HAS THE EXTENT OF LAND SURFACE BEEN INCREASING? 209 



known to be at least as much as ten times as great as the mean elevation 

 of the land, and perhaps somewhat more than that,* it becomes evident that 

 the depressions of the earth's crust are its really important features, so far 

 as relief of surface tvUh reference to sea-level is concerned, and that the por- 

 tions of the continental masses above that plane of reference are almost 

 insignificant in volume compared with the dimensions of space occupied by 

 water. 



From this fact we are enabled to draw conclusions of some importance 

 with reference to the question now before us. The great depth of the 

 ocean seems to indicate that the depressions in which its water is accumu- 

 lated have had, from the beginning of the earth's history, a character of 

 permanence. While the continental masses, over a large portion of their 

 areas, at least, are so low that comparatively slight oscillations of level 

 would raise them above and depress them beloAV the sea, we can hardly 

 conceive of the exertion of a force sufficiently intense to raise any consider- 

 able portion of the deep sea .so as to convert it into dry land. That the low 

 portions of the continents have been raised above and sunk below the water, 

 and that this operation has been more than once repeated in certain regions, 

 is a well-authenticated fact in geology. The areas thus affected have been 

 large as compared with the dimensions of tlie continents, but small in con- 

 trast with the magnitude of the ocean surface ; and, as before remarked, 

 the tendency has on the whole been to an increase of the continental masses, 

 although considerable portions of these are at the present slightly depressed 

 below the sea-level. t 



Dr. W. B. Carpenter was the first to call attention to the importance of 

 the argument in favor of the stability of existing continents derived from 

 the facts just stated in reference to the great mean depth of the ocean. He 

 thus concisely states the case : " The enormous depth of the Oceanic sea-bed, 

 as compared with the height of the Land above the sea-level, renders it very 

 unlikely that any subsidence of a Land-area should be compensated b)- such 



* The mean height of the laiid-niasses of the globe above the sea-level is usually taken, foUowiug Humboldt's 

 authority, at .about 1,000 feet. Recent investigations tend to make it somewhat more tliau this. The mean 

 de]ith of the ocean has been recently carefully computed by Knimmel, witli a resnlt of 11,280 feet. (See llorpho- 

 logie der Meeresraunie, Leijizig, 1878, and Kettler's Zeitschrift fiir wissenscliaftliche Geographie, Vol. 1. p. 40.) 

 A ratio of one to ten for the mean elevatiou of the continental masses as compared with the mean deptli of the 

 ocean is probably as fair an appro.ximation as cau be made at the present time. Dr. W. B. ('arpeuter, however, 

 puts this ratio at one to thirteen. He computes the volume of the ocean-water at thirty-six times that of the land 

 above the sea-level. 



t As, for instance, the region adjacent to Great Britain on the south and southeast. 



