the under-clay, or of those, together with others which may 

 have grown amongst them, or at a distance, and have been afterwards 

 drifted to the same position. 



Above the coal is the third soil, bearing evidence of the action of 

 a vigorous vegetation : this is the shale, which has all the appearance 

 of a quiet deposit from water charged with mineral matter, and into 

 which broken pieces of plants have fallen. Here there is so clear a 

 divisional line between the coal and shale, that it is still a disputed 

 point whether the plants contained in the latter actually grew upon 

 the former, or were drifted to that position in the fluid which depo- 

 sited the mineral matter. Amongst the shales are also interspersed, 

 in many cases, innumerable stumps of Sigillarice^ similar to those 

 whose roots occur in the under-clay, and which are themselves found 



* The absence of other parts of plants, and, indeed, of any plant but the roots 

 oi Sigillarice, in the under-clay, appears a fact of consideraJjle importance. In 

 the first place, it indicates that that soil was in a condition unsuited to the 

 growth of other vegetables (as mentioned above) whose seeds might have ac- 

 companied those of that genus on its previously naked-surface. In the second 

 place, this absence of other fossils in the under-clay is opposed to the theory of 

 the drift-origin of the vegetable matter comprising coal: for there is no interstra- 

 tification of coal with this subjacent deposit, which might have been expected 

 to occur over some portion of an extensive coal-field ; whereas the gradual 

 decay of these plants, whose roots struck into the under clay, would produce a 

 uniform bed of peat, perhaps adapted to the growth of those ferns and other 

 plants which are fossilized in the superincumbent shales. 



