Notices of Memoirs — P^^of. J. W. Gregory — Deserts. 419 



II. — Papers read in Section C {Geology), Meeting of British Association, 

 Australia, August, 191If. 



(1) Desekts (Definition, Causes, and Soils). By Professor 

 J. W. Gregory, F.R.S. 



rpHE term 'desert', according to modern usage, means a country 

 _L -which is unoccupied in consequence of having an arid climate. 

 Various exact limits of the conditions that established deserts have 

 been proposed. Thus Sir John Murray adopted the desert limit as 

 a 10 in. rainfall, and William Macdouald {Dry Farming, p. 91, 

 1911) that of 20 inches as the limit of the arid region. Goodchild, 

 on the other hand, rejected numerical limits as arbitrary and 

 impracticable, and proposed, a more elastic definition based upon 

 a general deficiency of moisture. Walther has pointed out that 

 deserts cannot be precisely defined on biological, morphological, or 

 climatic grounds. 



The factors that control the development of desert are not only 

 climatic. The geological structure of the district has an important 

 influence. The presence of rocks which are very permeable owing to 

 porosity of jointing, and which crumble into coarse grains, contributes 

 to desert conditions. Geographical situation is also important, for 

 a plateau which has a free drainage to adjacent lowlands is more 

 easily converted into desert than an area with no easy escape for its 

 subterranean water. 



The climatic influence depends not only upon the total amount of 

 rainfall but upon its distribution through the year. Thus a country 

 where the rainfall is in the late summer and autumn may be 

 economically desert, whereas the same rainfall at a more suitable 

 season would render the country fertile. The Transvaal, for example, 

 is hampered by its only certain rains falling between November and 

 March. Temperature and the complex group of factors which 

 determine the rate of evaporation also have an important influence. 



Hence the development of desert is not determined by any rigid 

 minimum of rainfall, but by the balance between the many conditions 

 which govern the utilization of the rain. 



Deserts are most easily produced and least curable where the rain- 

 fall is low, the temperature is high, the wind is strong, and the 

 country consists of a plateau where there is an easy drainage to the 

 adjacent lowlands. On a high plateau the elevation, by lowering 

 the temperature and cooling the air uplifted on to it, tends to secure 

 rain, though if it easily drains to the lowlands the main value of the 

 rain may be on them. 



Proximity to the sea is quite consistent with a desert development, 

 for if the sea-water be colder than the land, the capacity of the air 

 for moisture is increased as it blows ashore and a sea-wind may 

 therefore act as a drying and not as a moisture-carrying agent. 



Desert and non-desert conditions have frequently alternated in the 

 same district. Thus in the British area there is evidence that desert 

 conditions prevailed during the times of the Torridon Sandstone, the 

 Old lied Sandstone, the Permian, the Trias, and there is evidence of 

 an arid climate in Ireland during part of the Lower Kainozoic. It has 



