August 17, 1S83.] 



SCIENCE. 



197 



on the interior plateau, anil with those of intervening 

 lines of plication ami igneous disturbance. Stratig- 

 raphy, lithology, anil fossils arc all more or less at 

 faillt in dealing with these iiuestions; and, while the 

 general nature of the problem is understood by many 

 geologists, its solution in particular cases is still a 

 source of apparently endless debate. 



The causes and mode of operation of the great 

 movements of the earth's crust which have produced 

 mountain'', plains, and tablelands, are still Involved 

 in some mystery. One patent cause is the unequal 

 settling of the crust toward the centre; but it is not 

 so generally understood as it' should be, that the 

 greater settlement of the ocean-bed has necessitated 

 its pressure against the sides of the continents in the 

 same manner that a huge ice-floe crushes a ship or a 

 pier. The geological map of North America shows 

 this at a glance, and impresses us with the fact that 

 large portions of the earth's crust have not only been 

 folded, but bodily pushed back for great distances. On 

 looking at the e.Ktrcme north, we see that the great 

 Laureiilian mass of central Newfoundlanil has acted 

 as a protecting pier to the space immediately west 

 of it, and has caused the Gulf of St. Lawrence to 

 remain an undisturbed area since paleozoic times. 

 Immediately to the south of this, Nova Scotia and 

 New Brunswick are folded back. Still farther south, 

 as (iiiyot has shown, the old sediments have been 

 crushed in sharp folds against the Adirondack mass, 

 which has sheltered the tableland of the Catskills and 

 of the Great Lakes. South of this again, the rocks 

 of Pennsylvania aiul Maryland have been driven back 

 in a great curve to the west. Nothing, I think, can 

 more forcibly 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. 

 Comple-K and difficult to calculate though these move- 

 ments of plication are, they are more intelligible 

 than the apparently regular pulsations of the flat con- 

 tinental 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 secul.ir cooling of the earth, 

 or with the gradual retardation of its rotation, or 

 with both. Throughout these changes, e.ich succes- 

 sive elevation exposed the rocks for long ages to the 

 decomposing influence of the atmosphere. Each 

 submergence swept away, and deposited 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 pliints. Were it possible to restore ac- 

 curately the physical geograpliy 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 arrived at by the most careful obser- 

 vation and induction do not remain undisturbed, but 

 require constant vigilance to prevent them from being 

 overthrown. Somotimes, of course, this arises from 

 new discoveries throwing new light on old facts; but 

 when this occurs it rarely works the complete sub- 



version of previously received views. The more 

 usual case is, that some over-zealous specialist sud- 

 denly 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 

 liave referred to the mode of formation of coal as 

 one of the things most surely settled and understood. 

 The labors of many eminent geologists, raicroscopists, 

 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 

 tlMit grew upon them. Logan and I have illustrated 

 this in the case of the series of more than sixty suc- 

 cessive coal-beds exposed at the South Jogglns, and 

 have shown unequivocal evidence of land-surfaces at 

 the time of the deposition of the coal. Microscopical 

 examination has proved that these coals are composed 

 of the materials of the same trees whose roots are 

 found in the underclays, and their stems and leaves 

 in the roof-shales; that much of the material of the 

 coal has been subjected to sub-aerial decay at the time 

 of its accumulation; .and that in this, ordinary coal, 

 dilTers from bituminous shale, earthy bitumen, and 

 some kinds of cannel, which have been formed under 

 water; that the matter remaining as coal consists 

 almost entirely of epidermal tissues, which, being 

 suberose in character, are highly carbonaceous, very 

 durable, and impermeable by water,' and are hence 

 the best fitted for tlnv production of pure coal; and 

 finally that the vegetation and the climatal and geo- 

 graphical features of the coal period were eminently 

 fitted to produce in the vast swamps of that period 

 precisely the effects observed. All these points and 

 many others have been thoroughly worked out for 

 both European and American coal-lields, and seemed 

 to leave no doubt on the subject. But several years 

 ago certain microscopists observed on slices of coal 

 layers filled with spore-cases, — a not unusual circum- 

 stance, since these were shed in vast abundance by 

 the trees of the coal-forests, and because they contain 

 suberose matter of the same character with epidermal 

 tissues generally. Immediately we were informed 

 that all coal consists of .'pores; and, this being at 

 once accepted by the unthinking, the results of the 

 labors of many years are thrown aside in favor of this 

 crude and partial theory. A little later, a German 

 microscopist has thought proper to describe coal as 

 made up of minute algae, and tries to reconcile this 

 view with the appearances, devisingat the same time 

 a new and formidable nomenclature of generic and 

 specific names, which would seem largely to represent 

 mere fragments of tissues. Still later, some local 

 facts in a French coalfield have induced an eminent 

 botanist of that country to revive the drift theory 

 of coal, in opposition to that of growth in situ. A year 

 or two ago, when ray friend Professor Williamson 

 of Manchester informed me that he was preparing 

 a large scries of slices of coal with the view of revis- 

 1 Acailiiiii gt'ulugy, ttiJril L-Uiliuu, ttuppli-muut, ji. 6S. 



