Human beings have existed on earth for over a million years. It is only under extremely limited natural conditions that man and most land animals can survive on earth. Oxygen, which is essential to the existence of man and other forms of life, accounts for about 21% of the total volume of the present atmosphere. If the amount of oxygen were to fall below 15%, it would become difficult for human beings to maintain life on earth. On the other hand, some scientists have pointed out that if the amount of oxygen were to exceed 30%, great fires would occur on earth. It is only under this delicate balance in nature that the human race can exist. The fact that this balance is not permanent, but has changed continuously throughout geohistory, has been a consistent and repeated theme of this book.
KeywordsClay Dust Petroleum Sandstone Explosive
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