PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 807 



rocks of the earth is very great, amounting for many limestones 

 and sandstones to from tive to thirty per cent of their volume. 

 So that, without speaking of that in the pores of the unknown 

 rocks beneath the sea, it may be said that those of our present 

 continents include in their pores a voUmie of water which is a 

 large proportion of the bulk of our present ocean. We may even 

 calculate that a time Avill come, when from the cooling of our 

 globe, and the resulting porosity of the mass, the whole of the 

 water, not already included in the sedimentary deposits, will bo 

 absorbed, to be followed by the atmosphere itself ; so that our 

 planet will one day be like its satellite, without either sea or air. 



Ocean Currents. 



The different heating power of the sun in the intra-tropical and 

 polar regions, combined with the rotation of the earth, gives rise 

 to great oceanic currents. Beside the east-west equatorial cur- 

 rents, we have chiefly to notice in the northern hemisphere the 

 warm north-east (!urrents, like the gulf stream, and the cold south- 

 west currents. In the southern hemisphere these are so far 

 reversed that the course of the warm currents is south-east, and 

 that of the cold northwest. Thus to the north of the E. W. equa- 

 tori.al current we have currents running N. E. and S. W.; and to 

 the south, currents running N. W. and S. E., subject, however, to 

 great local deflections from continents, etc. These various ocean 

 currents are the great distributors of the products of erosion, 

 whether igneous or glacial, submarine or subaerial, and have 

 determined the lines of sedimentation, and hence of mountain 

 chains. 



Origin of Mountains. 



A given geological period will be found in one part of the sea 

 represented b}^ 500 feet, and in another by 10,000 feet of sedi- 

 ments ; in the former by soft cla^'^s or limestones, and in the latter 

 chiefly by coarse sand and gravel, a subsidence of the yielding 

 crust of the earth in the latter region having permitted the accu- 

 mulation of this great mass of material. A subsequent elevation 

 of the whole area to the level which prevailed at the beginning of 

 the period would then — but for disturbing causes — present a 

 mountain ridge 10,000 feet high. In such cases, however, the 

 strata in the thickest portion of the formation are generally more 

 or less contorted and depressed. The cause of this is to be found 

 in the fact that along the four or five narrow lines of ffrcat north- 



