AND THE WEST INDIAN ERUPTIONS OF 1002. 



219 



that have ever occurred ; yet volcanic action has gone on for 

 long periods of time in many areas without causing any surface 

 derangements other than tlie building up of conical hills or the 

 rupture of very small areas. 



Areas of great volcanic activity in Paheozoic and even in 

 Tertiary times, although still contiguous to the sea, are, and 

 have been for prolonged periods, perfectly unvolcanic, and 

 insular volcanoes in more recent times have become extinct 

 without change of geographical conditions, as in Madeira and 

 the Canary Islands. 



Lavas from different volcanic regions though having a general 

 resemblance, are not the same in composition, and some present 

 considerable differences ; while the products of the same 

 ^'olcanic centre at different periods may be respectively 

 trachytic and augitic, as those of the Alban Hills ; or may be 

 characterized by different mineralogical features, as those of the 

 Sonnna-Vesuvian centre; and moreover trachytic and augitic 

 lavas may respectively be emitted by each of two vents in the 

 same volcanic region. 



Lava solidifies with a small loss of heat, lava flows of little 

 volume solidifying rapidly, and many lava flows are small. 



Eruptive energy may suddenly occur where no volcanic vent 

 previously existed, in some cases followed by a continuance of 

 acti\'ity, as at Jorullo, and sometinies after a brief outburst 

 followed by perfect (quiescence, as at Monte Nuovo. 



Two volcanic craters on the same dome may not be sympathetic 

 in activity, as Kilauea and Mauna-Loa, yet the eruptive axis 

 of a volcano may alter its position as in Vulcano and A^ilcanello, 

 and the volcanic foci of Etna and Vesuvius were calculated l)y 

 Mallet to be only a few^ miles deej). 



The chief Nortlicrn European Tertiary volcanic outpouring, 

 that of the lavas of Antrim, Zona, Staff'a, and Mull, was in the 

 same geological epoch as the great Central European subsidence, 

 and the great Central European outpouring, that of the lavas of 

 Auvergne, Central Germany, Bohemia, and Hungary, was 

 contemporaneous with the principal Central European Tertiary 

 elevation. The Andean Central American and West Indian 

 volcanic regions are rising, or geologically recently elevated, 

 areas; and, generally, volcanic action is on r/si//<7 rather than 

 subsiding areas. 



Active volcanoes, with few exceptions, are near the sea cr 

 water areas, and inland extinct volcanoes were similarly situated 

 at the period of their activity, as in Auvergne and Hungary, 

 while extinction of activity has followed the removal of the 



