102 EARTH FEATURES AND THEIR MEANING 



The larger portion of the northwestern British Isles would 

 appear to have been at one time similarly blanketed by nearly 

 horizontal beds of basaltic lava, which beds extended north- 

 westward across the sea through the Orkney and Faroe islands 

 to Iceland. Remnants of this vast plateau are to-day found in all 

 the island groups as well as in large areas of northeastern Ireland, 

 and fissure fillings of the same material occur throughout large 

 areas of the British Isles. In many cases these dikes represent 



once molten rock which may never 

 have communicated with the surface 

 at the time of the lava outpouring, yet 

 they well illustrate what we might ex- 

 pect to find if the basalt sheets of 

 Iceland or Ireland were to be removed. 

 The floods of basaltic lava which in 

 the northwestern United States have 

 yielded the barren plateau of the Cas- 

 cade Mountains (Fig. 95) would appear 

 to offer another example of fissure erup- 

 tion, though cones appear upon the 

 surface and perhaps indicate the position of lava outlets during the 

 later phases of the eruptive period. The barrenness and desola- 

 tion of these lava plains is suggested by Fig. 96. 



NEVADA 1 UTAH 



FIG. 95. Basaltic plateau of the 



northwestern United States due 



to fissure eruptions of lava. 



Fio. 96. Lava plains about the Snake River in Idaho. 



Though the greater effusions of lava have occurred in pre- 

 historic times, and the manner of extrusion has necessarily been 

 largely inferred from the immense volume of the exuded materials 

 and the existence of basaltic dikes in neighboring regions, yet in 



