60 SECTIONAL ADDRESSES. 



This view of the origin of the seams of coal that enclose these bodies 

 is quite in accord with opinions long held by palseobotanists, that the 

 structure of the plants found in them is compatible with their growth in 

 brackish water, and corroboration is found in the fact that ' roof-balls ' 

 are found in the overlying shales that contain well-preserved remains of 

 a flora very significantly different from that of the seams. 



Boulders in Coal-seams. 



The occurrence of well-rounded masses up to several hundred- 

 weights of foreign roclis is well attested by many writers, and it is no 

 uncommon occurrence to see small specimens upon the mantelshelf in a 

 colliery of&ce. The subject is, as usual, thoroughly and almost ex- 

 haustively summarised by Stevenson,^ to whose pages any who desire 

 to study the subject further must be referred. 



These erratics have been found in coal-seams in many parts of the 

 world. They occur in every part of the seams from roof to floor, and 

 even penetrating the floor. Two forms of transport of these masses have 

 been suggested. The first ascribes it to floating ice, an hypothesis that 

 fails to take account of the smooth, rounded and water-worn appearance 

 of the stones, no less than the incompatibility of ice action on an ade- 

 quate scale with the climatic conditions indicated, in the judgment of 

 palsBobotanists, by the character of the vegetation. 



The other explanation, which ascribes the transport to floating trees, 

 is not without difficulty when the size, and particularly the shape, of the 

 baulders is considered. Stevenson comments upon the difficulty of 

 imagining a tree of sufficient magnitude carrying such a load with the 

 tenacious grip which would be required to maintain it from the source 

 of the boulder to its place of deposition. It is clear that thoroughly 

 rounded boulders of intensely hard quartzite could have been shaped 

 only by either a long journey in a mountain torrent or by prolonged 

 pounding on a beach. In either case a tree so burdened would need a 

 considerable depth of water for its flotation, and it is inconceivable that 

 it could steer its way through a forest, unless one deeply submerged. 

 I am disposed to think that such were indeed the conditions — that either 

 during a temporary flood or in the final submergence of the coal-swamp 

 some stray gymnospermous tree whose roots were adapted, as those of 

 Stigmaria clearly are not, to wrap round a smooth boulder, drifted over 

 or among the tree tops, and either came to a final anchorage or simply 

 dropped its burden. This explanation is not inconsistent with the 

 presence of boulders at all or any levels in the seam, for it will appear, 

 on reflection, that a mass of rock would readily sink into peat, the 

 rate of its descent being determined by the impetus of its fall, the 

 tenacity of the peat, the shape of the stone, and other factors. Some 

 might bring up against an embedded tree trunk, while others might 

 sink completely through the seam. That some stones have sunk in 

 this manner seems to be indicated by the fact that _ one of the large 

 stones preserved in the Manchester Museum occupied a vertical attitude 

 in the coal when discovered. 



= Op. cit., pp. 391 and 426-433. 



