34 The American Geologist. July, 1896 
covered with igneous and nietamorphic rocks. To ascertain 
the relative areas the United States and Evirope were selected 
as typical of the land surface. The United States was divided 
into three regions: (1), that east of the Mississippi river; 
(2), that between the Mississippi and Colorado ; and (3), that 
between Colorado and the Pacific. The first region was di- 
vided as to the relative amounts in each state and the results 
added. The central region was bulked as sedimentary rocks 
and the western region was called half sedimentary and half 
igneous and metamorphic. The results showed that 31.2 per 
cent, of the surface of the United States is covered with ig- 
neous and metamorphic rocks. 
In Europe each countrj^ was separately divided and the 
percentage of the respective sums taken. This proved to be 
19.8 per cent. An average of these results, by coincidence, is 
25.5 per cent, or, roughly speaking, three-fourths of the land 
surface of the earth is covered with sedimentary rocks, hav- 
ing an average thickness of one mile. 
Difficulty was encountered in ascertaining an average po- 
rosity. Sections were taken in various parts of this country, 
notably the 127,000 ft. generalized section of the Rocky 
mountains, a generalized section through New York, Pennsyl- 
vania and Ohio by various authorities and Fairchild's section 
at Rochester, N. Y. A mean and average rock would appear 
to be a fine-grained sandstone or limestone. 
The most accurate determination of the porosities of rocks 
has been made by Prof. Bauschinger of Munich. He found 
the average porosity of upw^ards of 300 specimens of sand- 
stones and limestones to be 20% of their volumes. Two per 
cent, may be taken as a low average for igneous and metamor- 
phic rocks. 
The most recent and careful computation of the respective 
areas of sea and land on the earth's surface is that by M. 
Thoulet in his '-Oceanographie." This he gives as 368,000,000 
kilg. for the sea and 112,000,000 kilg. for the land, or reduced 
to square miles, 112,084',860 and 51,826,200 respectively. 
Three-fourths of the land is 41,119,650 square miles, and one- 
fourth, 13.706,550 square miles. Taking 20% of the former 
and 2% of the latter and adding we get 8,498,061 cubic miles 
of water. 
