798 KEPOKT — 1892. 



the continental plateau appeared above the level of the sea. Insular conditions 

 everywhere prevailed. But as ages rolled on -wider and wider tracts of the 

 plateau were exposed, and this notwithstanding many oscillations of level. So 

 that one may say there has heen upon the whole a general advance from insular to 

 continental conditions. In other words, the sea has continued to retreat from the 

 surface of the continental plateau. To account for this change we must suppose 

 that depression of the crust has been in excess within the oceanic area, and that 

 now and again positive elevation of the continental plateau has taken place, more 

 especially along its margins. That movements of elevation, positive or negative, 

 have again and again afl'ected our land areas can be demonstrated, and it seems 

 highly probable, therefore, that similar movements may have been experienced 

 within the oceanic trough. 



Two kinds of crustal movement, as we have seen, are recognised by geologists. 

 Sometimes the crust appears to rise, or, as the case may be, to sinlc over wide 

 regions, without much disturbance or tilting of strata, although these are now and 

 again more or less extensively fractured and displaced. It may conduce to clear- 

 ness if we speak of these movements as regional. The other kind of crustal 

 disturbance takes place more markedly in linear directions, and is always accom- 

 panied by abrupt folding and mashing together of strata, along with more or less 

 fracturing and displacement. The plateau of the Colorado has often been cited as 

 a good example of regional elevation, where we have a ynde area of approximately 

 horizontal strata apparently uplifted without much rock-disturbance, while the 

 Alps or any other chain of highly flexed and convoluted strata will serve as an 

 example of what we may term axial or linear uplifts. It must be understood that 

 both regional and axial movements result from the same cause — the adjustment 

 of the solid crust to the contracting nucleus — and that the term elevation, there- 

 fore, is only relative. Sometimes the sinking crust gets relief from the enormous 

 lateral pressure to which it is subjected by crumpling up along lines of weakness, 

 and then mountains of elevation are formed ; at other times, the pressure is relieved 

 by the formation of broader swellings, when wide areas become uplifted relatively 

 to surrounding regions. Geologists, however, are beginning to doubt whether up- 

 heaval of the latter kind can affect a broad continental area. Probably, in most 

 cases, the apparent elevation of continental regions is only negative. The land 

 appears to have risen because the floor of the oceanic basin has become depressed. 

 Even the smaller plateau-like elevations which occur within some continental 

 regions may in a similar way owe their dominance to the sinking of contiguous 

 regions. 



In the geographical development of our laud movements of elevation and 

 depression have played an important part. But we cannot ignore the work done 

 by other agents of change. If the orographical features of the land everywhere 

 attest the potency of plutonic agents, they no less forcibly assure us that the 

 inequalities of surface resulting from such movement are universally modified by 

 denudation and sedimentation. Elevated plains and mountains are gradually 

 demolished, and the hollows and depressions of the great continental plateau 

 become slowly fiUed with their detritus. Thus inland seas tend to vanish, inlets 

 and estuaries are silted up, and the land in places advances seaward. The energies 

 of the sea, again, come in to aid those of rain and rivers, so that under the 

 combined action of all the superficial agents of change the irregularities of coast- 

 lines become reduced, and, were no crustal movement to intervene, would 

 eventually disappear. The work accomplished by those agents upon a coast-line 

 is most conspicuous in regions where the surface of the continental plateau is 

 occupied by comparatively shallow seas. Here full play is given to sedimentation 

 and marine erosion, while the latter alone comes into prominence upon shores that 

 are washed by deeper waters. When the coast-lines advance to the edge of the 

 continental plateau, they naturally trend, as we have seen, for great distances in 

 some particular direction. Should they preserve that position, undisturbed by 

 crustal oscillation, for a prolonged period of time, they will eventually be cut back 

 by the sea. In this way a shelf or terrace will be formed, narrow in some places, 

 broader in others, according to the resistance ofiered by the varying character of 



