108 J. W. DAWSON — SOME RECENT DISCUSSIONS IN GEOLOGY. 



sarily formed on dry land. They may be deposits from water, but may 

 have been raised-up or filled-m to constitute soils. The second is : " Un- 

 clerclays are distinctly stratified, showing that they have been deposited 

 under water." This is true of some of them at least, but is no argument 

 against their having become soils. The subsoils of many swamps and 

 marshes is a deposit from water, but land vegetation grows upon it. The 

 imperfection of such statements and the absurdit}^ of placing them in 

 contrast are sufficiently obvious, yet such objections have to be met in the 

 interest of scientific geology. They must be met exactly as they were met 

 by Logan so many jea,Ys ago in his observations on the underclays of 

 south Wales, which have been followed up by myself and others. We 

 have shown, in the first place, that the lycopods, ferns and calamites grow- 

 ing on these underclays were reall}^ land plants ; secondly, that their roots 

 penetrated the subjacent beds in such a way as to show that they have 

 grown upon them, and, lastly, that the coal itself, in all cases except that 

 of the cannel coals, bears evidence of subaerial accumulation, while the 

 erect trees associated with it show that they decayed and became hollow 

 by atmospheric action. No doubt the underclays were -usually swamp 

 rather than upland soils, but the occurrence of remains of land animals 

 in erect trees shows that in some cases the soil must have been elevated 

 ten feet or more above water level when the coal vegetation was growing 

 on it. I have myself studied and described these facts as evidenced in 

 the case of eighty successive beds of coal admirably exposed in the cliffs 

 of the south Joggins. 



In connection with all this we have the accumulation of five thousand 

 feet of sediments and organic beds, each of which must in turn have been 

 a land or shallow water surface, and the subsidence thus indicated must 

 have taken place by small downthrows, only sufficient to keep pace with 

 the accumulation of deposits, and this for a great lapse of time. The 

 coal-deposits of the great Carboniferous system thus mark a special stage 

 in the production of our continents, when they were less diff'erentiated 

 as to orography, and when a very uniform and equable climate extended 

 over the northern hemisphere, accompanied by a very peculiar vegeta- 

 tion. Such conditions did not occur in combination and to a like extent 

 in any succeeding period of the earth's history. 



Relation of V|:getation to continental Movements. 



This special position of the great coal-formation leads to a considera- 

 tion of the relation of vegetation and of fossil plants to the elevation and 

 depression of our continents, to changes of climate, and to the determina- 

 tion of geologic age, and of which we are reminded by Professor White's 



