X 



258 E. R. L. Schwarz — Hot Sjjriugs. 



Gala*pag08 Islands for instance, in whicli the principal craters lie on 

 T^oints where two sets of fissures cross each other.^ 



The idea that lavas are remelted portions of the crust is an old 

 one,' but seems to have been abandoned for the assumption that 

 volcanoes bring up to the surface material that has never been 

 there before, or at any rate since the crust became solid. A re- 

 discussion of the whole of the phenomena of volcanoes on the 

 principles of isostacy seems urgently called for, if only to settle 

 the following questions: — (1) Why do not volcanoes bring up the 

 heavy metals from the interior ? (2) Why is the temperature 

 increase measured in stable areas like the Witwatersrand, which 

 would seem to indicate the normal increase, enormously exceeded 

 in some areas, if these are not heated up by differential movements 

 of the crust? (3) Why cannot a force that is sufficient to crumble 

 up a resistant, infusible rock like quartzite, melt one which is fusible 

 and produce a volcano? 



The bearing of these speculations on the question of hot springs 

 is to endeavour to show that there is some reason for the explanation 

 that I have been giving for the origin of their heat ; for if it be 

 found that volcanoes do not get their material from the primordial 

 magma, then the question of original water will be ruled out. 

 What water the lavas do contain will on this hypothesis be simply 

 that which was once held up in its interstices when solid, with the 

 addition of any that the breaking of the rocks may bring them into 

 communication with. 



In this connection it is interesting to notice what very large 

 underground conduits must exist which dischai'ge their waters in 

 the bottom of the sea. We have very few large springs in the 

 Colony, the largest being that at Uitenhage ; there are, however, 

 large tracts of country similarly situated in respect to their geology, 

 superficial area, and rainfall, which do not contain anything like 

 so large an output of spring-water. Boreholes also are continually 

 tapping large sources of water without lessening the flow of 

 neighbouring springs, and it seems certain, though difficult of perfect 

 demonstration, that a large part of the water that sinks underground 

 does find an outlet in the bottom of the sea. On the coast we have 

 several fountains that emerge below high-water mark ; for instance, 

 all along the sandy coast east of Cape Agulhas. An inverse case 

 occurs also at Eastbourne in England, where an increased pumping 

 from wells situated a mile from the sea brought the salt-water 

 soaking through the greensand.^ In the Colony, however, we do 

 not have to deal with porous rocks ; all underground seepage, 

 outside the infinitesimally slow one through the substance of the 

 rock, takes place through fissures. Conceive now a system of 



^ See C. Darwin, " Observations on Volcanic Islands," in "Geological Observa- 

 tions," 1851, p. 116 ; and also " More Letters of C. Darwin," 1903, vol. ii, p. 143. 



2 C. E. Dutton: " High Plateaus of Utah," 1880, p. 125. 



^ It would be interesting to know in this connection whether increased pumping 

 from the boreholes that were put down in granite near the sea along the Swedish 

 coast would bring the salt water through the crevices. See C. E. Markham : Geogr. 

 Journ., vol. x (1897), p. 465. 



