Page:Popular Science Monthly Volume 22.djvu/397

Rh The salts that form on the surfaces of playas are composed principally of the chloride, sulphate, and carbonate of soda, but sometimes contain borate of soda, sulphate of potash, sulphate of magnesia, and other salts in smaller proportion. In places the surface of a playa is sometimes formed of sulphate of soda several feet in thickness, as near the Buffalo Salt-Works on the Smoke Creek Desert, Nevada. Again, crystals of sulphate of lime (selenite), forming a bed more than six feet thick, cover hundreds of acres of the playa-surface, as on the eastern border of the Sevier Desert in Utah. Sometimes a playa for many square miles in extent is covered by a layer of salt a few inches in thickness, as was the case when Sevier Lake in Utah evaporated to dryness a few years since, and as is shown also by the large salt-field in Osobb Valley, Nevada. At other times the beds composing the playa contain brine beneath the surface, which yields large quantities of nearly pure salt upon evaporation; the supply of salt from this source in Nevada is practically without limit.

When by a change of climate a playa is no longer flooded, the subaërial gravels that are constantly moving down toward the bottom of a valley eventually overflow the entire surface of the playa, and the valley acquires a rounded instead of a horizontal floor. The same action tends to obliterate the beach-marks that a lake makes along its shores, so that in time all records that a lake has once occupied a valley become buried and erased: where once a broad, clear lake existed in which glacial covered peaks were reflected, there now stretches an arid desert, bearing only a scanty growth of artemisia. This, in brief, is the history of a large number of the valleys of the Great Basin.



HE primary and leading object of the experiments with animals, which have been conducted at Rothamsted during the past thirty-five years, was the solution of practical agricultural problems; but, as in the case of the field experiments already noticed, the practical lines of inquiry have naturally led to the investigation of a wide range of topics belonging to the science of biology, which, in themselves, are of more particular interest to the physiologist, or even to the student of sanitary or of social science, than to the farmer.

From the number of animals under experiment, and the well-planned and thorough methods of investigation, in all departments of the experimental work, the results obtained have been of great value,