8/2 PRINCIPLES OF STRATIGRAPHY 



ing the former cinder cones are buHt up which are destroyed again, 

 in part or entirely during the violent periods, when crater formation 

 is the marked characteristic. It is during this period of activity 

 that the extravasative eruptions are in the ascendency, and at this 

 time also Assuring of the volcano takes place, with all the varied 

 activities which accompany such a state. 



Submarine Cones. Submarine cones of pure extravasation are 

 apparently illustrated by the Hawaiian Islands, though the early 

 history of many of these volcanoes is shrouded in obscurity. Sub- 

 marine cones of the composite type are well known, however. 

 Probably many of the volcanoes of the Mediterranean began as 

 submarine volcanoes and subsequently reached the surface. Vol- 

 canoes of this type are also known from the Aleutian island group 

 (Jaggar-2i), while volcanoes apparently rising from the abyssal 

 portions of the sea abound in the western Pacific. A singular ex- 

 ample of a volcanic peak projecting from mid-ocean is seen in the 

 little island of St. Paul, which rises from the Indian Ocean mid- 

 way between the southern end of Africa and the west of Australia 

 and more than 2,000 miles distant from Madagascar, the nearest 

 mass of dry land. This little island, scarcely 2)^ geographical 

 miles long and about i^ miles broad, is the mere summit of a 

 volcano. The crater has been breached by the waves and is now 

 occupied by the sea, the break in the rim being nearly dry at low 

 tide. (Figs. 236, 237.) 



Mud Volcanoes. Of an origin fundamentally the same as that 

 for lava volcanoes are the mud volcanoes found in various regions 

 of the world, but not associated with igneous eruptions. They 

 occur in Sicily, the Apennines, Caucasus, and on the peninsulas of 

 Kertch and Taman bordering the Black Sea, as well as in India. 

 They find their chief activity in the escape of various gases, which 

 play much the same part as does the escaping steam in igneous 

 volcanoes. Hydrocarbons, carbon dioxide, nitrogen, and naphtha 

 are some of the gases emitted. The mud volcanoes of Sicily have 

 been explained as due to the slow combustion of sulphur beneath 

 the surface. Whatever the causes, these volcanoes are manifested 

 on the surface in mounds or hillocks of mud. They generally 

 occur in groups and range in elevation up to several hundred feet, 

 while during periods of explosion they throw mud and stones up 

 into the air to much greater heights. They are built up by succes- 

 sive outpourings of mud, which harden and form a foundation for 

 later mud flows. "In the region of the Lower Indus, where they 

 are abundantly distributed over an area of 1,000 square miles, some 

 of them attain a height of 400 feet, with craters 30 yards across." 



