THE MEDITERRANEAN NATURALIST 
35 
northern border of which he had recently visited. 
He pointed out that the arid regions of the world 
were distributed in two bands, north and south 
of the equator. They were all inland drainage 
areas, or areas where the streams had no con- 
nection with the sea. They were also regions 
where evaporation was in excess, for if the latter 
were in excess the water would rise till it could 
flow into the sea, as in the case of the great lake 
district of North America, and the area would no 
longer be one of inland drainage. The largest of 
the deserts, the Sahara, was about three and a 
half million square miles in area, and the area of 
all the deserts of the world to-gether was about 
11,500,000 square miles. That was to say, over 
one fifth of the land of the world had no outlet 
for drainage to the sea, and in all that area 
evaporation was greater than precipitation. 
This area corresponded very closely with the 
regions of the world where the rainfall was less 
than 10 inches annually. In no place in the world 
could there be got such enormous ranges of tem- 
perature as in the deserts. In the Sahara the tem- 
perature sometimes fell from 100 degrees during 
the day to the freezing point during the night. 
That arose from the great dryness of the atmo- 
sphere, and from the radiation that took place 
from the burning soil after the sun had set. These 
inland drainage areas corresponded very much in 
their barometric phenomena. In all desert regions 
during summer ail the winds blew in to them. 
In winter the reverse took place — the winds flowed 
out of them, and that held good both for the 
northern and the southern hemispheres. This 
let to the low rainfall, for the great majority of 
these regions were more or less bounded by high 
hills. The winds came into the deserts over these 
hills, and the vapour was precipitated from the 
atmosphere by the hills, with the results that 
when the winds reached the interior regions there 
w r as nothing left to be deposited. If there were 
not hills all round any desert area, as in the case 
of Northern Asia, the winds passed from a colder 
to a warmer climate, and as they got to warmer 
regions they were able to contain more vapour, 
and none was precipitated. Dr. Murray then pro- 
ceeded to give an account of his own views and 
impressions as to the Sahara. During the Chal- 
in the bed of the Atlantic for a long distance west 
of the African coast opposite the Sahara, and in 
the bed of the Indian Ocean to the south ot 
Australia, small grains of red quartz sand, and 
they had found scarcely a trace of such in the 
sea-bed in any other part of the world. He 
suspected this quartz sand had been blown out 
from the Sahara in the one case, and from the 
Australian desert in the other. On his journey 
southward through Algeria, he found the country 
as far as Tougourt converted into a garden by 
means of artesian wells. At Tougourt the real 
sandy part of the desert began, and he made 
excursions into it, with that town as his head- 
quarters. He exhibited to the meeting a specimen 
of the sand, of a light yellowish-brown colour, and 
exceedingly fine in the grains. There were, he 
said, a good many clay particles in it, and the 
quartz particles, which were also numerous, were 
identical with those they had got in the bottom of 
the Atlantic. There was no doubt that the winds 
from the desert carried the sand a long way out to 
sea. He had also examined the region geologically, 
and the formation of the rocks was entirely that 
of fresh water, and of quaternary date. The great 
majority of geographers and geologists Had. expres- 
sed the belief that th,e whole of the Sahara was an 
old sea-bed, but he was of opinion that it had 
never as a whole been covered by the sea since 
Cretaceous or Devonian times; and no part of it, 
he believed, had been, covered by the ocean since 
Tertiary times. The whole question about the 
discovery of shells seemed to rest upon one com- 
mon species being found very rarely in one region 
of the desert. He thought that, owing to recent 
researches, the opinion as to the Sahara, being an 
old sea bottom was very likely to disappear from 
our text-books. He considered that the features 
of the region had been produced by atmospheric 
conditions. The sand was the product o: f the 
disintegration of the rocks in situ. The existing 
rock was not far below the surface, and by digging 
down to it, the hard sandy particles were found 
embedded in the stone. The sun shone or the 
locks and they expanded. The sudden cooling at 
night broke them up, the wind carried away the 
smaller particles, and so continually were the 
rocks being disintegrated by means of changes 
other than water, although water perhaps had in 
