On some of the Modes of Formation of Coed Seams. 99 



undergoing seaward transport from the land. The zone 

 where deposition of this kind is in progress must necessarily 

 remain constant the whole time that the relative position of 

 sea and land, and the physical conditions generally, remain 

 unchanged; so that there is practically no limit to the 

 quantity of material that might be sorted out and deposited 

 by itself in this manner. 



But both the specific gravity and the power of resisting 

 maceration of different vegetable organisms vary within 

 wide limits. As a result, there is not only a constant 

 separation of the vegetable from the mineral matter going 

 on, but there is, further, an equally constant sorting out of 

 the different vegetable organisms themselves. Those that 

 become waterloo-aed earliest reach the sea-bottom nearest to 

 the land; whilst those that, from various causes, resist decom- 

 position longer, remain longer in suspension, and are there- 

 fore drifted to greater distances by the submarine currents 

 before they finally come to rest on the sea-bottom. 



With oscillations of level, or any other causes bringing 

 about changes of physical conditions, the absolute position of 

 these various zones of deposition must necessarily change, 

 and so variation in the constituents of the coal, or the sub- 

 stitution for it of mineral sediments on the one hand, or of 

 oceanic deposits on the other, may be brought about. 



This mode of accounting for coal seams does not by any 

 means preclude any one of the others ; on the contrary, as 

 deltas advance seaward over areas where deeper water 

 conditions had previously obtained, it would be almost a 

 necessary consequence that lagoon vegetation of some kind 

 or other should prevail over the spot where, at an earlier 

 period, thalassic coal seams had been formed. And it is 

 equally clear that coals originating in any other manner 

 might be inter-stratified with the beds whose history is more 

 especially noticed here. 



It appears to the writer that this last-mentioned explana- 

 tion will enable us to account for the remarkably fine 

 lamination seen in so many coal seams ; it will also explain 

 their freedom from admixture with impurities of inorganic 

 origin. We need not longer marvel at the fact that the 



