FISSURE ERUPTIONS Sb/ 



The most gigantic outpouring of lavas from fissures occurred in 

 late Tertiary or early Quaternary time in western North America. 

 There lava floods formed the great plains of the Snake River region 

 in southern Idaho, and the vast basaltic plateau of Washington, 

 Oregon, and northern California. This lava field has more or less 

 interrupted extensions through Nevada, Arizona, New Mexico, 

 and the western half of j\Iexico south into Central America and 

 northward through I^ritish Columbia to the Alaskan Peninsula and 

 the Aleutian Islands. (See the Geological Map of North America.) 

 The main lines of fissures were probably parallel to the Pacific coast, 

 but of this nothing is visible, except the general trend of the lava 

 sheets from north to south. The area covered by the lava outpour- 

 ings aggregates 200,000 square miles, wdiile the thickness of the 

 sheet averages 2.000 feet and reaches in some places 3.700 feet. 

 The comparatively recent origin and the location of the lava plateau 

 have precluded much destructive work by the surface agents, al- 

 though the Snake River has cut a series of picturesque gorges 

 through it. The cones now rising from this surface indicate 

 localization of eruption subsequent to the outpouring of the lava 

 floods. Prismatic structure is well developed in parts of these 

 lava sheets. Intercalated river sediments often separate successive 

 flows. 



Remnants of early Tertiary basaltic lavas are now found in 

 numerous places in northeast Ireland, western Scotland, the lower 

 Hebrides, the Faroe islands, and faraway Iceland. These, famous 

 for their columnar partings (Giants' Causeway, Fingal's Cave, etc.), 

 were probably part of a once continuous lava field, now dismem- 

 bered by the agents of erosion, not the least of which is the sea. 

 Numerous dikes of similar material occur in regions from which 

 this lava has apparently been eroded, and these dikes probably 

 mark the fissures through which this welling-up of the lava took 

 place. 



These dikes are extremely abundant in the northwest of Scot- 

 land (Peach and Horne-28) and range eastward across Scotland 

 and the north of England and Ireland. They have been traced 

 from the Orkney Islands southward to Yorkshire and across Britain 

 from sea to sea over a total area of probably not less than 100,000 

 square miles. This may indicate the former wide extent of this 

 basaltic lava field which tlicn rivaled the younger one of western 

 America. When erosion has been carried far enough in the great 

 lava plateau of western North America, to remove a considerable 

 portion of the lava sheet, there will no doul)t appear an equally 



