86 



ISLAND LIFE. 



[part I 



It is there that nature has always strewn the dust of continents 

 to be." 



The Movements of Continents. — As we find these stratified 

 rocks of different periods spread over almost the whole surface 

 of existing continents where not occupied by igneous or meta- 

 morphic rocks, it follows that at one period or another each 

 part of the continent has been under the sea, but at the same 

 time not far from the shore. Geologists now recognise two 

 kinds of movements by which the deposits so formed have been 

 elevated into dry land — in the one case the strata remain 

 almost level and undisturbed, in the other they are contorted 

 and crumpled, often to an enormous extent. The former often 

 prevails in plains and plateaus, while the latter is almost always 

 found in the great mountain ranges. We are thus led to picture 

 the land of the globe as a flexible area in a state of slow but 

 incessant change ; the changes consisting of low undulations 

 which creep over the surface so as to elevate and depress limited 

 portions in succession without perceptibly affecting their nearly 

 horizontal position, and also of intense lateral compression, 

 supposed to be produced by partial subsidence along certain 

 lines of weakness in the earth's crust, the effect of which is to 

 crumple the strata and force up certain areas in great Gontorted 

 masses, which, when carved out by subaerial denudation into 

 peaks and valleys, constitute our great mountain systems.^ In 

 this way every part of a continent may again and again have 

 sunk beneath the sea, and yet as a wbole may never have 

 ceased to exist as a continent or a vast continental archipelago. 



^ Professor Dana points out that the regions which, after long under- 

 going subsidence, and accumulating vast piles of sedimentary deposits, 

 have been elevated into mountain ranges, have thereby become stiff and 

 unyielding, and that the next depression and subsequent upheaval will be 

 situated on one or the other sides of it; and he shows that, in North 

 America, this is the case with all the mountains of the successive geological 

 formations. Thus, depressions and elevations of extreme slowness but 

 often of vast amount, have occurred successively in restricted adjacent 

 areas ; and the effect has been to bring each portion in succession beneath 

 the ocean but always bordered on one or both sides by the remainder of 

 the continent, from the denudation of which the deposits are formed which, 

 on the subsequent upheaval, become mountain ranges. {Manual of Geology^ 

 2nd Ed., p. 751.) 



