460 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



The formation of the chamber containing the Prospect 

 rock would seem to have been due f o the hydrostatic (or 

 magmastatic) pressure produced in the magma by com- 

 pression of a deep-seated reservoir by earth movement. As 

 the shales did not fracture, and thus allowed the magma 

 no way through them, they were forced up. The pressure 

 of contained gases cannot, of course, become effective in 

 moving a magma or containing rock until the pressure has 

 fallen sufficiently for the gases to be released in steam- 

 holes, which often never happens. The escape of contained 

 gases played a very unimportant part at Prospect for 

 vesicles, empty or filled, are found but rarely, and conse- 

 quently pressure caused by deep-seated compression must 

 be regarded as having been the operative force. The action 

 might be well illustrated by filling a strong india-rubber 

 bag completely full of soda-water under pressure (to repre- 

 sent the contained gases), so that no gas-filled space 

 remained. Then, if the lower part were squeezed, the 

 upper part would perforce swell out wherever it were 

 weakest. The squeezing produces an increase of pressure, 

 which is thus at once relieved, and the contained gas plays 

 no part whatever in the expansion of the upper part of the 

 bag. 



On the other hand, should a fracture form in the overly- 

 ing strata a magma will rise through it to the surface, or 

 as high as the pressure will raise it. The fissure must be 

 about 60 cm. (two feet) wide at the least, however, or the 

 magma cools so rapidly that the passage is soon sealed up. 

 Any block of strata surrounded by fissures wide enough to 

 contain liquid magma may be floated up if the magma rises 

 high enough. At Prospect, if the strata were fractured 

 after being bent, as suggested, (fig. 3), it is unlikely that 

 the fracture would produce a fissure wide enough to allow 

 the magma to rise high enough in it all round to float the 



