130 NEOZOIC TIME. [CHAP. VIII. 



the tracts now covered by the Bunter were flat lands on to 

 which the torrents from the surrounding hills swept the 

 detritus which they carried, at first dropping the pebbles in 

 the mountain valleys, and only carrying sand on to the 

 plains. 



The final arrangement of this sand seems frequently to 

 have been accomplished by the wind (see p. 119), and we 

 may therefore suppose that during the greater part of the 

 year the plains formed bare and arid deserts, over which 

 hot winds whirled clouds of sand, and on which no living 

 creature could find sustenance. Such conditions would 

 account for the total absence of organic remains in the 

 Lower Triassic sandstones of Britain. When the period of 

 increased rainfall came on the torrents would carry the 

 pebbles to greater distances, and there may have been a 

 time when the streams were strong enough to sweep out all 

 the detritus which had accumulated in the upper valleys, 

 and to spread it over the lowlands in the shape of the 

 pebble beds. Depression and decreasing rainfall would 

 bring back the former conditions, and a partial recurrence 

 of sandy deposits. 



That the mountain torrents should only fill their chan- 

 nels in the winter or rainy season is not at all improbable, 

 for this is the case with many watercourses in eastern 

 Egypt and Abyssinia at the present day. In those coun- 

 tries immense quantities of debris are swept out of the 

 hill-valleys by the heavy rains which occasionally occur in 

 winter time, and are spread out over the plains and wider 

 valleys which occur at lower levels, and which in the dry 

 season form arid tracts of sandy and pebbly desert. It is 

 probably to such regions that we must look for a type of 

 the conditions which prevailed in Britain during the earlier 

 part of the Triassic period. 



When the depression which ushered in the conditions of 

 the Keuper epoch took place, an inland sea of considerable 



