142 PROCEEDINGS OF THE BOSTON MEETING 



of the microscopic determination of plagioclase in igneous roclis, namely, the 

 statistical method of finding the maximum equal extinction angle in the zone 

 perpendicular to 010. This method is based on the assumption that the plagio- 

 clase feldspars of a slide are all of the same composition. 



It is reasonable to assume that the law of association of feldspars charac- 

 teristic of igneous rocks also holds true of most schists or gneisses derived 

 from igneous rocks, from considerations which follow. The feldspars of a 

 schist or gneiss derived from igneous rocks may have the following sources : 



(a) Primary crystallization from a magma — the case of primary gneisses. 



(&) Recrystallization, granulation, slicing, and rotation of the primary 

 feldspathic material of the igneous rocks — processes which leave the compo- 

 sition of the feldspars the same as in the original igneous rocks. 



Dynamic metamorphism commonly causes a loss of lime, but this does not 

 necessarily imply a change in the composition of the feldspars, but a change 

 in the lime-bearing ferromagnesian minerals to those containing less lime, 

 such as the change from augite to amphibole or biotite. In the saussurization 

 of basic igneous rocks, however, to quartz, albite, or sodic micas, all stages 

 of this decomposition are generally visible. In some cases the dynamic meta- 

 morphism of acid igneous rocks destroys the feldspars, one of the principal 

 products of decomposition being mica. 



(c) Recrystallization of the weathered igneous rock. The feldspars in the 

 recrystallized portion would be chiefly residual feldspars. The weathered 

 material is generally deficient in bases ; hence its recrystallization would 

 probably fail to produce feldspars. In the case of basic igneous rocks, car- 

 bonation is sometimes an important process of weathering. It is, therefore, 

 conceivable that recrystallization of the weathered basic igneous rock might 

 produce limy feldspars. Their occurrence, however, would be no radical viola- 

 tion of the law of association of feldspars characteristic of igneous rocks, 

 since the primary feldspars of such rocks are basic. It is conceivable that 

 their relation to carbonates and their distribution might indicate their second- 

 ary nature. Regional weathering rarely extends to a depth of more than 100 

 feet ; hence its relation to the unweathered portions might be observed in the 

 field. As yet, no schists derived from weathered igneous rocks have been 

 positively identified ; hence this proposition is only of theoretical interest. 



(d) Recrystallization of hydrothermally altered igneous rocks. In a broad 

 way, the results of hydrothermal alteration are similar to weathering. Such 

 changes generally destroy the feldspars. The feldspars present in the schist 

 and gneisses derived from them may be (1) residual, (2) recrystallized from 

 the altered product, or (3) may have been introduced by the thermal solu- 

 tions. It is conceivable that the feldspars due to introduction or recrystalliza- 

 tion may violate the law of association of igneous rocks. Their secondary 

 nature, however, might be recognizable. As in the preceding cases, schists 

 and gneisses of this type have not as yet been identified. 



The source and characteristics of the feldspars of gneisses and schists of 

 sedimentary origin are chiefly as follows : 



1. They may represent the primary feldspars of feldspathic sediments — re- 

 crystallized, granulated, sliced, and rotated. If the sediments were derived 

 from a single kind of igneous rock, the feldspars present will show the same 



