294 Barrell — Physical Effects of Contact Metamorphism. 



sufficiently high, the rock would be coarse-grained up to the 

 walls. The metamorphism would be practically completed 

 before solidification began and during a period when the 

 magma was able to act hydrostatically and transmit lateral 

 pressures. 



Under these conditions the shrinkage of the vertical walls 

 would be largely lateral and result in a corresponding lateral 

 expansion and vertical subsidence of the magma. 



The shrinkage of both the intrusion and the metamorphic 

 zone due to the progressive cooling after solidification had 

 been completed, would result in the partial relief from lateral 

 pressure and tend to mask any effects due to metamorphism 

 alone. Thus a number of phenomena, such as the formation 

 of pegmatite dikes and fissure veins near the margins of 

 intrusive masses, though chiefly owiug their origin to the 

 contraction due to the crystallizing and cooling of the igneous 

 rock, may find a minor cause in the shrinkage of the meta- 

 morphic zone, and the latter cause may be the factor which 

 determines why at Elkhorn, Montana, aplite dikes several 

 hundred feet wide are found localized at or near the contact 

 between the granite and its sedimentary walls, Similar 

 intrusions, though not on so great a scale, have been observed 

 at other places near the margin of this batholith and have 

 been noted in other localities and discussed by Pirsson.* 



It may be readily seen that the relations of the meta- 

 morphosing walls to the igneous activities have considerable 

 importance, but the development of the subject would be 

 largely speculative and the facts at hand do not at present 

 warrant carrying it to any greater degree of refinement. 



Infiltrated strata. — Although it has been observed that as a 

 rule, metasomatic infiltration of the metamorphosing strata 

 does not take place, yet there are special instances where the 

 evidence shows that it occurs. Two such which the writer has 

 noted are due to strata of limestone containing considerable 

 amounts of arenaceous and argillaceous impurities being con- 

 fined between others of purer limestone. In both instances 

 the strata are tilted and the inclination has made them favor- 

 able channels for the escape of mineralizing waters. The 

 differential shrinkage has given them the porosity of a burned 

 brick, and the analogy is the more appropriate, since in both 

 cases the action is one of thermal metamorphism without 

 sufficient pressure to result in a close texture. 



The first instance occurs upon Elkhorn Mountain, at Elk- 

 horn, Montana, at an elevation of 9000 feet above the sea, 

 where two slabs of Madison limestones metamorphosed to 



*" Complementary Eocks and Eadial Dikes," this Jour., vol. 1, pp. 394- 

 399, 1895. 



