SOME UNSOLVED PROBLEMS IN GEOLOGY. 65 



main an undisturbed area since palaeozoic times. Immediately to 

 the south of this, Nova Scotia and New Brunswick are folded back. 

 Still farther south, as Guyot has shown, the old sediments have been 

 crushed in sharp folds against the Adirondack mass, which has shel- 

 tered the table-land of the Catskills and of the Great Lakes. South of 

 this, again, the rocks of Pennsylvania and Maryland have been driven 

 back in a great curve to the west. Nothing, I think, can more forci- 

 bly show the enormous pressure to which the edges of the continents 

 have been exposed, and at the same time the great sinking of the 

 ocean-beds. Complex and difficult to calculate though these move- 

 ments of plication are, they are more intelligible than the apparently 

 regular pulsations of the flat continental areas, whereby they have 

 alternately been below and above the waters, and which must have 

 depended on somewhat regularly recurring causes, connected either 

 with the secular cooling of the earth, or with the gradual retardation 

 of its rotation, or with both. Throughout these changes, each succes- 

 sive elevation exposed the rocks for long ages to the decomposing in- 

 fluence of the atmosphere. Each submergence swept away, and de- 

 posited as sediment, the material accumulated by decay. Every 

 change of elevation was accompanied with changes of climate and 

 with modifications of the habitats of animals and plants. Were it 

 possible to restore accurately the physical geography of the earth in 

 all these respects, for each geological period, the data for the solution 

 of many difficult questions would be furnished. 



It is an unfortunate circumstance that conclusions in geology, ar- 

 rived at by the most careful observation and induction, do not remain 

 undisturbed, but require constant vigilance to prevent them from being 

 overthrown. Sometimes, of course, this arises from new discoveries 

 throwing new light on old facts ; but when this occurs it rarely works 

 the complete subversion of previously received views. The more usual 

 case is, that some over-zealous specialist suddenly discovers what seems 

 to him to overturn all previous beliefs, and rushes into print with a 

 new and plausible theory, which at once carries with him a host of 

 half -informed people, but the insufficiency of which is speedily made 

 manifest. 



Had I written this address a few years ago, I might have referred 

 to the mode of formation of coal as one of the things most surely set- 

 tled and understood. The labors of many eminent geologists, micro- 

 scopists, and chemists in the Old and the New Worlds had shown that 

 coal nearly always rests upon old-soil surfaces penetrated with roots, 

 and that coal-beds have in their roofs erect trees, the remains of the 

 last forests that grew upon them. Logan and I have illustrated this 

 in the case of the series of more than sixty successive coal-beds exposed 

 at the South Joggins, and have shown unequivocal evidence of land- 

 surfaces at the time of the deposition of the coal. Microscopical ex- 

 amination has proved that these coals are composed of the materials 



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