438 SCIENCE PROGRESS. 



spread deposit is by no means confined to coal seams ; 

 among the Palaeozoic strata sheets of fine-grained muds may 

 be followed over considerable tracts of country without any 

 great change in lithological structure. If we take into 

 account the lightness of vegetable fragments, and of the 

 ulmic substances which would be carried away by water 

 flowing over the partially decayed accumulations on forest- 

 clad surfaces, it is not difficult to understand how such 

 sediments might have been spread out over an area of 

 wide extent, with little or no admixture of heavier detrital 

 material. Fayol's instructive experiments 1 on the condi- 

 tions of sedimentation fully bear out the assertion that in 

 many coal basins the manner of occurrence of coals, sand- 

 stones, and shales, is exactly analogous to that of a series of 

 subaqueous deposits laid down in a quiet fresh-water lagoon, 

 or on the floor of a current-swept area of deposition. The 

 same observer has made out a very good case with regard 

 to the vertical stems well known to those familiar with Coal- 

 Measure stratigraphy ; many of these have undoubtedly been 

 drifted, and have finally settled down in the sand or mud in 

 an erect position. Instances of calamitean and other trunks 

 preserved where they grew in a swampy or submerged 

 region are by no means unknown. Grand' Eury and other 

 French writers have figured several examples of such stems, 

 showing a series of adventitious roots developed at succes- 

 sively higher levels on the axis, and radiating out into the 

 surrounding and growing sediment. 



The enormous amount of time necessitated by the 

 growth-in-place theory would seem to be a matter of some 

 difficulty. Geologists have been warned by physicists, 

 whether on good grounds or not cannot here be discussed, 

 that it is no longer possible to draw unlimited " time 

 cheques " in support of any theory which involves long 

 periods of time. The sedimentation method of formation 

 does not make any such exorbitant demands on the duration 

 of the coal-producing era. Many authorities have laid 

 stress on the gradual passage of pure coal through impure 



1 P. 365, et seq. 



