Barrett — Physical Effects of Contact Metamorphism. 293 



enter into the problem at that point. The thermodynamics of 

 metamorphism, however, have been discussed by Tan Hise* 

 and need not detain us. It is evident that a massive igneous 

 intrusion would quickly impart large quantities of heat to the 

 adjacent rocks, more than sufficient for these operations, as 

 fast as the conductivity of the rocks could convey it. 



The result would be the rapid liberation of the water and 

 carbon dioxide as gases far above their critical temperatures. 

 If they were retained within the original volume, the maxi- 

 mum pressure would be realized, and to that degree by which 

 their rapid escape is hindered by the friction of percolation 

 through the rocks, this maximum would be approached. Thus 

 the internal pressures would vary directly with the rapidity of 

 metamorphism and the frictional resistances of the channels by 

 which the gases escape. As they gradually became dissipated, 

 the external gravitational and lateral pressures would cause the 

 rock mass to shrink under conditions which it is believed 

 would effect a continuous recrystallization, accounting for the 

 dense character and absence of infiltration. 



Effects of wall-shrinkage upon the magma. — The effects 

 of the metamorphism upon the magma depends upon a 

 number of factors, the first to be considered being the form 

 of the intrusion. In the case of sheets and laccoliths, the 

 cover being the principal region undergoing metamorphism, a 

 partial subsidence of the surface would result, following shortly 

 after the original uplift. 



On the intrusion of dikes, volcanic stocks, and batholiths 

 with vertical walls, the contraction of the margins would result 

 in a certain relief from lateral pressure, the direction of maxi- 

 mum shrinkage depending upon the ratio of the gravitational 

 to the lateral forces. This leads to the consideration of two 

 cases. 



First : If the magma were intruded at the lowest tempera- 

 ture consistent with fluidity, solidification would begin as soon 

 as the latent heat of crystallization had escaped into the walls. 

 This quantity of heat would presumably not be sufficient to 

 produce a wide contact zone, and the alteration and shrinkage 

 would continue after solidification. The igneous mass could 

 no longer freely follow the shrinking wall and a partial relief 

 from pressure would occur which might determine lines of 

 new intrusions or open the walls of dikes or veins already 

 present. The original intrusions would show chilled margins 

 and coarser-grained centers with veins or dikes parallel to 

 their walls. 



Second : If the intrusion were large and the temperature 



* Metamorphism of Eocks and Eock Flowage, by C. E. Van Hise, Bull. 

 Geol. Soc. America, vol. ix, p. 269. 



Am. Jour. Sci. — Fourth Series, Yol. XIII, No. 76. — April, 1902. 

 20 



