400 SCIENCE PROGRESS. 



masterly work of the Brothers Rogers, when studying the 

 Appalachian range. The full results of their labours were 

 brought before the British public by Professor H. D. 

 Rogers in 1857 (1), and it is interesting therein to note the 

 great advance already made towards a true knowledge of 

 the elements composing a mountain range. These ob- 

 servers early recognised that every tract of the earth's crust 

 which had undergone upheaval had a wave-like form, and 

 that faults were merely separated, disarranged portions of 

 what were originally continuous undulations ; more than 

 this — that in regions of great disturbance the strata were 

 arranged in belts of parallel waves, the region of maximum 

 disturbance being marked by a closer folding of the same. 

 Each primary undulation itself might be thrown into second- 

 ary waves, which, though not necessarily parallel to the 

 primaries, nevertheless preserved a constant parallelism 

 between themselves, and a third class might also be pro- 

 duced, giving rise to the so-called rolls of strata. But these 

 were not merely simple waves of undulation, for the folding 

 changed its character from symmetrical flexures, dipping 

 equally on both sides, to normal flexures, where the dip was 

 steeper on one side than the other. Finally, in regions of 

 greatest disturbance the flexures are overfolded, there being 

 an actual inversion, or doubling under, of the steeper side 

 of each curve. Thus the lines bisecting the half curve, the 

 axis-planes, usually dip at a very low angle, and towards 

 the region of maximum disturbance. 



Further, they recognised that a close series of plications 

 may appear as one of simply conformable deposits, es- 

 pecially in cases where slaty cleavage is set up. Failure to 

 recognise this principle has in subsequent years given rise 

 to many difficulties, and complicated theories of migration 

 have been invented to account for the repetition of zones 

 of the same fossil at different levels in the same group of 

 strata. 



Unlike Elie de Beaumont, they came to the conclusion 

 that crust waves were not straight only, but also curvilinear, 

 and that flexures grade away from the districts of maximum 

 disturbance and contortion, being highly plicated in those 



