322 THE DESERT SANDSTONE. 



alkaline particles which saturate the atmosphere at such times. 

 Dust-storms are common on the deserts during the arid season, 

 and impart to the atmosphere a peculiar haziness that lasts for 

 days and perhaps weeks after the storms have subsided. Whirl- 

 winds supply a characteristic feature in the atmospheric phenomena 

 of the Far West especially during calm weather, and frequently 

 form dust columns of two or three thousand feet, even more in 

 height, which may many times be seen moving here and there over 

 the valleys. The loose material thus swept about at the caprice 

 of the winds tends to accumulate on certain areas, and forms dunes 

 or drifts which at times cover many square miles of surface. 

 During its journey across the country the material which finds a 

 resting place in the dunes becomes assorted with reference to size 

 and weight, so that the resulting sand drifts are usually homogene- 

 ous in their composition, but are characterised by extreme 

 irregularity of structure when seen in section. In the Lahontan 

 basin the sub-aerial deposits are usually composed of fine sharp 

 quartz sand ; but in some instances small drifts are principally 

 formed of the cases of ostracoid crustaceans." 



Without following the author into all the details, some further 

 peculiarities of these eolian sands may be inserted here. A few 

 miles further north there is a belt of drifting sand about 40 miles 

 long by 10 wide. The drifts are fully 75 feet thick and the whole 

 vast field of sand is slowly travelling eastward. The sand is of a 

 light creamy-yellow colour and forms beautifully curved ridges 

 and waves. Another area is south of the Carson desert. This 

 train of sand dunes is 20 miles long and four or five wide. In a 

 sheltered recess of Alkali Valley the sand drifted by eddying wind 

 currents has formed a mountain 200 to 300 feet above the plain. 

 This great sand hill changes its outline from year to year while 

 the winds modify the rounded domes and gracefully curving crests 

 of creamy yelloAv sand. This trace is also slowly travelling east- 

 ward across mountains and deserts, unaffected by the topography 

 of the country. The sands find temporary resting places on the 

 terraces in the black basalt on the shores of Lake Lahontan, 

 " bringing out the horizontal lines in strong relief and accentuating 

 the minor sculpturing of the cliffs." (op. cit. p. 155.) 



I have given this quotation rather fully, to show how the 

 volcanic character of these sands does not prevent them from being, 

 in particular cases, eolian. In my former paper on the Hawkesbury 

 Sandstone I laid stress upon this mode of formation, but as a rule 

 the ash sands do not always remain loose and drifting. Probably 

 their consolidation depended upon the amount of water that was 

 discharged from the volcano which sent them forth. 



The colour of the volcanic sands is another thing to which 

 attention may be drawn. The creamy yellow sands around Sydney 



