CONTACT ZONES 245 



source, together with that of iron and minor elements, is proven both by 

 the absence of the cavities, which we would infer from the recrystalliza- 

 tion of earthy limestones, and by the absence of the silica and iron in 

 analyses of what seem to be representative samples of unaltered lime- 

 stone. 28 Tests of the latter character have been made by Waldemar Lind- 

 gren, on a very elaborate scale, in the mines at Carr Fork of Bingham 

 Canyon, Utah. Samples taken by channeling across hundreds of feet of 

 exposures in the cross-cuts, when analyzed, showed that the metamor- 

 phosed limestone had a decided excess over the unchanged prolongation 

 of the same stratum. The results were reported to the National Academy 

 of Sciences in May, 1920, but will ultimately appear in detail. 



Observations by one of my old students, the late Prof. Charles A. 

 Stewart, 29 of the University of Idaho, led us years ago to the conclusion 

 that the silica and iron for the garnets must penetrate at times the 

 minutest pores of the calcite in the limestone and develop small, isolated 

 garnets having no visible connection with supply cracks or with larger 

 masses. 



The study of contact zones has been especially stimulated in later years, 

 because they are so often the locus of ore deposits. The mining opera- 

 tions undertaken along them have enormously facilitated and extended 

 our opportunities for study. Earlier observers walked over the more or 

 less weathered surface exposures. Later observers follow the phenomena 

 underground, it may be through many miles of drifts and cross-cuts. In 

 most localities observers concluded that the lime silicates are the first 

 eifects in time. Next follow the magnetite-specularite bodies of iron ore, 

 often of huge size, seldom entirely failing as scattered minerals. The 

 almost universal veinlets of sulphides of iron, copper, and less often of 

 other metals, which cut across the silicates and the iron ores, come third. 28 

 In one or two cases other observers have thought that the iron ores pre- 

 ceded the lime silicates. Among the lime silicates we may note in in- 

 stances that garnet, epidote, and vesuvianite favor positions next or near 

 the intrusive; farther away, wollastonite and diopside predominate, grad- 

 ually giving way to marble, which itself fails by interfingering passages 

 to unaltered limestone. 



Mineral Veins 

 igneous after-effects 



Frequent reference has been earlier made in this address to pegmatites. 

 I may remind you that geologists have been long of two minds, whether 

 to speak of pegmatites as dikes or veins. Some have compromised with 



