388 Transactions of the South African Philosophical Society. 



age, made a similar miscalculation when he postulated the existence 

 of an ice-cap, ten, or even twenty miles thick, at the South Pole, 

 whereas it is impossible for a column of ice more than 1,600 feet high 

 to support its own weight ; if more were piled on the top the bottom 

 would melt. 



Bernachi observed in icebergs that the maximum thickness, namely, 

 1,600 feet is not requisite to produce this melting, but that very much 

 less will do. His observation is so interesting that I will quote his 

 own words : — " On the 30th August, the temperature in the shade 

 was — 15° Fahrenheit, with a perfectly clear sky and a glaring 

 sunshine beating on the north walls of the icebergs. From the 

 south, upon which at that time of year the sun never shone, drops 

 of water were oozing from top to bottom."''' There was no 

 possibility of bringing in external sources of heat to produce the 

 melting, the source must have been internal, and here at once 

 we have an illustration of the general principle, that there are 

 causes which produce the crushing of materials other than that 

 due to the intensity of the lode. 



It may be objected that an illustration taken from ice has nothing 

 to do wath hard rocks, as the two are so different in nature, but it is 

 manifest that if the rock were sufficiently heated, the cohesion of the 

 particles would become similar to that of ice under the temperature 

 at the surface of our globe. The main source of heat is supposed to 

 be that left over from the cooling of earth, and it is a fact that the 

 lower we go down the hotter the rocks become. The great amount of 

 variation in the rates of increase, however, leads one to suspect that 

 the internal heat of the globe is not the only source which produces 

 this temperature. Thus, of the 57 observations given in Prof. 

 Lebour's list, the lowest increase is, for every degree Fahrenheit : — 



157*2 feet at Minas Geraes, Brazil, 



and the highest : — 



28'1 feet at Anzin, N. France, 



or in other words, to get a temperature of 1000° F., in the one case 

 one would have to go down 29 miles, and in the other case only five 

 miles, or about the depth of sea-level below Mount Everest. In the 

 experiments of Professors Eiicker and Eoberts-Austenf on the 

 melting point of dolerite, it was found that the rock became soft at a 

 temperature of 760° C. or 1,400° F., while complete fusion took place 

 at 920° C. or 1,688° F. The melting point of quartz or silica is not 



* " To the South Polar Regions," 1901, p. 218. 



t Carried out at tlie suggestion of Rev. 0. Fisher. See that author's " Physics of 

 the Earth's Crust," Appendix, 1891, pp. 18-28. 



