Stratigraphy

Age of the Earth.— Much interest has al ways attached to the determination of the length of geologic time. Physicists computing the length of time which it has taken the earth to cool at the present rate from the highly heated condition assumed under the nebular hy pothesis arrive at a 'figure of about 20,000,000 years. Geologists have approached the problem in other ways. One of these is the computa tion of the length of time it might take to ac cumulate the known thickness of sedimentary rocks. By careful studies it is possible to get an average figure for the rate of accumulation of sediments in the ocean at the present time. It is also possible to determine the average thickness of sedimentary rocks in many regions. Assuming the present rate of accumulation to have been constant throughout geologic time, the required time to accumulate the known thickness of rocks is about 100,000,000 years.

Still another method of attack is based on the salinity of the ocean. All rivers carry com mon salt to the ocean, and it is believed that all the salt of the sea has been so derived. The area, average depth, and percentage of salinity of the ocean being known, its volume can be computed and the total number of tons of con tained salt be determined. The composition and amount of water delivered to the sea each year by all the larger rivers of the world is so the total amount of salt added to the ocean each year is determinable. Dividing the total salt in the ocean by the amount added each year, gives 70,000,000 years as the time it has taken to reach the present degree of salinity. This computation is based on the two assump tions that little salt is ever removed from the ocean, and that rivers have always carried salt in about the present amount. These assump tions are not strictly true and necessary cor rections have been attempted. Such computa tions involve so much that is uncertain that they are to be accepted with the utmost caution. They establish, however, beyond much doubt that geologic time is enormously long. All geologists agree that it is to be measured in millions of years, rather than in hundreds of thousands.

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