396 VEGETATION OF THE CARBONIFEROUS PERIOD 



very frequently pervading all the succeeding beds, though so many as 

 thirty may be interposed between the highest and the lowest. 



3. Of the relations between the soil and the plants nourished by it, 

 little more is recognizable, than that the Sigillarice have been particu- 

 larly abundant on the under- clay, which, judging from the absence of 

 any other fossils but Sigillarice roots (Stigmaria), seems to have been 

 either, in itself, unfriendly to vegetation, or so placed (perhaps from 

 being submerged) as to be incapable of supporting any other. The latter 

 is- the most probable, because both Sigillarice and their Stigmaria roots 

 occur in other soils besides under-clay, and are there accompanied by 

 Calamites, Ferns, Sfc. The Coniferce, again, are chiefly found in the 

 sandstones ; and their remains being exceedingly rare in the clays, 

 shales, or ironstones, it may be concluded that they never were asso- 

 ciated with the Sigillaria and other plants which abound in the coal 

 seams ; but that they flourished in the neighbourhood, and were at times 

 transported to these localities. The quantity of moisture to which 

 these plants were subjected must remain a question, so long as some 

 authors insist upon the Sigillaria being allied to plants now character- 

 istic of deserts, and others, to such as are the inhabitants of moist and 

 insular climates. The singularly succulent texture and extraordinary 

 size of both the vascular and cellular tissue of many, possibly indicate a 

 great amount of humidity. The question of light and heat involves a 

 yet more important consideration, some of the coal-plants of the Arctic 

 regions being considered identical with those of Britain. How these 

 can have existed in that latitude, under the now-prevailing distribution 

 of light and heat, has not been hitherto explained : they are too bulky 

 for comparison with any vegetables inhabiting those regions at the pre- 

 sent time, and of too lax a tissue to admit of a prolonged withdrawal 

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

 4. The consequence of the existence of the coal -pi ants 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, materially altered by the 

 presence of the vegetable matter which they contain. The iron-clays 

 again present a third modification of this mixture of organic and in- 

 organic 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. 



