328 RELATION OF STEAM TO UPHEAVAL. 



five or six miles at Pompeii. It is necessary to assume the stone to 

 have been thrown about 7000 feet above the summit of Vesuvius, or 

 more than two miles above the sea, and the force to do this work 

 may be supposed to have been seated one, two, or three miles below. 

 American volcanoes have thrown bombs to far greater distances. 



That the steam or gas power, thus estimated for intensity, is also 

 of enormous volume and magnitude, appears from the continuity of 

 some eruptions, the amazing mass of rock which has been ejected, 

 the clouds of ashes, the rapidly formed hills, the land upheaved, and 

 the islands raised. Thus in forty-eight hours the volcanic forces 

 seated about the Lucrine lake raised by showers of ashes a hill called 

 Monte Nuovo, 440 feet high and 841 feet deep in the middle (1538). 

 Skaptar Jokul, or a great Icelandic volcano thus indicated, is reputed 

 to have thrown out three streams of lava, eight miles apart, which 

 covered an area of 1200 square miles; though it has been affirmed 

 that Icelandic records show that the mountain so named has never 

 been in eruption. 



The pressure of steam equal to raise felspathic lava five miles 

 high may be called in round numbers 2000 atmospheres, a pressure 

 attempted at the request of the British Association by Mr. Hopkins 

 and Sir W. Fairbairn. These experimentalists accumulated pressures 

 equal to that of the highest mountains nay, equal to thirty-three 

 miles of water. Such a pressure, unrelieved by volcanic vents, might 

 lift large tracts of solid land. 



Probably the western coast of South America, raised in 1822 for 

 i ooo miles in length, and in places for three or four feet, was uplifted 

 less by the explosive power of steam than by the crumpling of the 

 earth's crust, which takes place owing to lateral east-to-west contrac- 

 tion and vertical expansion of the rocks beneath the Andes, con- 

 sequent upon the formation of a vacuity by the discharge of volcanic 

 products ; and elevation in most cases must result in pressure, which 

 increases the heat beneath mountain chains, and thus generates steam. 



Hopkins' Theory of Elevation. Mr. William Hopkins, F.K.S., 

 gave a mathematical form to the experiences of miners and geologists, 

 which had recognised the existence in a limited area of two sets of 

 dislocations placed at right angles to one another, which often yield 

 metallic matters of different kinds. These fractures may depend on 

 one system of movement under that district. Suppose an expansive 

 force gradually augmenting under the whole of a limited tract, and 

 capable of bearing up the whole mass of the strata there. Let these 

 strata be capable of extension, so that they should swell up into an 

 arch, but let their extensibility be limited, so that at last the arch 

 must break. It will depend mainly on the outline or figure of the 

 ground raised what shall be the direction of the fractures. If the 

 area be indefinitely very long as compared to the breadth, and the 

 sides be parallel, there will, in the first place, be one or more frac- 

 tures parallel to the length of the figure across the lines of greatest 

 tension and, secondly, other fractures depending on them at right 

 angles to them. Thus, in the mining districts of Alston Moor, the 



he 



