THE EXTRUSIVE PROCESSES. 605 



body of the earth. ThevSe stresses are possibly an essential factor in 

 'eruptions. 



2. Relations to one another. — A most significant feature of volcanic 

 action is the degree of concurrence or of independence of action in ad^ 

 jacent volcanoes. In some instances they act as though in sympathy, 

 as in the recent outburst in Martinique and Saint Vincent^ and the con^ 

 current symptoms of activity in other places. On the other hand, the 

 independence of neighboring vents is sometimes extraordinary. The 

 group of volcanoes near the center of the Mediterranean, of which 

 Vesuvius and Etna are the most conspicuous examples, usually act 

 with measurable independence of one another, an eruption in the one 

 not being habitually coincident with an eruption in the others. But 

 the most conspicuous instance of independence is found in the great 

 craters of Mauna Loa and Kilauea in Hawaii. They are only about 

 twenty miles apart, the one on the top and the other on the side of the 

 same great mountain mass. The crater of Mauna Loa is about 10,000 

 feet higher than the crater of Kilauea, and yet, while the latter has been 

 in constant activity as far back as its history is known, the former is 

 periodic. The case is the more remarkable because of the greatness of the 

 ejections. The outflow of Mauna Loa in 1885 formed a stream from 

 three to ten miles in width, and forty-five miles in length, with a probable 

 average thickness of 100 feet, and some of its other outflows were of nearly 

 equal greatness; indeed its outflows are among the most massive that 

 have issued from volcanoes in recent times. Besides this massiveness 

 there have been extraordinary movements of the lava within the crater, 

 if the testimony of witnesses may be trusted. But throughout these 

 great movements in the higher crater, the lava-column of Kilauea, 10,000 

 feet lower, continued its quiet action without sensible effects from its 

 boisterous neighbor. The bearing of such extraordinary independence 

 upon the sources of volcanic action is very cogent, for the lavas are of 

 the same type, both being basalts, that of Mauna Loa being notably 

 basic and probably as high in specific gravity as that in Kilauea. No 

 difference in specific gravity that could at all account for a difference 

 in height of 10,000 feet can be presumed, unless their ducts remain 

 separate to extraordinary depths. Nor does it appear possible that a 

 superior amount of gas mthin the column of Mauna Loa could account 

 for such an extraordinary difference in height, for the hydrostatic pres- 

 sure of such a column is not far from 10,000 pounds to the square inch. 



