358 SCIENCE PROGRESS. 



rence of the underclay and its stigmarian fossils under every 

 bed of coal ; (2) the remarkable absence of arenaceous or 

 argillaceous impurities, and the uniformity of some coals in 

 composition and thickness over a wide tract of country ; (3) 

 the not uncommon occurrence of upright stems of trees in 

 strata associated with the beds of coal. Various authors 

 have successively passed on these duly accredited arguments, 

 without always pausing to think whether or not they form a 

 fatal objection to some other mode of origin than the car- 

 bonisation in situ of a semi-decayed mass of forest debris. 

 A glance at a series of chemical analyses 1 of anthracite, coal, 

 lignite, peat and wood, shows a gradual increase in the 

 percentage of carbon, and a corresponding decrease in other 

 elements. From these and other classes of facts, it has been 

 argued that we have in coal and anthracite the extreme 

 terms of a fairly continuous series of vegetable deposits, 

 which, speaking generally, are richer in carbon, according as 

 they belong to older rocks, and have been longer exposed 

 to slowly acting chemical changes. A connection between 

 an increase in carbon percentage and the amount of earth 

 movement to which the strata have been exposed, lends 

 support to such opinions. 



The unusual character of Carboniferous lignitic deposits 

 in Central Russia, made up of paper-like laminae of little 

 altered corticular tissues, 2 has been attributed to the 

 escape of the beds from the effects of earth movements, 

 and from the influence of those potent factors, heat 

 and pressure, which, in other cases, have accelerated and 

 extended the chemical changes to a much more advanced 

 stage in the process of carbonisation. Among Tertiary rocks 

 we occasionally find carbonaceous deposits which would be 

 placed in the category of ordinary coal, if they were not 

 members ol a much more recent geological system. 

 Pressure and heat may have played important parts in the 

 production of coal; but the series of changes involved in the 

 alteration of plant tissues into compact coal, have been far 



1 Toula, p. 22, and in many other works on coal. 



2 Figures of these tissues are given by Zeiller in a paper published in 

 the Ami. Sri. Nat. (Bot.), No. 6, vol. xiii., p. 213, 1882. 



