Barrell — Physical Effects of Contact Metamorphism. 279 



Art. XXIII. — The Physical Effects of Contact Metamorph- 

 ism; by Joseph Barrell, Ph.D. 



Introduction. — Although much has been developed in past 

 years concerning the physical, chemical and mineralogical effects 

 of the metamorphism produced in sedimentary beds by the con- 

 tact of igneous masses, but little has been said concerning the 

 wholesale liberation of gases from the sediments so affected, 

 attended by shrinkages of volume and the possible results in the 

 formation of vein fissures, impregnation deposits and new intru- 

 sion of igneous matter, owing to these causes and the changes in 

 pressure which accompany them. Certain of these questions 

 were suggested to the writer in 1899 while studying the 

 geology of the Elkhorn District in Montana as a field assistant 

 for the U. S. Geological Survey, and the following article was 

 written in the petrographic laboratory of the Sheffield Scientific 

 School of Yale University under the supervision of Prof. 

 L. Y. Pirsson as one chapter in a thesis on the Geology of the 

 Elkhorn District, prepared in partial fulfillment of the require- 

 ments for the degree of doctor of philosophy. 



Excluding for the moment the possible impregnation and 

 metasomatic effects of mineralizing vapors and heated waters 

 c rrying dissolved materials into the contact rocks, the chemical 

 effects consist in the more or less complete expulsion of carbon 

 dioxide and combined water and the formation of the remain- 

 ing constituents into new minerals. In addition, there are 

 physical effects of -considerable magnitude ; the strata not 

 only assume a greater hardness and density, but in beds of 

 certain compositions it will be shown that there may be a 

 shrinkage of from 25 to 50 per cent, in volume, attended with 

 the evolution of great quantities of gases which at surface 

 pressures and temperatures would amount to several hundred 

 times the volume of the original sediments. 



The reasons why such changes in volume have not been 

 noted in the field probably lies in the special compositions 

 necessary to produce the most striking effects and the fact 

 that intrusions have often greatly disturbed the adjacent 

 strata. 



The kinds of rocks which will be least affected are those of 

 igneous origin. In the presence of later intrusives and dis- 

 regarding the temporary expansions due to the high tempera- 

 tures, these will naturally suffer no appreciable change in 

 volume and none at all in mass. The most that would be 

 expected to occur would be the acquisition of certain charac- 

 teristics due to the minerals having been exposed to long and 

 intense reheating. On the other hand, those subject to the 



