380 PLEASANT WA YS IN SCIENCE. 



less than eighty-three feet. Of course in inhabited places 

 the deposition of dust is checked, though not so much as 

 most persons imagine. There is not probably in this 

 country a single building five hundred years old, originally 

 built at a moderately low level, the position of whose founda- 

 tion does not attest the constant gathering of matter upon 

 the surface. The actual amount by which the lower levels 

 are raised and the higher levels diminished in the course of 

 a thousand years may be very much less, but that it must 

 amount to many feet can scarcely be questioned. 



And as in considering the action of rain falling over a 

 wide range of country, we have to distinguish between the 

 slow but steady action of ordinary rains and the occasional 

 violent action of great storms of rain, so in considering the 

 effects of drought following after rain which has well saturated 

 the land we have to distinguish between ordinarily dusty 

 times and occasions when in a very short time, owing to the 

 intensity of the heat and the violence of the wind large quanti- 

 ties of dust are spread over a wide area, Darwin thus describes 

 the effect of such exceptional drought, as experienced in the 

 years 1827-1832 in Buenos Ayres : "So little rain fell that 

 the vegetation, even to the thistles, failed ; the brooks were 

 dried up, and the whole country assumed the appearance of 

 a dusty high road. This was especially the case in the 

 northern part of the province of Buenos Ayres, and the 

 southern part of Santa Fe\" He describes the loss of life 

 caused by the want of water, and many remarkable circum- 

 stances of the drought which do not here specially concern 

 us. He then goes on to speak of the dust which gathered 

 over the open country. " Sir Woodbine Parish," he says, 

 " informed me of a very curious source of dispute. The 

 ground being so long dry, such quantities of dust were 

 blown about that in this open country the landmarks became 

 obliterated, and people could not tell the limits of their 

 estates." The dust thus scattered over the land, whether 

 left or removed, necessarily formed part of the solid material 

 brought from higher to lower levels, indirectly (in this case) 



