SCIENTIFIC FARMING AT ROTHAMSTED. 383 



The salts that form on the surfaces of playas are composed prin- 

 cipally 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 thi3 

 source in Nevada is practically without limit. 



When by a change of climate a playa is no longer flooded, the 

 subaerial 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 val- 

 ley 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. 



-**- 



SCIENTIFIC FAKMING AT EOTHAMSTED. 



Br MANLY MILES, M. D. 



II. 



THE 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, 



