BY H. I. JENSEN. 515 



the continents have had their entity wholly restored from time 

 to time, there should be even less sedimentary rock left than we 

 see at present, especially if the continental segments have under- 

 gone uplift after uplift, pinched in between subsiding segments. 



(2) Chalk and many other formations formed in moderately 

 deep sea are found in the heart of many continents. Abyssmal 

 deposits if formed at the present slow rate would play such a 

 minor part in the sedimentary series that when metamorphosed 

 they might easily be overlooked. Such rocks as the Tarn worth 

 radiolarian tuffs and limestones may very well have been formed 

 under abyssmal conditions. 



(3) If only continental platforms and oceanic borders are 

 heavily sedimented and a creep of land-segments takes place 

 towards those zones (as suggested by Chamberlin and Salisbury 

 in their 'Geology,' Vol. ii.) it follows that the continental shelves 

 become overloaded and subside; rock-flowage of a compensating 

 nature would then take place in the zone of shear from beneath 

 them towards the continental area and restore the balance. The 

 old continental platform, being then further sedimented and 

 super! oaded, and squeezed in between higher segments, tends to 

 become folded, a process which would be aided by rise of isogeo- 

 therms. Consisting largely of sedimentary rock, it would 

 eventually become specifically lighter than adjacent segments 

 which would have had their gravity increased by rock-flowage 

 from beneath the subsiding area. By a gradual subsidence of 

 the heavy areas and an elevation of the lighter, the old conti- 

 nental shelf might become a lofty mountain range and the old 

 continent an oceanic area. A continent might by this process 

 gradually transplant itself from one part of the earth's crust to 

 another.* 



The volcanic sequence of alkaline areas tends, as will be seen 

 later, to lend support to such an hypothesis. Alkaline rocks are 

 continental and occur in areas of normal faulting above and pos- 



* Compare T. Mellard Reade, " Geomorphic Changes " in " Evolution of 

 Earth Structure." 



