THE DEEP SEA AND ITS CONTENTS. 3^3 



South America. There is generally a very marked contrast in 

 elevation between the slightly submerged portions of this land- 

 platform and the deep sea-floors in its neighbourhood, the descent 

 from the former to the latter being very abrupt. 



Now these parts remarkably confirm the doctrine long since 

 propounded by the distinguished American geologist Professor 

 Dana, when reasoning out the probable succession of events 

 during the original consolidation of the earth's crust, and its sub- 

 sequent shrinkage upon the gradually contracting mass within — 

 that these elevated areas now forming the Continental platforms, 

 and the depressed areas that constitute the existing Ocean-floors 

 wej-e formed as such in the first instance, and have remained 

 unchanged in their general relations from that time to the 

 present, notwithstanding the vast disturbances that have been 

 produced in each by the progressive contraction of the earth's 

 crust. For this general contraction, coupled with the unequal 

 bearing of the different parts of the crust upon one another, has 

 been the chief agency in determining the evolution of the earth's 

 surface-features ; producing local upheavals and subsidences aUke 

 in the elevated and depressed areas ; so that lofty mountains and 

 deep troughs have been formed, with plications and contortions 

 of their component strata; metamorphism of various kinds has 

 been produced, and volcanic action, with earthquake phenomena 

 involving extensive dislocations of the crust, has been repeated 

 through successive geological periods, mostly along particular 

 lines or in special areas, without making any considerable altera- 

 tion in the position of the great Continent, or in the real borders 

 of the Oceanic areas, though the amount of the continental areas 

 that might be above water, and the position of their coast-lines, 

 might vary greatly from time to time. 



This idea of the general permanence of what we used to call 

 the great " ocean basins " had, in fact, struck me forcibly, as 

 soon as the soundings of the Challenger and Tuscarora, in the 

 Pacific, enabled me to work out the enormous disproportion 

 between the mass of land above the sea-level and the volume of the 

 waters beneath it. At the end of our first {Lightning) cruise in 

 1868, my colleague, Professor Wyville Thomson, had pointed out 

 to me that there is no adequate reason for supposing that the 



