550 WALDEMAR LINDGREN 
district; other observers have come to the same conclusion.! 
A great quantity of CaO and CO, are carried away, but the volume 
of the mass remains the same; even drusy textures are rare. 
Accessions from the magma to form the contact metamorphic 
silicates have evidently balanced any shrinkage. If there were no 
accessions from the outside and if the silica and alumina combined 
with the lime to contact metamorphic silicates, considerable shrink- 
age would undoubtedly result provided always that opportunity 
were available for the gas phase to escape. In such a case the 
volume changes could be calculated by means of the appropriate 
simple chemical equations and Lepsius’ volume law.? As a matter 
of fact such rocks do not show evidence of shrinkage, such as would 
be afforded by drusy texture, and no field evidence indicating such 
contraction has been brought forward. I must conclude that here, 
too, calculations of volume relations on the basis of Lepsius’ law 
applied to the ordinarily used equations are worthless as indicating 
the actual process. It is believed that eventual tendency to con- 
traction is equalized by additions of substance from circulating 
waters at the time of metamorphism. 
In the case of hornfels derived by contact metamorphism from 
shales the mineralogical relations have been fully covered by 
V. M. Goldschmidt and earlier writers, but neither the microscope 
nor the field examination corroborate an assumption of reduction 
of volume. As heavy aluminum silicates have been formed it 
seems that contraction would take place unless counterbalanced by 
additions. Here also I believe that the replacing solutions, not 
necessarily of magmatic origin, have carried a certain amount of 
material into the rock and that when a heavy silicate is formed the 
remainder of the space will be filled by some other substance, say 
quartz or feldspar. 
Replacement without liquid solutions.—Water solutions certainly 
circulate in the rocks of the upper metamorphic zones. It would 
1 W. Lindgren, U.S. Geol. Survey, Prof. Paper 43, 1905; F.C. Calkins, ibid., Prof. 
Paper 78, 1913, p. 132; B.S. Butler, ibid., Prof. Paper 80, 1913, p. 90; J. B. Umpleby, 
tbid., Prof. Paper 97, 1917, Pp. 71. 
2 Joseph Barrell, U.S. Geol. Survey, Prof. Paper 57, 1907, p. 149; also ‘‘The 
Physical Effects of Contact Metamorphism,” Am. Jour. Sci., Fourth Series, XIII 
(1902), 279-96. 
