2 PRINCIPLES OF STRATIGRAPHY 



sea-level is about 15 pounds ( 14.7 pounds)* per square inch, or about 

 a ton per square foot, and this is spoken of as the normal at- 

 mospheric pressure or o}ie atmosphere. It becomes more tenuous 

 upward ; above 50 miles from the earth's surface it becomes so ex- 

 cessively thin that it is incapable of producing measurable pres- 

 sure, or of deflecting perceptible amounts of sunlight. (Davis- 

 6:7j.)t Above 100 miles it practically ceases to exist so far as ob- 

 servations have been made, though it is supposed that it might ex- 

 tend in an excessively attenuated state to the limit of the earth's 

 gravitative control, which is at about 620,000 miles from the sur- 

 face of the lithosphere. (Chamberlin and Salisbury-4, \:6.) 



II. THE HYDROSPHERE. This is a fairly continuous en- 

 velope, concentrated chiefly in the larger depressions of the litho- 

 sphere, where it constitutes the oceans, but also permeating the 

 upper part of the earth's crust as a more or less continuous layer. 

 The surficial extent of the sea water on the earth is estimated at 

 361. 1 million square kilometers, while the total land surface is esti- 

 mated at 148.8 million square kilometers. This makes a ratio of 

 land surface to sea surface of i : 2.43 or 29.2% land and 70.8% 

 sea (Krummel-17, i:8),X or in round numbers the sea surface is 

 2.5 times that of the land. When taken in hemispheres, the 

 northern one has 60.7% water and the southern 80.9% 

 (17, i:/j?). The greatest known depth, that of the Nero deep, 

 opposite the island of Guam in the Ladrone Islands east of the 

 Philippines, is 9,636 meters (5,269.13 fathoms or 31,615 feet). 

 The area of the ocean bottom below 4,000 meters comprises about 

 185 million square kilometers, that below 5,000 meters of depth 72 

 million square kilometers or about half the area of the dry land 

 ( i/5th of the sea surface); that below 6,000 meters about 5.4 

 million square kilometers or about the area of European Russia. 

 Among the greater deeps, which are more circumscribed, we have 

 that of the Marian depression opposite the Marian or Ladrone 

 Islands (in which the Nero deep occurs) with an area of 49,000 

 square kilometers (equal in area to Sardinia and Sicily) below 

 7,000 meters and about 22.500 square kilometers below 8,000 



* I -0333 kgm. per sq. cm., or the weight at sea-level of a column of mercury 

 760 mm. (29.921 inches) high. 



t The Arabic figure refers to the number of the article in the bibliography 

 at the end of the chapter, the italicized number refers to the page. 



X Penck in Scobel's Geographisches Handbuch makes the surface area of the 

 water 366 million square kilometers, and that of the land 144 million square 

 kilometers, including the Antarctic continent, which has an estimated area of 

 8 to 9 miUion square miles, giving in percentages 71.8% water and 28.2% 

 land, a ratio approximately of 2.5 : i. 



