ANCIENT AMERICAN LAKE— BRADLEY 285 



body and, under the influence of the sun's warmth and the breathmg 

 of minute plants, combined with other elements to form tiny white 

 flecks of mineral — ^particles of lime carbonate. These settled to the 

 bottom as a gentle rain the year around, but most plentifully in the 

 early summer, and formed a light-colored granular deposit that sepa- 

 rated the dark organic layers from one another. Thus, because the 

 dark organic layers were formed at a certain time each year and 

 because the organic matter was then abundant enough to mask out 

 the light-colored particles, each dark layer told off the passage of a 

 year. 



The ancient lake continued to serve as an annual calendar in this 

 manner for thousands and thousands of years, interrupted only at 

 long intervals by a fall of volcanic ash or by a storm of extraordinary 

 vigor that stirred even the deep bottoms. The layered deposit was 

 ultimately changed into rock, but the thin velvety dark bands still 

 stand out sharply from the light-buff matrix. These layers are very 

 thin — 'about 150 of them to the inch. This means that it required 

 about 1,800 years for enough material to accumulate on the bottom 

 of the ancient lake to make a slab of rock 1 foot thick. As might be 

 expected, the varieties of rock consisting of the coarser mineral 

 particles were built up somewhat more rapidly than this, and the 

 finer-grained rocks, those consisting predominantly of organic sub- 

 stance, much more slowly. The measured rates of accumulation 

 range from 250 to 8,200 years to the foot. By applying the rate at 

 which each kind of sediment accumulated to the quantity of that 

 kind of sediment throughout the body of material deposited in Lake 

 Uinta it has been possible to estimate that the lake was in existence 

 approximately 7,500,000 years. In this long period evolution had 

 time to remold some of the more impressionable races of animals 

 living in the neighborhood. For instance, ancestors of the horse 

 family that lived at about the time Lake Uinta vanished were larger 

 and had notably better grinding teeth than their forebears that lived 

 just before the lake came into existence; moreover, in that interval 

 evolution also altered somewhat the design of their toes. 



In the record which the ancient lake kept year by year, we find the 

 suggestion that the lake's volume and temperature varied in sym- 

 pathy with the changing face of the sun — 'that is, with the number of 

 sunspots. Admittedly this correlation is no more than a suggestion, 

 yet there is a fairly soimd theoretical basis for believing it to be real. 

 Foregoing all effort to explain the steps, we may present the argu- 

 ment in its briefest form somewhat as follows: Sunspots are most 

 numerous at intervals of about every 11 years, and these cycles 

 signify changes in the amount of radiant energy that the sun emits. 

 It has been observed that the levels of lakes which lose most of their 

 water by evaporation and relatively little by overflow show a much 



