318 DYNAMICAL GEOLOGY.  [Boox III, | 
motion, it is of interest to note the ascertained velocity and pressure 
of wind as expressed in the subjoined table :— 
Velocity in Miles ._ Pressure in Pounds 
per hour. per square foot. 
Calm . : : : : ger) 
Light breeze : : : Pe 0 i 
Strong breeze A ‘ ; . 42 9 
Strong gale. : : 5 tO 25 
Hurricane . ; : - sue OX 36 
While the paramount importance of the atmosphere as the 
vehicle for the circulation of moisture over the globe, and con- 
sequently as powerfully influencing the distribution of climate and 
the growth of plants and animals, must be fully recognized by the 
geologist, he is specially called upon to consider the influence of the 
air in directly producing geological changes upon the surface of the 
land and in augmenting the geological work done by water. - 
§ 1. Geological work of the air on land. 
Viewed in a broad way the air is engaged in the twofold task of 
promoting the disintegration of superficial rocks and in removing 
and redistributing the finer detritus. These two operations however 
are so intimately bound up with each other that they cannot be 
adequately understood unless considered in their mutual relations. 
1. Destructive action.—Still dry air not subject to much 
range of temperature has probably little or no effect on minerals and 
rocks, The chemical action of the atmosphere takes place almost 
entirely through dissolved moisture. This subject is discussed in 
the section devoted to Rain. But sunlight produces remarkable 
changes on a few minerals. Some lose their colours (celestine, 
rose-quartz), others change it, as cerargyrite does from colourless to 
black, and realgar from red to orange-yellow. Some of these 
alterations may be explained by chemical modifications induced by 
such causes as the loss of organic matter and oxidation. Certain sub- 
aerial changes though not properly atmospheric may be most appro- 
priately considered here. 
Effects of lightning.—Hibbert has given an account of 
the disruption by lightning of a solid mass of rock 105 feet long, 10 
feet broad, and in some places more than 4 feet high, in Fetlar, one 
of the Shetland Islands, about the middle of last century. The 
dislodged mass was in an instant torn from its bed and broken into 
three large and several lesser fragments. “One of these, 28 feet long, 
17 feet broad, and 5 feet in thickness, was hurled across a high point 
of rock to a distance of 50 yards. Another broken mass, about 40 
feet long, was thrown still further, but in the same direction and 
quite into the sea, ‘There were also many lesser fragments scattered 
up and down.” ? 
The more usual effect of lightning, however, is to produce in 
* Hibbert’s Shetland Islands, p. 889, quoting from the MS. of Rev, George Low. 

