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THE FLORA OF THE INDIAN DESERT. 

 (JODHPUR AND JAISALMER.) 



BY 



E. Blatter, S. J. and Peof. F. Hallberg. 

 Part VII. 



With 3 plates. 

 {Continued from -page 279 of Vol, XXVII.) 

 PART 11— (concluded). 

 Ecological Notes. 



Sand Formation. 



A considerable portion of Western Rajputana is covered by blown sand 

 chiefly consisting of well ground quartz grains, but also of flakes of hornblende 

 and felspar, as well as fragments of the local rocks. In addition, grains of carbo- 

 nate of lime have been found, some of which are casts of minute foraminifera, 

 proving that some of the sand, at least, has been carried by the wind from the 

 distant Umestone hills of Cutch. In fact, the whole sand mass is slowly moving 

 in the direction SW-NE, the prevailing winds of the district being the south- 

 west and north-east monsoons, of which the former is the strongest. 



The action of the wind on the sand results in the formation of dunes of various 

 shapes, depending on the local configuration of the country, on the variation of 

 strength and direction of the wind, and on the supply of material. Sometimes 

 they form with extreme rapidity, a railway track being covered in a few hours, 

 as often happens south of Phalodi. Sometimes they may be nearly stationary, 

 as in a hollow between two hills, or when protected from the wind by a ridge. 

 PI. XXXVI-A. shows such a stationary dune at Barmer on which traces of 

 water erosion may be distinguished, and which supports a shrubby vegetation. 

 The Lighter patches visible aU round the large dune are secondary recent dunes, 

 devoid of vegetation, as is the small sandy river-bed visible to the left. When 

 the leeward slopes of the protecting hill are steep, there is generally a deep 

 trench between the hill and the dune, due to eddy currents, and without vege- 

 tation on the sandy side on account of constant shifting. Where the hill-slope 

 is gentle, the protected dune may form close to the slope, as is shown on PI. 

 XXIV-A. The view is taken towards SW. The dune crest shows clearly a 

 case of reversible action of the wind, and the plant family of Gyperus arenarms 

 on the right, previously partly swamped by the reversed crest, is seen recover- 

 ing the lost ground. 



Where a dune is advancing its leeward slopes are invariably steep, and without 

 vegetation, if the motion is rapid enough. If the sand is wet, the grains carried 

 by the wind erode the surface in a curious manner, an instance of which is shown 

 on PI. I- A. Note the small cubical block of wet sand on the right. That the 

 blown sand is capable of eroding a far harder material as well, may be gathered 

 from PI. I-B. The photo shows one of several small exposed limestone crags 

 on a wind-swept ridge near Phalodi. The rock is whitish, hard, and fine-grained. 

 The SW surface is sloping, polished, and very finely furrowed longitudinally. 



