296 PHYSICAL GEOLOGY 



The principal gases given off during volcanic eruptions are sul- 

 phureted hydrogen (H 2 S), sulphur dioxide (S0 2 ), carbon dioxide 

 (CO2), carbon monoxide (CO), hydrochloric acid (HC1), hydrogen 

 (H), oxygen (O), nitrogen (N), argon (A), and water. It is stated that 

 the gases emitted from Vesuvius in 1906 contained so much ammonia 

 and hydrochloric acid that the glowing lavas were shrouded in a veil 

 of ammonium chloride (NH 4 C1) vapor, and that the " pine tree " 

 cloud of yellowish " smoke " which hangs over that volcano during 

 eruptions consists chiefly of ammonia compounds. The glare of the 

 red-hot lava in the crater is reflected from this cloud and gives the 

 appearance of a burning mountain. 



The composition of the vapors depends upon the state of activity 

 of the volcano ; chlorine is more abundant in the energetic phases, 

 while sulphurous and carbonic gases characterize the dying out of 

 activity. 



According to recent investigations, steam seems to be in smaller 

 quantities than formerly thought. This contention is supported by 

 the facts (1) that the amount of steam in craters decreases as the 

 center of the crater is approached ; (2) that the white cloud which 

 hangs over volcanoes during eruptions is a mixture of solids and 

 gases, and not steam as it appears ; (3) that volcanic ash is invariably 

 white and consequently has the appearance of steam when in sus- 

 pension in the air; (4) that the volcanic cloud never produces rain- 

 bows or aureoles. 



The great quantity of steam rising from some parasitic cones and 

 from some lavas, however, is enormous. For example, it has been 

 estimated that from one of the many parasitic cones of Etna suffi- 

 cient steam was ejected during one period of one hundred days to 

 form, if condensed, 462,000,000 gallons of water. The steam of 

 fumaroles is, however, apparently largely of surface origin, as is 

 shown by the increase in quantity after rains. ] 



Fragmental Majj^als. — All of the substances thrown into the air | 

 by volcanic explosions, which fall to the ground in a solid state, are 

 included under the term fragmental materials, and are classified as (1) 

 dust, (2) ash, (3) cinders, (4) bombs, ana (5) blocks of rock. These solid 

 ejections are either portions of the rock which has been broken into pieces 

 by the force of the explosions, or lava which was hurled into the air 

 in a liquid condition but which solidified before reaching the ground. 

 The size of the fragments varies from rocks weighing many tons to 

 five finest dust, which may remain in the air many months. The term 



