y2 PRINCIPLES OF STRATIGRAPHY 



ELECTRICAL PHENOMENA OF THE ATMOSPHERE. 



By thunderstorms, or other electrical discharges in the atmos- 

 phere, some of its oxygen is probably converted into the allotropic 

 modification ozone. Hydrogen dioxide and oxides of nitrogen are 

 further formed, the latter yielding, with the moisture of the air, 

 nitric and nitrous acids, which, when brought to the earth by rains, 

 become agents of corrosion of the rocks. Both ozone and hydrogen 

 dioxide are powerful oxidizing agents, and will transform organic 

 matter suspended in the air into carbon dioxide, water, and probably 

 ammonium nitrate. 



Lightning may further act as a disrupter of rock masses. In 

 Fetlar, one of the Shetland Islands, a solid mass of rock 105 feet 

 long, 10 feet broad, and in some places more than 4 feet high, was 

 in an instant torn from its bed by lightning and broken into three 

 large and several small 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 farther, but in the same direc- 

 tion, and quite into the sea. There were also many lesser fragments 

 scattered up and down." ( Hibbard-49 : j5'p. ) 



Fulgurites. When the electric spark strikes loose sand or rock, 

 a coating of fused vitreous material may be formed, or a mass of 

 vitreous drops or bubbles. More generally, slender tubes of fused 

 material mark the path of the electric spark, especially in loose 

 sand. These tubes, known as fulgurites * consist of fused grains, 

 and range up to 2^ inches in diameter. They descend vertically 

 or obliquely into the rock or sand, occasionally branch, and dimin- 

 ish in size to a point. The summit of Little Ararat, which is ex- 

 posed to frequent thunderstorms, is riddled with fulgurites, which 

 here occur in a porous andesite and consist of irregular tubes hav- 

 ing an average diameter of 3 centimeters, and being lined with a 

 blackish-green glass. Humboldt obtained these fulgurites from 

 trachyte peaks in Mexico, and found that in two cases at least the 

 fused mass overflowed from the tubes onto the surrounding sur- 

 face. Other cases of lightning-fused rocks have been reported in 

 hornblende schist of Mont Blanc (De Saussure) ; in mica schist 

 and limestones from the Pyrenees (RaViiond; Arago) ; in basalt of 

 Mount Thielson, Oregon, and Mount Shasta, California (Diller) ; 

 in glaucophane schist of Monte Viso (J. Eqcles) ; in gneiss of Lake 



* For extended bibliography including the ca^es cited, see the article by 

 Barrows (5) 1910. 



