436 SCIENCE PROGRESS. 



their luxuriant growth, also supplied their contingent in the 

 form of the parts which died and sank to the bottom." 



Without attempting to give any full account of the facts 

 on which this particular theory has been founded, a brief 

 summary of the main arguments, advanced by the supporters 

 of the allochthonous origin of coal, may serve to draw atten- 

 tion to some of the more recent views which, on the whole, 

 seem more adequate to explain the method of coal formation 

 than those usually accepted. If the plant fragments, from 

 which coal has been formed, had gradually accumulated on 

 the floor of coal period forests, we should expect to find in 

 them more commonly a close association of fragments of the 

 same species or genus of plant, but, as a rule, we have a 

 heterogeneous assemblage of all kinds of plants, much more 

 suggestive of drifted vegetable ddbris than a collection in 

 situ of forest vegetation. 



In a mass of plant ddbris, covering the site of a former 

 forest, we should look for the occurrence of large stems, and 

 more complete specimens of leaves or branches, instead of a 

 fine vegetable paste containing a few scattered fragments of 

 plant tissues. 



The nature and manner of occurrence of the underclay 

 stigmarias are matters of great importance from our present 

 point of view. The serious difficulty to be explained by the 

 supporters of the growth-in-place theory is the absence in 

 nearly every case of the stems of Sigillaria or Lepidoden- 

 dron, in direct connection with the dichotomously branched 

 stigmarias. One or two instances have been recorded of 

 sigillarian stems passing up into the coal from a stigmarian 

 root in the underlying rock, but there is a striking absence 

 of satisfactory instances of such a mode of occurrence of 

 these plants. 



Grand' Eury regards many of the stigmarian fossils as 

 aquatic rhizomes, and in several instances he admits there can 

 be little doubt but that they occur in place. Potonie l has 

 recently called attention to the manner of occurrence of Stig- 

 mai'ia, with its delicate appendages radiating in all directions 



1 Potonie (i). 



