COAL: ITS STRUCTURE AND FORMATION. 437 



through the unstratified underclay, as a strong argument in 

 support of the autochthonous views ; but he apparently 

 overlooks the fact that it is generally admitted that these 

 plants are often found in their position of growth, a mode of 

 occurrence which does not at all preclude the deposition of 

 coaly sediment on a submerged floor, penetrated by roots 

 or creeping rhizomes. An old land surface may easily be- 

 come covered with water, or form the floor of a lake in 

 which vegetable sediment is being accumulated, or, as some 

 hold, the stigmarian soils may very likely have been under 

 water during the growth of the plants. It must not be 

 forgotten that in some districts stigmarian underclays are 

 not found below seams of coal, and even in England their 

 occurrence is by no means universal. On the other hand it 

 not unfrequently happens that the underclays are overlain 

 by grits or sandstones, and not by a bed of coal. The fre- 

 quent irregularity in the floor surface of a coal seam, and the 

 distinct unconformity between the coal and the underlying 

 rock, clearly points to an interval between the deposition of 

 the floor rock and the formation of the coal. Denudation 

 must have taken place in this interval between the deposi- 

 tion of the two sets of beds, and the stem portions of the 

 trees removed before the carbonaceous sediments were laid 

 down on the submerged underclays. Grand' Eury includes 

 in the family of Stigmarece two forms of plants : the true 

 stigmarias he regards as rhizomes, which floated in water 

 or grew on the surface mud, into which their branches 

 penetrated ; the true roots of Sigillaria he speaks of under 

 the name Stigmariopsis, and sees in them certain well-marked 

 structural peculiarities. Some further light has lately been 

 thrown on this vexed question by the results set forward in 

 an interesting memoir by Solms-Laubach ; x but a detailed 

 consideration of the stigmarian question must be deferred to 

 a later article. 



The occurrence over large areas of uniformly pure beds 

 of coal is frequently urged against the theory of drifted 

 vegetable ddbris. This fairly constant character in a wide- 



1 Solms-Laubach (2). 



