20 SCIEXCE PROGRESS. 



of a belt girdling the Atlantic, analogous to the much better 

 defined " Circle of Fire " which surrounds the Pacific Ocean. 

 Judd, following Nordenskjold, long ago suggested that the 

 volcanic rocks seen at so many points from Jan Mayen to 

 Tristan d'Acunha may be regarded as portions of a ridge, 

 now for the most part submerged, bordering the eastern 

 continents as the volcanic belt of the Rocky Mountains, 

 Cordilleras, and Andes does the western. 



Taking now as an established principle the intimate 

 relation between igneous rocks and crust-movements, 

 whether of the mountain-building or of the continent- 

 building order, we pass on to another branch of our 

 subject, V12., the idea of historical sequence in the 

 various phenomena of igneous action. This may be 

 viewed either from the physical or from the petrological 

 aspect. The conception of a " life-history " of a volcano, 

 divided into successive stages of development and decline, 

 is not a new one, and it is capable of extension to districts, 

 provinces, and regions of igneous rocks. The question 

 whether such traces of a connected history are to be 

 detected in the oreoloo-ical record as a whole is a more 

 dubious one, and need not detain us at present. Those 

 who see in this record evidences of a general decline in 

 the energy of vulcanicity from Archaean times to the 

 present day have perhaps been influenced to some extent 

 by a prioid reasoning on the physics of a gradually cooling 

 globe. 



The earlier stages in the igneous history of a disturbed 

 region seem to have been marked usually by the intrusion 

 of considerable bodies of molten magmas, often by violent 

 superficial outbursts of a paroxysmal character, and generally 

 by the approximately simultaneous extrusion of great quant- 

 ities of lava at various centres over very extensive areas. 

 The closing event as regards intrusive rocks has been the 

 injection of fissures by dykes, which are often seen to 

 traverse the earlier plutonic and large intrusive masses ; 

 while surface outbursts have been restricted in their later 

 stages to diminishing and often isolated districts. The 

 modern volcanoes of the ^Mediterranean area and of 



