642 CHARLES GORDON CARLSON 



metamorphic rocks to which it is appHcable, and this means any 

 rock containing recognizable feldspar constituents. The studies 

 show that metamorphic rocks in general, except where they have 

 suffered alteration due to ordinary weathering or hydrothermal 

 alteration, contain such constituents. Where hydrothermal altera- 

 tion has been effective, as in the proximity of the intrusive por- 

 phyries of the west, some other criteria must generally be resorted 

 to. Even here, however, the alteration is not likely to have pro- 

 ceeded far from the main intrusive, so that by following a forma- 

 tion into its unweathered portion, recognizable feldspars may 

 often be found. The feldspar method is to be preferred to the 

 heavy residual or "zircon" criterion. The theory upon which 

 the heavy residual method is based is undoubtedly a valid one, yet 

 the studies of a large number of sediments show that any inter- 

 pretations as to the origin of metamorphic rocks which are based 

 upon its use, cannot be but uncertain. In the examination of 

 marine beach sands from South Carolina and Anticosti Island, 

 crystals of zircon and titanite were found which retained perfectly 

 their crystal outline. Dr. W. H. Twenhofel reports similar results 

 from a study of coral beach sands from the Hawaiian Islands. 

 Such a sediment after conversion to a metamorphic rock would, on 

 the basis of the zircon method, have been interpreted as suggestive 

 of igneous origin. It must be borne in mind, however, that of all 

 the criteria at present available for the determination of the origin 

 of schists and gneisses, the use of field relations, where possible, is 

 by far the most conclusive. Chemical and mineralogical criteria 

 must therefore be subordinated to it. On the basis of practical 

 usefulness and reliability the feldspar criterion should supply a 

 valuable addition to our present laboratory methods. 



