RISE OF MOLTEN ROCK TO THE EARTH'S SURFACE 103 



Iceland we are able to observe the connection between the dikes 

 and the lava outflows. Professor Thoroddsen has stated that in 

 the great basaltic plateau of Iceland, lava has welled out quietly 

 from the whole length of fissures and often on both sides without 

 giving rise to the formation of cones. At three wider portions of 

 the great Eld cleft, lava welled out quietly without the formation 

 of cones, though here in the southern prolongation of the fissure, 

 where it was narrower, a row of low slag cones appeared. Where 

 the lava outwellings occurred, an area of 270 square miles was 

 flooded. 



The composition and the properties of lava. In our study of 

 igneous rocks (Chapter IV) it was learned that they are com- 

 posed for the most part of silicate minerals, and that in their 

 chemical composition they represent various proportions of silica, 



FIG. 97. Characteristic profiles of lava volcanoes. 1, basaltic lava mountain; 

 2, mountain of siliceous lava (after Judd). 



alumina, iron, magnesia, lime, potash, and soda. The more 

 abundant of these constituents is silica, which varies from 35 to 

 70 per cent of the whole. Whenever the content of silica is rela- 

 tively low, basic or basaltic lava, the cooled rock is dark in 

 color and relatively heavy. It melts at a relatively low tempera- 

 ture, and is in consequence relatively fluid at the temperatures 

 which lavas usually have on reaching the earth's surface. Further- 

 more, from being more fluid, the water which is nearly always 

 present in large quantity within the lava more readily makes its 

 escape upon reaching the surface. Eruptions of such lava are 

 for this reason without the violent aspects which belong to extru- 

 sions of more siliceous (more " acidic ") lavas. For the same 



