394 Transactions of the South African Philosophical Society. 



rate is one-teath that of glass, from Barus' experiments we find that 

 water would dissolve and redeposit its own volume of minerals in 

 five hours. If now the volume of water is one-tenth per cent, 

 of the whole mass, the whole rock at or below the level at which a 

 temperature of 180° C. exists, could be dissolved and redeposited in 

 5,000 hours, or considerably less than a year. As a matter of fact, 

 however, most crystalline rocks on analysis give more than one- 

 tenth per cent, of water, some schists and gneisses go up to 2% by 

 weight or over 5% by volume, which means that they could be 

 wholly recrystallized every 100 hours, or, say, every week. 



Taking the extremes in the rate of increase of underground 

 temperature as between 157'2 and 28*1 feet for every degree 

 Fahrenheit, the temperature 180° C. or 356° F. would be obtained at 

 a depth between 10*6 and 1-9 miles. 



Whether our deductions from laboratory experiments are correct 

 we cannot say, but the extreme distortion of many crystalline rocks 

 is certainly very favourable to our conclusions. Instances of these 

 distortions occur in the gneisses of Eobertson and Prieska, and the 

 old rocks of Johannesburg and Prieska, the Hospital Hill slates and 

 the Griquatown Series especially ; some cause has allowed these 

 rocks to adapt themselves with the greatest ease to the most com- 

 plicated stresses, and solution under high temperature is an adequate 

 cause. Eocks that are being deformed by this means are solid 

 throughout during the operation, yet they respond to small stresses 

 like a plastic body without loss of their crystalline character ; each 

 grain of the rock, while retaining its own individuality, becomes 

 wholly new formed by crystallization, somewhat after the manner of 

 the fleshy parts of animals which are periodically entirely replaced 

 by new material. 



We have, then, three elements causing deformation in rocks, any 



one of the three being able to carry out the deformation by itself. 



They are : — 



Pressure. 



Heat. 



Solution. 



Each by itself can render deformation possible at the following 

 depths : — 



Pressure 5 to 7 miles. 



Heat 8 to 45 miles. 



Solution 1'9 to 10-6 miles. 



The average diameter of the globe is 7,912 miles, so that a very 

 small proportion is outside the zone in which deformation must go 



