THE FLUCTUATIONS OF THE OTERO SODA LAKE. 41 



inile in scarcely 30 years, it seems probable that the dunes would have spread farther than 

 the present limits if equally favorable conditions had prevailed for any great length of time. 

 The part of the White Sands which is now advancing is not the main body of the dunes, 

 but merely a small, superficial portion. The main body is of the same type as the portions 

 now in motion, but the sands are partially fixed by scattered bushes and other small forms 

 of vegetation. Therefore they do not move. In some places it can be seen that in recent 

 times certain portions of them have been freed from the restraint of vegetation, and have 

 begun to move, adding their quota to the supply of sand derived from the gypsum crystals 

 of the floor of the playa. Clearly the older portion of the WTiite Sands, which is decidedly 

 the major portion, was at one time in motion. At that time the cUmate must have been 

 approximately as dry as now. Then came a time of changed conditions, when the moving 

 dunes were fixed. In certain cases, such as the shores of the Atlantic Ocean in JNIassa- 

 chusetts, or the south shore of Lake Michigan, dunes become fixed because they are driven 

 so far from the source of supply that new sand is not furnished in sufficient quantity to 

 prevent the growth of vegetation. In such cases the dunes close to the abundant and 

 constantly renewed supply of sand along the shore can not become covered with vegetation 

 simply because there is such a constant influx of sand, although vegetation would quickly 

 appear if the amount of sand brought in by the waves and wind should diminish. Conse- 

 quently we find the dunes more and more covered with vegetation as we proceed inland, 

 until a few miles back from the coast they are completely fixed. Among the White Sands, 

 on the contrary, the conditions are quite different. Here there is no gradation from fixed 

 to unfixed dunes. The two types exist side by side, both at the outer edge of the dune area 

 and at the inner edge close to the playa. Everywhere the new moving dunes are over- 

 riding the old stationary ones. The only explanation seems to be either that the supply 

 of gypsum has recently increased or that the amount of vegetation has decreased so that 

 the fixed dunes have in part become free. Either alternative demands a change of climate. 

 The supply of gypsum would be greatly increased by a diminution in the amount of water 

 flowing into the lake. If a considerable portion of the floor of the playa were covered 

 with water, as happened at the time of the formation of the 4-foot strand, the supply of 

 gypsum would be much less than now, for at present practically all of it comes from the 

 floor of the playa during the diy season. If the rainfall were great enough to change 

 the playa into a shallow lake, the amount of moisture among the dunes would probably 

 be so great as to cause the growth of vegetation, and thus the dunes would be fixed. In 

 other parts of the world such an increase in vegetation followed by a later decrease seemy 

 to be sufficient to cause the fixation and freeing of dunes without any change in the suppls 

 of sand. I have seen instances of this in several places, especially in the desert south of 

 Palestine near Beersheba, and on the borders of the great desert of Transcaspia. Whether 

 the twofold aspect of the White Sands is due chiefly to a change in the supply of gypsum or 

 to a variation in the amount of vegetation, its ultimate cause seems to be the same. The 

 older phase seems to indicate a period of aridity much like the present; the fixation of the 

 dunes apparently points to a greater supply of water and a higher stand of the lake; and the 

 free dunes of the present are in motion because the climate is dry, the lake has become a 

 playa, and the amount of vegetation is limited. Here, then, we seemingly have evidence 

 that the last series of climatic changes has not been a mere increase in aridity, broken by 

 a period of uniformity, but has been a pulsation from dry to moist and back again to dry. 

 Two deposits of gypsum older than those just discussed appear to be the remains of 

 earlier fields of dunes, which in their day were like the present White Sands. One of them, 

 called by Mr. Free the Intermediate Gypsum, still shows the characteristic topography of 

 dunes, together with occasional traces of cross-bedding. It covers about the same area 

 as the modern Sands, and can frequently be seen coming out from under them and extending 

 for half a mile or so. The sharper forms of the dunes have been smoothed off, and a con- 



