VOLCANOES. 747 



very wide limits. The Mesozoic eruptions of doleryte (" trap ") on the 

 Atlantic border of North America show by their extent (p. 417) that 

 the source of liquid rock was not less than a thousand miles in length 

 from northeast to southwest ; or else, that isolated areas existed at the 

 time corresponding to the principal areas of eruption, the Nova Scotia, 

 the Connecticut valley, and each of the more southern from New 

 Jersey to the northern border of South Carolina, — a view not proba- 

 ble considering the uniformity in constitution of the trap through- 

 out. 



They teach also that other forces have produced ejections to the sur- 

 face besides those that work in the throat or conduit of a volcano. 

 For the ascent of the liquid rock cannot depend on the slow ascension 

 of expauding gas or steam within it, the outflow from fissures being 

 usually as sudden as the Assuring. Two other agencies are appealed 

 to : The pressure (1) of vapor (steam) in the deep subterranean re- 

 gion of the lavas ; and (2) of the earth's crust upon the liquid mass 

 below. The first mentioned source of ejecting force, has its limit, as 

 explained by Bischof (in 1839), in the fact that "the elastic force of 

 steam cannot surpass a certain maximum which it reaches when its 

 density is equal to that of water." Using the formula of Mayer, he 

 deduced for this limit in pressure nearly 8,320 atmospheres, which 

 corresponds, he observes, to a temperature of 2,786° F. and, for the 

 height of the column of lava, of specific gravity, 3*0, which steam at 

 its maximum elasticity is capable of supporting, 88,747 feet, or less 

 than 17 miles. Experiments have not been carried far enough to 

 make such calculations even approximations; yet they show that there 

 is undoubtedly heat enough for the best that steam can do. But this 

 elastic force of steam is that of saturated steam, and it demands that a 

 constant and full supply of water be kept up — a condition not possible 

 in the depths of the earth, unless the sea enter freely, which is not at 

 all probable, and not indicated by any products along the walls of 

 dikes. It is not even certain that there is much if any moisture in 

 the plastic depths of the globe (p. 814). 



The second cause, pressure of the earth's crust, may have often been 

 sufficient, and the actual cause. The fact that the melted rock has been 

 pushed upward with great force, as if from such pressure, is sometimes 

 apparent in the displacements or increased dip given to beds adjoining 

 fissures, especially in cases of oblique fractures. Upliftings take place 

 from this cause in volcanic regions ; and the pressure of escaping va- 

 pors contribute to the effect, these vapors opening chambers into which 

 the melted rock is forced and becomes congealed. Such vapors may 

 have often aided also in fissure-ejections intersecting sedimentary 

 strata, since these always contain moisture and may include subterra- 



