CONTINUITY. 189 



[olivine, herzolite, and serpentine, for instance, closely resemble 

 them] ; that as we approach the surface rocks having similar 

 components with meteorites are found, but in a state of oxida- 

 tion, which necessarily much modifies their mineral character, 

 and which, by involving secondary oxygenised compounds, 

 must also change their chemical constitution. By experiments 

 he has succeeded in forming from terrestrial rocks substances 

 very much resembling meteorites. Thus close relationship, 

 though by no means identity of composition, is established 

 between this earth and those wanderers from remote regions, 

 some evidence, though at present incomplete, of a common 

 origin. 



Surprise has often been expressed that, while the mean 

 specific gravity of this globe is from five to six times that of 

 water, the mean specific gravity of its crust is barely half as 

 great. It has long seemed to me that there is no ground for 

 wonder here. The exterior of our planet is to a considerable 

 depth oxidated ; the interior is in all probability free from 

 oxygen, and whatever bodies exist there are in a reduced or 

 deoxidated state ; if so, the specific gravity of most of them must 

 necessarily be higher than that of their oxides or chlorides, &c. ; 

 we find, moreover, that many of the deep-seated minerals have 

 a higher specific gravity than the average of those on the sur- 

 face ; olivine, for instance, has a specific gravity of 3*3. There 

 is therefore no a priori improbability that the mean specific 

 gravity of the earth should notably exceed that of its surface ; 

 and if we go farther, and suppose the interior of the earth to be 

 formed of the same ingredients as the exterior, minus oxygen, 

 chlorine, bromine, &c., a specific gravity of 5 to 6 would not 

 be an unlikely one. Many of the elementary bodies entering 

 largely into the formation of the earth's crust are as light or 

 lighter than water for instance, potassium, sodium, &c. ; 

 others, such as sulphur, silicon, aluminium, have from two to 

 three times its specific gravity ; others, again, as iron, copper, 

 zinc, tin, seven to nine times ; while others, lead, gold, plati- 

 num, &c., are much more dense ; but, speaking generally, the 

 more dense are the least numerous. There seems no impro- 

 bability in a mixture of such substances producing a mean 

 specific gravity of from 5 to 6, although it by no means 



