290 Barrell — Physical Effects of Contact Metamorphism. 



Consequences of the Escape of G-ases. 



Degree to which metasomatic additions are prevented. — 

 From sediments of suitable composition the escape of these 

 large quantities of gases can not be questioned, but it may be 

 asked how much material, especially silica, may be added to 

 the rock mass by circulating heated waters and thus the 

 shrinkage in volume be in part prevented. The question has 

 greater force since such transfer of material is believed to take 

 place where rocks are brought within the zone of weathering, 

 large amounts of silica being taken into solution and carried 

 downward and away from surface action to be deposited in 

 a zone of cementation.* 



The extent to which this takes place in contact meta- 

 morphism can best be judged by field observations. Harkerf has 

 noted in the metamorphic zone observed by him that the distance 

 of transfer of material has not exceeded one-sixteenth of an 

 inch and the writer has noted similar features at a number of 

 localities near the Boulder batholith in Montana. In the latter 

 region a number of separate intrusions broke through the 

 sedimentary rocks in the early Tertiary and the igneous activ- 

 ity finally culminated in the eruption of the Boulder batholith, 

 a mass of granitic rock which under the present depth of 

 erosion possesses a surface extension of seventy miles in lati- 

 tude by forty in longitude. At;}: Elkhorn the granite contact 

 breaks on the whole across the strata, at right angles to the 

 strike, so that the metamorphic action may be observed on 

 sediments of widely differing composition. Within three or 

 four feet of the contact, absorption effects from the granitic 

 magma are sometimes observed, but beyond this narrow limit 

 no addition of material is to be noted. 



The metamorphism has transformed the purer sandstones 

 into massive lustrous quartzites sometimes shattered by a mesh 

 of aplite dikelets, but showing nothing but quartz and dis- 



9-8- 



-2-6 



= 3-8 



vol. 



of quartz. 



41-7- 



-2-5 



=16-7 



vol. 



of kaolinite. 



48-5- 



-2-6 



=18-7 



vol. 



of calcite. 









392 



vol. 



of sediments. 



72-8- 



-3-5 



=20-8 



vol. 



of garnet. 



21-4- 



- -00197 



=10830 



vol. 



of carbon dioxide 



5-8- 



- -00081 



= 7148 



vol. 



of water vapor. 



Dividing each of these volumes by 39 "2, to compare them to the volume of 

 the sediments as a unit, gives the composition by volume as indicated in the 

 table. 



*C. R. Van Hise, Metamorphism of Rocks and Rock Flowage, Bull. Geol. 

 Soc. Amer., vol. ix, p. 282. 



fA. J. Harker, Quart. Jour. Geol. Soc, vol. xlix, p. 368. 



X Geology and Ore Deposits of the Elkhorn Mining District, by Walter 

 Harvey Weed, with an appendix on the Microscopical Petrography, by 

 Joseph Barrell, 21 Ann. Rpt. U. S. Geol. Surv., p. 400. 



