THE FELDSPAR METHOD 641 



in feldspar composition. Studies of metamorphic rocks of known 

 igneous and sedimentary origin showed that the former retained 

 their limited feldspar composition, whereas the metamorphic- 

 sedimentary rocks included feldspar combinations such as would 

 characterize the original sedimentary rock. The conclusion, as 

 based upon the work done, is that there is no decided change, 

 during anamorphism, of the feldspar composition possessed by the 

 original unmetamorphosed material. 



7. The usefulness of the feldspar method as compared with the 

 present criteria used in the determination of the origin of metamorphic 

 rocks. — The present criteria which are used to determine the 

 igneous or sedimentary origin of metamorphic rocks are dependent 

 upon field relations, together with chemical and mineraldgfcal com- 

 position. Field evidence consists chiefly in tracing metamorphic 

 rocks into the less altered igneous or sedimentary equivalents. 

 Thus a basalt has often been observed to grade into a chlorite or 

 micaceous schist. Similarly banded gneisses are often associated 

 with and grade into granites. Chemical evidence suggestive of a 

 sedimentary origin consists, according to Bastin' ''of a dominance 

 of magnesia over lime, potash over soda, excess of alumina and 

 high silica. If the chemical composition is essentially that of an 

 igneous rock this fact favors igneous origin." Mineralogical 

 evidence favoring a sedimentary origin consists of a high content 

 of quartz as does also an abundant development of aluminum 

 silicate minerals. The presence of graphite probably denotes a 

 sedimentary rather than an igneous origin.^ Rounded grains of 

 such minerals as garnet, sphene, and especially zircon have been 

 taken as evidence of sedimentary origin. These minerals are 

 especially resistant to weathering and will remain after the other 

 minerals have been completely altered. 



The plagioclase feldspar method is an addition to our mineral- 

 ogical criteria. The results obtained prove the feldspar method 

 to be a valid and reliable method for the determination of the 



' Edson S. Bastin, "Chemical Composition as a Criterion in Identif^'ing Meta- 

 morphosed Sediments," Jour. Geol., XVII (1909), p. 472. 



^ J. D. Trueman, "The Value of Certain Criteria for the Determination of the 

 Origin of the Foliated Crystalline Rocks," Jour. Geol., XX (191 2), pp. 228-58, 

 300-15- 



