380 THE POPULAR SCIENCE MONTHLY. 



such a work as Claus's speculation upon the origin of the Crustacea, or 

 by the critical study of the problem of the vertebrate skull, or by the 

 study of the literature upon the affinity between the vertebrates and 

 the annelids, is a better training in that logic of probabilities which 

 is the basis of all conduct than can be found in the study of the other 

 sciences ; and from this point of view it is plain that good may result 

 from honest but erroneous attempts at morphological speculation, for 

 the logical restrictions of sound reasoning are often studied to the best 

 advantage in the errors of acute thinkers. 



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PLAYAS AND PLAYA-LAKES. 



Br ISRAEL C. EDSSELL. 



OF the many characteristic features of the arid region of the far 

 West known as the Great Basin, none attract the attention of 

 the traveler more forcibly than the desert mud-plains that have been 

 left by the evaporation of former lakes. 



These areas are known locally as mud-flats, salt-flats, salt-marshes, 

 borax-flats, alkali-flats, deserts, sinks, etc., the name usually indicating 

 some peculiar feature of the valley to which it is applied. As these 

 desert regions have an almost identical history, the Spanish word playa 

 meaning shore or strand has been adopted by geologists as a generic 

 term under which the various desiccated lake-basins may be grouped. 

 Valleys more absolutely desolate than the playas of the Great Basin 

 can not be found, even in the midst of Sahara. They occur as mud- 

 plains, occupying the lowest portion of arid valleys, and form a hori- 

 zontal, even floor that is soft and perhaps covered by a shallow lake 

 during the winter, but in the summer and fall becomes hard and dry, 

 and crossed by innumerable shrinkage-cracks that give the whole 

 broad surface the appearance of a tessellated pavement of cream- 

 colored marble. At other times, after the water has evaporated, the 

 salts contained in the mud of the playa are brought to the surface in 

 solution by the action of capillary attraction, and a saline incrustation 

 is formed on the surface of the desert when the water that held the 

 salts in solution evaporates. In such instances the playa appears as 

 white and dazzling as if covered with drifting snow. A journey 

 across such a plain during the heat of summer, when the mirage renders 

 even the most familiar land-marks uncertain, becomes the most weary 

 and trying that the explorer is called upon to make. 



Examples of playas of broad extent are furnished by the desert 

 region that borders Great Salt Lake on the west, which was left as a 

 vast mud-plain by the evaporation of Lake Bonneville. This is an 

 absolute desert, more than a hundred miles long by thirty or forty 



