716 DYNAMICAL GEOLOGY. 



are small cones over the very region where the large part of the cone has sunk, the 

 fracture must have occurred before the volcano was extinct. 



Mount Somma is part of an outer wall to Vesuvius; and it is supposed, with good 

 reason, that the fracture of the mountain at an eruption reduced the mountain to its 

 present size. 



The Val del Bove is a gorge or valley, with precipitous sides, 1,000 to 3,000 feet high, 

 in the upper slopes of Mount Etna. Fresh-looking lavas cover the bottom; and dikes 

 intersect the sides. It has been regarded as the result of subsidence. It is probable, as 

 suggested by the author in his Report on Volcanoes, that at its head was once a crater, 

 like Kilauea or the summit-crater of Maui. The conditions within and about the great 

 depression accord with this view. 



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 ; and the dikes vary- 

 in width, from a fraction of a foot to many yards or even rods. Some 

 of the characteristics of non-volcanic igneous rocks and dikes are 

 mentioned on pages 107 and 112. 



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

 but especially along their mountainous border-regions. Examples in 

 New England, and along other portions of the Atlantic Border of 

 North America, have been mentioned (p. 418), and others in the Lake 

 Superior region (p. 185). But, over the larger part of the Mississippi 

 basin, they are wanting. They abound in many parts of Europe, the 

 larger part of which is the mountain-border region of the east side 

 of the North Atlantic. They are very numerous in western Great 

 Britain, especially in Cornwall, Wales, and portions of Scotland and 

 Ireland. Fingal's Cave and the Giants' Causeway are noted ex- 

 amples. 



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 

 were not so large as to remain open vents. 



The columnar form which the rocks often assume — not unfrequent 

 in volcanic regions — is well illustrated in the accompanying sketch 

 (Fig. 1116) of a scene in 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 the mineral tachylite. The heavy basic 

 rocks, doleryte and peridotyte, and the lighter feldspathic or acidic 

 kinds, trachyte and others allied, are the most common. They are 

 sometimes cellular, owing to inflations by steam or other vapors ; but 

 the cellules have generally a smooth or even surface within, and are 



