740 



DYNAMICAL GEOLOGY. 



2. Non-volcanic Igneous Eruptions. 



Non-volcanic igneous eruptions are those that take place through 

 fissures, in regions remote from volcanoes. The modes of eruption are 

 not essentially different from those of true volcanic regions. The 

 cooled rock occupying the fissure is called a dike, as explained and de- 

 scribed on pages 107, 112 ; and the dikes vary in width from a foot 

 or less to hundreds of feet. 



These eruptions have occurred on various parts of all the continents, 

 but especially along their mountainous border-regions. Examples in 

 the Lake Superior region are mentioned on page 185; in Eastern 

 America, on page 418; in Western, page 524. Along Snake River, 

 the southern fork of the Columbia, a single field covers 24,000 square 

 miles ; and in Oregon, on the upper Columbia, another has an area of 

 30,000 square miles, or, with the Mt. Hood region included, 40,000. 

 In India, the great basaltic area of the Deccan covers 200,000 

 square miles. Areas are very uumerous in western Great Britain, 

 especially in Cornwall, Wales, and portions of Scotland and Ireland. 

 Fingal's Cave and the Giant's Causeway are noted examples. 



The fissures for the ejections were formed by a fracturing of the 

 earth's crust, down to a region of liquid rock. They have thus the 

 same origin as volcanoes, — but with this difference : that the fissures 

 did not remain open vents for successive outflows. 



The columnar form which the rocks often assume — not unfrequent 

 in volcanic regions — is well illustrated in the accompanying sketch 

 (Fig. 1119) of a scene in New South Wales. 



Fig. 1119. 



Basaltic columns, coast of Illawarra, New South Wales. 



The rocks include nearly all the igneous rocks mentioned on pages 

 76-79, except the scoriaceous and glassy kinds ; and even the latter 

 occur at times in forms like pearlstone and tachylite. The heavy basic 



