CRUSTAL BLOCK MOVEMENTS 249 



appear to reveal a relation both to the belts of earthquakes and to4he 

 zones of geosynclinals as these have been mapped by de Montessiis and 

 Hang respectively. We may thus add to the correspondencies already 

 mentioned that of an actual record of recent differential vertical move- 

 ment of crustal blocks within the earthquake zones, at least for the areas 

 of the coral seas. 



The elevations since the Tertiary, which are given in figures on the 

 map, indicate not uniform values throughout any considerable area, but 

 each island group or individual island shows an elevation which generally 

 differs widely and quite erratically from that of its nearer neighbors, thus 

 confirming the view of Mawson* that the groups are here separated by 

 so-called Graben depressions. 



The bearing of the facts above brought in review is to show that the 

 ocean basins of the present day have been formed largely as a result of 

 sinking of great orographic blocks bounded by submerged scarps which 

 are even today the seat of progressive movement. 



IsosTATic Adjustment 



As regards the coastlines of the continents, the resultant movement 

 which has been going on since the Tertiary is, as has been already said, 

 quite generally an upward one; but where large rivers are depositing 

 their burden of sediment, a local settling is quite generally now to be 

 noted. The way in which isostatic adjustment may locally modify the 

 general vertical movement of the crust which takes place in relatively 

 large masses McGee has well expressed in his study of the gulf of Mexico 

 as a measure of isostacy.f Speaking of the greater movements of the 

 crust, he said: 



"Now in contrasting these great oscillations with the gentle modern move- 

 ment of the crust, they are found to differ widely ; the modern subsidence Is 

 a gentle warping in such a direction as to deepen the basin and gradually 

 submerge its perimeter, while the old oscillations were widespread and in- 

 volved both sea bottom and continent. The modern movement is slight and 

 commensurate with the simple and uniform processes of erosion and sediment- 

 ation, while the old movements were cataclysmic and utterly transcended the 

 influence of rain and rivers." 



Great oceanic Eevolutions and geological Periods 



The unique importance which for later geological time the close of the 

 Tertiary acquires by reason of its having so profoundly changed the face 



* D. Mawson : The geology of the New Hebrides. Pi-oc. LinnKan Soc. New South 

 Wales, vol. 30, pt. iii, 1905, pp. 400-484, pis. 14-29. 

 t Bull. Geol. Soc. Am., vol. 3, 1892, pp. 501-503. 

 XXI — Bull. Geol. Soc. Am., Vol. 18, 1906 



