A HISTORY OF CUMBERLAND 
manner, the same envelope of aqueous vapour which acts as a sunshade 
during the day, acts equally as a blanket by night, by checking the radi- 
ation from the earth. Therefore, where there is but little aqueous 
vapour present in the air, the rocks exposed to the sun very soon become 
hot during the day, and cool down with equal rapidity during the night. 
Hence the difference between the daily maximum temperature and the 
nightly minimum is much greater in an arid than in a humid district. 
As a consequence of a great diurnal range of temperature, rocks expand 
greatly during the heat of the day, and contract to an equal extent during 
the cold of the night. Hence they crack under these conditions and fly 
to splinters in a manner and to an extent both very different from what 
we are accustomed to meet with in more humid climates. Moreover, 
in an arid region wind comes into action to a much greater extent than 
water, from which cause the character of the rock fragments is different 
in many important respects from those found in sedimentary rocks of the 
ordinary type. 
(4) The history of the events that ensued is much as follows : 
First there set in continental conditions, with the land rising locally at a 
faster rate than it was wasted by denudation. In the earlier part of the 
period there was a moderate rainfall, so that river-courses were shaped 
on the flanks of the uplands much as they are shaped here now. With a 
further uprise of the land, and a consequent recession of the sea margin 
to a still greater distance, the average quantity of aqueous vapour in the 
atmosphere steadily decreased, and the rainfall therefore became very ir- 
regular. The total quantity per annum must have been small in amount, 
and perhaps rarely exceeding ten inches, and most even of that was pre- 
cipitated in connection with the torrential rain accompanying thunder- 
storms. The waste of the rocks took place chiefly through the strains 
set up by the rapid expansions and contractions arising from the great 
diurnal range of temperature. In other words, the rocks were splintered, 
shattered, and broken up by mechanical causes instead of being, as they 
are here now, mainly decomposed by chemical means. As the volume of 
the streams diminished with the rainfall, so that their channels remained 
dry except during heavy rain, there was little or none of that rounding 
which characterizes river gravel in general. Each rock fragment, as it 
split up into smaller and smaller pieces, remained angular from first 
almost to last, and most of the material detached from the rocks at the 
sides of the valleys remained as screes until a violent flood shifted them 
in great masses to the low ground. As the fragments split up the finer 
chips were driven by the wind against each other, or used in the natural 
sand blasts by which rock erosion was partly accomplished, until the 
chips were reduced to the finest dust. In this state they served to load 
the atmosphere with fine particles, which occasionally gathered into 
clouds and were then blown far and wide as desert dust. ‘The more 
durable rock materials, chiefly quartz, which long resisted reduction in 
size, were blown to and fro by the wind so that the grains became worn 
by attrition against each other and eventually assumed that rounded form 
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