702 DYNAMICAL GEOLOGY. 



volcanic mountains united at base, and Hawaii, of three mountains. In the 

 case of both Oahu and Maui, the northwestern of the two volcanoes became 

 extinct long before the southeastern, — as is apparent in the profound valleys of 

 denudation that intersect its slopes and almost obliterate its original features, 

 while the lavas and parasitic cones of the latter look fresh and recent. In 

 Hawaii, also, Mount Kea, the northern volcano, is the extinct one. Again, in 

 the whole Hawaian group the only active volcanoes are in the southeastern 

 island, Hawaii, while the northwestern island, Kauai, shows in its features that 

 its extinction was among the earliest, if not the very earliest, of the whole 

 number. It appears, therefore, that each, Oahu and Maui, stands over a fissure 

 which was largest towards the southeast, since the fires of the southeast extre- 

 mity of each were last extinguished ; that Hawaii had a similar origin, but 

 with probably a second more western fissure as the origin of the volcano of 

 Hualalai ; and that the whole Hawaian group originated in a series of fractures 

 which increased in extent from the northwest to the southeast; for Maui conti- 

 nued in eruption long after Oahu (a more western island in the group), and 

 Hawaii, the southeasternmost, is the only island now active, and the one that 

 through its prolonged activity has attained the greatest height above the sea. 



These facts illustrate a general principle with regard to the fractures of the 

 earth's crust, as well as the origin of volcanic groups. 



2. IGNEOUS ERUPTIONS NOT VOLCANIC. 



Non-volcanic igneous eruptions are those that take place through 

 fissures in regions remote from volcanoes. The cooled rock occu- 

 pying the fissure is called a dike. Some of the characteristics of 

 non-volcanic rocks and* dikes are mentioned on pages 117 and 122. 



These eruptions have occurred on various parts of all the con- 

 tinents, but especially along their mountainous or hilly border- 

 regions. Examples in New England and along other portions of the 

 Atlantic border of North America have been mentioned (p. 430), 

 and others in the Lake Superior region (p. 195). But over the larger 

 part of the Mississippi basin they are wanting. They abound in 

 many parts of Europe, and are very numerous in western Great 

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

 well as in Ireland. Fingal's Cave and the Giants' Causeway are 

 noted examples. They may be not less abundant in eastern Eng- 

 land, beneath the covering of Mesozoic and Cenozoic formations 

 which there prevail. 



The columnar form which the rocks often assume — not un- 

 known even in volcanic regions — is well illustrated in the accom- 

 panying sketch (fig. 975) of a scene in New South Wales. 



The dikes differ in width from a fraction of a foot to several 

 yards or even rods. 



The rocks include nearly all the igneous rocks mentioned on 



