270 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



dimensions go) pockets which are quite variable in ^as content, 

 pressure, and conditions of chemical eciuilibrium. 



There were times even in late geolo<^i(' history when quite diti'erent 

 conditions prevailed, when masses of fluid magma were larger and 

 poured out through crustal rifts several miles in length without 

 signs of explosive activity or any restraint upon complete freedom 

 of crystallization and release of their volatile content. Such condi- 

 tions gave us the successive flows known as the Deccan ti'aps, the 

 Stormberg lavas of South Africa, and the great basaltic outflows of 

 the Snake River Basin and adjacent territory in this country, which 

 covered nearly 250,000 square miles. Latest of these and appro- 

 priately smallest in extent is the flow from the Laki rift in Iceland 

 in 1783. It may well be that such outpourings are past. It is a 

 rare volcanic outburst in modern times which yields as much as a 

 cubic mile of lava. Matavanu, in Samoa, is reported to have re- 

 leased this quantity in a continuous flow through the six years 

 ending in 1912. 



Through all of these studies our conclusion seems to stand fast 

 wherever it is applied, namely, that the outstanding factor in deter- 

 mining the character of modern volcanism is the gas content of the 

 crystallizing magma. If this be mainly of steam released in a closed 

 chamber, as a liassen Peak, then only steam explosions are to be 

 expected as the surface manifestation of the crystallization of the 

 magma below; if to the steam are added such chemically active gases 

 as chlorine, sulphur, hydrogen and the hydrocarbons, then chemical 

 reaction between these will be a sufficient cause of higher tempera- 

 tures and lava flows of the character well known at Vesuvius, Strom- 

 boli, or Kilauea. 



