THE EARTH AS A WHOLE 13 



tinct because it is the zone of fusion and of the formation of vol- 

 canic matter. Its existence is revealed by the manifestation of vol- 

 canic phenomena, and it passes insensibly on the one hand into the 

 Lithosphere and on the other into the Centrosphere, of which it 

 may indeed be a part. Its depth varies for different rocks, and it 

 cannot be regarded as constituting a continuous sphere, as do the 

 others so far discussed. Its consideration as a distinct sphere is 

 rather more for the sake of convenience of discussion. 



V. THE CENTROSPHERE. So far as actual observation is 

 concerned, the greater part of the geosphere is unknown to us. Be- 

 yond the relatively insignificant thickness represented by the known 

 part of the earth's crust or lithosphere open to observation, and the 

 inferred pyrosphere, there is the vast mass of the earth's interior, 

 forever withdrawn from direct observation and approachable only 

 in an indirect manner. This is the centrosphere which may be the 

 ultimate storehouse of the earth's internal heat. The following 

 diagram (Fig. 4), adapted from Crosby,* will serve to illustrate the 

 relation between the known and the unknown parts of the earth. 

 The diagram represents a sector of the earth, two degrees or about 

 140 miles broad. It is drawn to a radius of 75 inches, or a scale of 

 53^ miles to the inch. Assuming the crust to have a thickness of 75 

 miles, and the greatest depth of the atmosphere at 100 miles, these 

 would be represented by i^ and ij-s inches, respectively. The 

 extreme depth of the ocean is taken as 31,600 feet (o.ii inches), 

 the mean depth as 12,000 feet (0.043 inches), the mean height of 

 land as 2,300 feet (0.009 inches) and the greatest height of land 

 as 29,000 feet (0.03 inches). The length of radius on this scale 

 being 75 inches, it follows that the two radii will meet at that 

 distance from the surface of the water line, i. e., the distance 

 to the center of the earth on this scale is 634 feet from the line 

 representing sea-level. This shows well the relative insignificance 

 of the surface features as compared with the size of the earth as a 

 whole. 



Temperature of the Earth's Interior. (Giinther-io, 'w^28.') 

 From observations in deep mines, artesian wells, etc., it appears 

 that there is an increase in temperature downward, this being about 

 1° Fahrenheit for every 53 feet vertical descent, or, in round num- 

 bers, 100° per mile. (2.5° to 3° C. per 100 meters, or about one 

 degree for every 40 meters.) Considerable variation is, however, 

 shown in dift'erent mines or wells. Thus the Sperenberg bore hole 

 in North Germany (south of Berlin), which went to a dejith of 



* Collections of Dynamic and Structural Geology in the Museum of the Bos- 

 ton Society of Natural History. 



