Page:Popular Science Monthly Volume 22.djvu/394

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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.



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 horizontal, 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