INTRODUCTORY OBSERVATIONS. 
XV 
portant science, to Mr. BakewelPs Introduction, 
as one of the most popular, lucid, and highly in- 
teresting elementary works on the subject ; and to 
Mr. Lyell’s Principles of Geology, as presenting 
the most able, comprehensive, and philosophical 
view of the science, that has hitherto apjieared. 
The superficies of our planet is computed to 
contain 190 millions of square miles, of which 
three fifths are covered by the ocean ; another 
large portion by polar ice, eternal snows, and vast 
bodies of fresh water : the space habitable by man 
and terrestrial animals being thus reduced to scarcely 
more than one fifth of the whole.* The strata of 
which the earth is composed, have been examined 
through a thickness nearly equal to eight miles, 
calculating from the summits of the highest moun- 
tains to the greatest natural or artificial depths : 
but as the earth is nearly 8000 miles in diameter, 
the entire series of strata that has been explored, 
is but as the paper which covers a globe 12 inches 
in diameter ; and the highest mountains, and the 
deepest valleys, may be compared to the inequalities 
and fissures in the composition with which the 
surface of such an instrument is coated. To render 
this more obvious, let us assume that the height of 
this page of letter-press is equal to the radius of 
the earth ; then this letter, I, will represent a thick- 
ness of 100 miles, and a mere line, — , the greatest 
elevations in the world. It is therefore clear that 
disturbances in the earth’s surface to ten times the 
depth to which the researches of man can extend. 
• Bakewell’s Litroduction to Geology, 3d edit., p. 6. 
