The question of time, expressed in years, has been a puzzle which attracted some attention even in the earliest days of investigation. Its solution was attempted by several methods long before there was sufficient information to make them work satisfactorily, which accounts in part for the extreme variation in results of the calculations. Even now it is to be expected that changes will have to be made as long as pertinent studies are continued. Two of the most promising methods of investigation in late years have been producing figures which are surprisingly large. More accuracy than ever before is probably present in modern estimates but, except for comparatively recent time, there is yet no way of knowing within a range of millions of years when a creature lived. Astronomy and physics were used in early calculations but, although taken seriously by some geologists, it was soon recognized by others that certain events revealed by earth history could not be explained with so short a time allowance as these methods indicated. One of the first estimates provided a total of only twenty-five millions of years and included a great stretch of time during which the earth, according to prevailing theory, was more sun-like than rock-like, a time when planets were being born and the earth could not have been in its present physical condition, which is the chief concern of the geologist. Since those earlier conditions could not have supported life as we know it, our knowledge of cosmic history renders small service in the study of fossils. Among the methods suggested by astronomy and the laws of physics is one which is based on the probable rate at which the earth cooled from its molten condition to present temperature. It is believed now that the heat of the earth is not necessarily due to an original molten state and that a steady rate of cooling cannot be ascertained. Any figures based on such procedure, therefore, are discredited today. The amount of salt in the oceans, and the time required for its concentration there by natural processes, offers another way of attacking the problem. It is a well known fact that salt is being added to the seas at a fairly constant rate; sea water, then, must become saltier from year to year. The salt comes from rocks exposed on land surfaces and is transported by the rivers which drain these areas. By analyzing the river waters it is possible to estimate the amount of salt annually dumped into the oceans and, also by chemical analysis, it is a comparatively simple matter to figure the total amount now present in the oceans. Some recent calculations indicate that thirty-five million tons of salt are being added each year, and this figure divided into the total amount for all the years places the age of the oceans at three hundred sixty millions of years.