816 



DYNAMICAL GEOLOGY. 



stations than at the coast stations. This difference in gravity between 

 continental and oceanic portions of the globe means difference in den- 

 sity. 



As the plumb-line is made to deviate from a perpendicular by inequalities of mass in 

 the country around, — arising from ridges and depressions, and from differences in den- 

 sity in the material of the earth whether the surface be even or not, — so the mobile 

 oceanic waters are drawn aside, and, consequently, have a higher level on a mountain- 

 ous coast than they would have on the same coast if it were backed by a low and level 

 country. Archdeacon Pratt, in his work on the Figure of the Earth, thence derives the 

 conclusion that the position of the earth's oceans is evidence as to the greater density of 

 the rocky material underlying the oceanic areas, and, hence, that the waters have their 

 distribution largely determined by gravity underneath. 



The difference in density in the common rocks of the globe is due chiefly to the 

 amount of iron present. If, then, the oceanic crust abounds most in the heavier (it 

 may be basic) rocks, in which iron minerals carry up the specific gravity nearly to or 

 above 3*0, and the continental part most in the feldspathic (chiefly orthoclase) kinds 

 containing less iron, some of the difference of density is explained, and also the differ- 

 ence in fusibility (p. 743); and the same condition accords with the idea that the 

 oceanic areas continued in fusion for a time after the continental, or less fusible, areas 

 had become solid. It is consonant with this that feldspathic (oithoclase) lavas, trach- 

 ytes, etc., are more abundant in continental regions of igneous ejections than in oceanic. 



The globe, when its continental area had become in the main terra 

 firma, may hence have had other great areas unsolidified. In that case 

 the surface of the former would at first have been, owing to gravity, 

 nearly on the same level with that of the latter ; but as the latter 

 solidified they would have become depressed in surface through the 

 contraction attending the cooling (which amounts in basalt to about 

 eight per cent.) ; and thus oceanic depressions and continental plateaus 

 would have been initiated. Moreover, the depressions in this way be- 

 gun would have gathered in the chief part of the waters of the globe, 

 as they became condensed with the progress of the earth's cooling ; and 

 the heat taken up by the waters would have hastened the cooling of 

 the areas, and so aided in the deepening process. The continuation 

 of the contraction through ages since may account for the greater part 

 of the difference which now exists between the depressions and pla- 

 teaus in mean level. 



The difference between great areas in the constitution of the earth's 

 material not only localized areas of cooling, but, in consequence of 

 this, determined also differences of direction in the lateral thrust from 

 contraction, and fixed the positions of the areas of least movement 

 or virtual resistance. And it made those conditions permanent for all 

 time ; so that oscillations of level took place and mountains came forth, 

 in subsequent eras, parallel to the primal lines of the Archaean age. 



III. Mode of "Working and the Effects. 



The chief effects of the lateral pressure which contraction from 

 cooling has generated are, as already explained, in part, (1) Flexures 



