VOLCANOES AND IGNEOUS INTRUSIONS 



3" 



other hand, the surfaces of some of the flows were exposed to the 

 action of the weather many years before the next outpouring 

 occurred, as is shown by the 



Fig. 304. — Lava fields in Washington, 

 Oregon, Idaho, and California. 



thick layers of soil between 



the lava flows. Previous to 



the extrusion of the lava the 



region was a deeply dissected 



one, but the lava filled the 



valleys, buried the lower hills, 



and surrounded some of the 



mountains, leaving them as 



islands in a molten sea. The 



border of the lava plateau is 



very irregular, since ridges 



and spurs extend into it 



from the higher land, and it 



in turn protrudes long fingers 



between the mountain masses. The edge of the sheet can best be 



compared to the shore line of a submerged coast (p. 226). 



Recent Icelandic Lava Sheets. — Much of the nature of such lava plains as those 

 described can be learned from a study of recent eruptions in Iceland, a region which 

 exhibits marks of igneous activity in greater variety and magnitude than any other 

 spot in the world. In 1783 lava welled out for several months from the great Laki 

 fissure. This fissure is 20 miles long, and on it were formed more than one hundred 

 low craters, from which sheets of lava were spread out on either side (Fig. 291, p. 298). 

 From the place of eruption the lava stream flowed 47 miles on one side and 28 miles on 

 the other, covering an area of 220 square miles to an average depth of 100 feet. The 

 longest flow on record in Iceland is 90 miles, the slope of which is so gentle as to be 

 almost imperceptible, the angle being only a little more than one half of a degree. In 

 some cases lava has welled up from fissures in Iceland without the formation of cones ; 

 the longest flow of this class is 19 miles. In other parts the lava has built up great 

 domes similar to those in Hawaii; one of these is 4600 feet high, with an elliptical 

 crater about three quarters of a mile across at its widest point. 



In 1913 a fissure three miles long was formed in Iceland from craters on which lava 

 poured forth and covered the plains. In some cases, the lava shot up in a jet like a 

 geyser ; in others, it flowed out like a fiery waterfall. 



Characteristics of Volcanic Cones 



Profiles of Volcanoes. — The slope of a volcanic cone, as has been 

 seen, depends upon the character of the material of which it is made. 

 If it is composed entirely of cinders and ash, the slope will be at the 



