326 • H. C. Wilme7\ — The Relation of the Sand Dune [July 31, 



THE RELATION OF THE SAND DUNE FORMATION ON 

 THE SOUTH WEST COAST OF AFRICA TO THE 

 LOCAL WIND CURRENTS. 



By H. Carrington Wilmer. 



[Read 31st July, 1889.] 



The prevailing winds on this coast from the mouth of the Orange 

 River to Walfish Bay are as follows : — 



First, in the summer months, namely from November to April, from 

 early morning to midday North to North-west, from midday light 

 South-west, increasing in force till from 7 p.m. to midnight, then 

 dying away gradually and coming round to North before day -break. 

 North to North-west wind brings the fog-bank on shore and some- 

 times carries it far inland, South to South-east carries it out to sea. 



Secondly, winter months, May to October, mornings calm with light 

 North-west to North, fog on land, mid-day to midnight light South- 

 west dying away to calm. Occasional strong northerly wind for from 

 three to four days in succession and once at least in each month for a 

 period of from two to eight days, hot and very dry East wind 

 frequently blowing a full gale. This wind generally comes on about 

 2 a.m., increasing in force till 10 to 11 a.m., dying away to calm in the 

 afternoon ; it carries clouds of fine dust, consisting, for the most part, 

 of tiny grains of quartz and flakes of mica. 



From Angra Pequena to Walfish Bay the coast is lined by a belt 

 of sand hills, varying between five and fifteen miles in width and from 50 

 to 300 feet high. At the back of these hills is a desert strip from 

 thirty to eighty miles wide, devoid of vegetation and covered by 

 nodules of white quartz and pinkish white felspar. This gravel 

 results from the decomposition of the granite, gneiss and mica schist 

 which form the predominant rocks of the district. As the rocks 

 decompose, the softer constituents, that is to say the mica and the 

 smaller particles of quartz and felspar, are, owing to the want of rain 

 to consolidate them, carried away by the winds, leaving the larger 

 inodules covering the surface, which is consequently white and glaring. 

 These fragments present the appearance of being water-worn, but are 



