GEOGRAPHICAL EVOLUTION. 597 



of uprise or relief, where the rocks were crumpled and pushed out of 

 the way. Paradoxical, therefore, as the statement may appear, it is 

 nevertheless strictly true that the solid land, considered with reference 

 to the earth's surface as a whole, is the consequence of subsidence 

 rather than of upheaval. 



Grasping, then, this conception of the real character of the move- 

 ments to which the earth owes its present surface configuration, we are 

 furnished with fresh light for exploring the ancient history and growth 

 of the solid land. The great continental ridges seem to lie nearly on 

 the site of the earliest lines of relief from the strain of contraction. 

 They were forced up between the subsiding oceanic basins at a very 

 early period of geological history. In each succeeding epoch of move- 

 ment they were naturally used over again, and received an additional 

 push upward. Hence we see the meaning of the evidence supplied by 

 the sedimentary rocks as to shallow seas and proximity of land. These 

 rocks could not have been otherwise produced. They were derived 

 from the waste of the land, and were deposited near the land. For it 

 must be borne in mind that every mass of land, as soon as it appeared 

 above water, was at once attacked by the ceaseless erosion of moving 

 water and atmospheric influences, and immediately began to furnish 

 materials for the construction of future lands, to be afterward raised 

 out of the sea. 



Each great period of contraction elevated anew the much-worn 

 land, and, at the same time, brought the consolidated marine sedi- 

 ments above water as parts of a new terrestrial surface. Again a long 

 interval would ensue, marked perhaps by a slow subsidence both of the 

 land and sea-bottom. Meanwhile the surface of the land was chan- 

 neled and lowered, and its detritus was spread over the sea-floor, until 

 another era of disturbance raised it once more with a portion of the 

 surrounding ocean-bed. These successive upward and downward 

 movements explain why the sedimentary formations do not occur as a 

 continuous series, but often lie each upon the upturned and worn edge 

 of its predecessors. 



Returning now to the chronological sequence indicated by the 

 organic remains preserved among the sedimentary rocks, we see how 

 it may be possible to determine the relative order of the successive 

 upheavals of a continent. If, for example, a group of rocks, which, as 

 before, may be called A, were found to have been upturned and covered 

 over by undisturbed beds C, the disturbance could be affirmed to have 

 occurred at some part of the epoch represented elsewhere by the miss- 

 ing series B. If, again, the group C were observed to have been sub- 

 sequently tilted, and to pass under gently-inclined or horizontal strata 

 E, a second period of disturbance would be proved to have occurred 

 between the time of C and E. 



I have referred to the unceasing destruction of its surface which 

 the land undergoes from the time when it emerges out of the sea. As 



