684 DYNAMICAL GEOLOGY. 



to a whole hemisphere, and therefore sufficient evidence as to the 

 nature of the whole globe. Volcanoes occur also through Java 

 and Sumatra ; in central Asia in the Thian-chan Mountains ; about 

 the Mediterranean and Eed Seas ; in western Asia, and southern, 

 central, and southwestern Europe; in Iceland, and the West Indies. 



The ejection of melted rock through fissures has taken place 

 over all the continents ; in Nova Scotia, Canada, New England, 

 New Jersey and the States south, the region of Lake Superior, the 

 Eocky Mountains, and western America; in Ireland, Scotland, 

 and various parts of Europe ; and so over much of the globe. 



These evidences combine to prove that the interior of the earth 

 is a source of heat. 



It is, however, still an open question whether the internal heat 

 is that of fusion ; or, if there is fusion, whether the whole interior 

 is fused, or whether there are only interior seas of liquid rock ; or, 

 if the interior is fused, what is the thickness of the crust. From 

 a survey of the facts, the most probable conclusion is that the 

 crust is not over 100 miles thick. Within the crust there may be 

 isolated seas of melted rock, feeding volcanoes. 



Professor Perrey, of Dijon, has inferred from his extended re- 

 searches that there is a periodicity in earthquakes dependent on 

 tides in the internal igneous material of the globe. (See beyond, 

 on Earthquakes.) 



Recent mathematical calculations have made the thickness of the crust to be 

 at least 800 miles. But the results of figures should not be allowed to suspend 

 or throw discredit on observations until it is absolutely certain that all the 

 data required for fhem are known and thoroughly understood in their various 

 bearings. 



2. EFFECTS OF HEAT. 



The effects of heat considered in this place are the following : — 

 1. Volcanoes and related phenomena ; 2. Non-volcanic igneous 

 ejections ; 3. Metamorphism and production of mineral veins. 



Heat is also one at least of the causes of the elevation of mountains, and 

 of earthquakes. These topics are considered in the following chapter. It is 

 an important agent also in most chemical changes ; and hence its effects belong 

 in part to Chemical Geology. 



1. VOLCANOES. 



The facts relating to volcanoes are here presented under the fol- 

 lowing heads : — (1) General nature of volcanoes, and their geogra- 

 phical distribution ; (2) Kinds of volcanic cones ; (3) Volcanic action ; 



