296 Barr ell— Physical Effects of Contact Metamorphism. 



the ore stratum has been somewhat greater than in either the 

 foot or hanging wall, since the feldspars and biotite are 

 minerals which, as shown by the alkalies present, were formed 

 from sediments not fully hydrated or carbonated. Moreover a. 

 microscopic examination of the ore stratum shows parallel 

 sinuous cracks due to tension, and not to shear, and now filled 

 with calcite. Elsewhere in the section the calcite exists as a 

 sponge, holding garnet, diopside and ore grains within it, and 

 its secondary nature is not so clear. The ore is associated with 

 the calcite and also with a certain coarser crystallization of the 

 diopside. 



Thus besides the physical effects of metamorphism which 

 have been emphasized, the problem is seen to have an eco- 

 nomic value in furnishing a guide in the search for strata 

 impregnated with valuable minerals. 



Itelation to subordinate volcanic 'phenomena. — It is com- 

 monly accepted that in regions of hot springs, geyser, and 

 fumarole action that the causes are to be found in the circula- 

 ting underground waters coming in contact with still heated 

 masses of igneous rocks, or in the gases escaping from solidify- 

 ing magmas. While these are undoubtedly the chief factors, 

 the nature of the sedimentary rocks may be such, that in the 

 presence of igneous intrusions they may be undergoing meta- 

 morphism and evolving considerable quantities of gases and 

 heated waters. 



These effects would be more noticeable in certain regions than 

 in others and particularly in western Montana, where the 

 abundant Tertiary intrusions have frequently come in contact 

 with great thicknesses of Algonkian and Paleozoic sediments, 

 consisting in large part of mixed arenaceous, argillaceous and 

 calcareous materials. 



Lehigh University, 

 South Bethlehem, Penn. 



