THE GEOSPHERES 229 



tinents and islands and on the floor of the 

 ocean. We know the continental rocks by 

 borings and mines to the depth of several 

 thousand feet, but that portion (three-fourths 

 of the whole) on which the ocean rests is known 

 by actual observation only to the depth of a 

 few feet, since our sounding-tubes and dredges 

 rarely penetrate deeper than three feet into 

 the marine deposits. The rocks of the litho- 

 sphere are heterogeneous in structure and 

 composition so far as they are open to direct 

 observation : at some places there are great 

 extrusions of both acid and basic lavas, at 

 other places there are granites and horn- 

 blendic and other gneisses, as well as vast 

 strata of sandstones and limestones, and 

 in the preceding chapter it is shown that 

 marine deposits are heterogeneous. 



The deep-sea soundings recorded during the 

 last half-century now permit us to draw some 

 wide general conclusions concerning the topo- 

 graphy and composition of the external 

 surface of the rocky crust. The knowledge 

 we have acquired, relating to the depth of 

 the ocean below sea-level and the height of 

 the dry land above sea-level, shows that the 

 surface of the lithosphere, whixjh has been 

 calculated to have an area of about 197 

 millions of English square miles, may be 

 regarded as consisting of ; — 



