REGENERATION OF CLASTIC FELDSPAR 523 



the form of scattered or banded grains of oxide of iron in the regenerated crystal. 

 The bright rims of fresh feldspar surrounding the old feldspar grains he does 

 not unhesitatingly ascribe to new growths on the original grains, but suggests that 

 they are in part due to an actual replacement of the outer part of the detrital 

 grain by the new feldspar. This suggestion is borne out by the fact, observed in 

 the same conglomerate and in other rocks, that the fresh feldspar penetrates along 

 fissures well within the original old feldspar, forming tongues and irregular areas 

 connected with the rim in all optic characters, and demonstrating that the new 

 feldspar, which is characteristically abundant in the rims, pervades more or less 

 the entire clastic grain. Mr Wolff also called attention to the fact that probably 

 botli albite and microcline had formed independently of any preexisting nuclei. 



The same conglomerate schist was the subject of a more general treatment by 

 Dr C. E. Whittle in 1892.* Connected with the description of the occurrence of 

 various secondary minerals, such asottrelite, sericite, anatase, etcetera, he describes 

 fully the alteration which the clastic feldspars have undergone. Mr Whittle calls 

 attention to an important feature, due to what he calls the " clearing action of 

 sericite," this mineral lying in belts parallel with the exterior clear rim, and along 

 other clear belts that penetrate within the grain. He hence is disposed to attribute 

 the clear rims not so much to enlargement of the original grains as to some subse- 

 quent action that eliminated the impurities. He also shows that in such meta- 

 morphism microcline is sometimes altered into plagioclase. 



In the same year Dr W. H. Hobbs, in discussing the metamorphic schists of 

 western Massachusetts, fdwelt at length on the so-called porphyritic feldspars of 

 the albite schists of that region. He gives numerous cases of plagioclase feldspar, 

 more or less oval in shape, exhibiting various kinds of alteration and regeneration. 

 He figures an old feldspar that shows a granophyre structure That this structure 

 is not preserved from some former igneous condition of the feldspar is evidenced 

 by the identity of extinction which it bears with the quartz surrounding the grain. 

 He inclines to interpret the granophyre structure here as a secondary feature due 

 to decay of the original feldspar. In other cases feldspar of different composition 

 had partially replaced the original, and this new feldspar not only formed the rim, 

 but constituted mottlings throughout the original grain. Doctor Hobbs mentions 

 the same phenomenon as Doctor Ha worth, namely, that frequently the cores of en- 

 larged feldspars show crystal boundaries, while the enlargements are irregular. He 

 considers this an indication that they are not of detrital origin, though the rocks 

 are undoubtedly clastic. These changes, and others exhibited by the formation 

 of staurolite, tourmaline, ottrelite, etcetera, Doctor Hobbs does not attribute to 

 dynamic metamorphism, there being little or no evidence of crushing and shear- 

 ing, but to " static metamorphism " — that is, a metamorphism due to pressure com- 

 bined with heat and moisture acting over an extended area. 



Besides the publication of these studies on enlargements of clastic feldspars, 

 there are no others in America, so far as I know, prior to my own, recently pub- 

 lished in the final report of the Minnesota Survey, volume v. It is the purpose of 

 this paper to call attention to certain conditions in the alteration of clastic feld- 



*Some dynamic and metasomatic phenomena in a metamorphic conglomerate in the Green 

 mountains. Bull. Geol. Soc. Am., vol. 4, 1892, p. 147. 



f Phases in the metamorphism of the schists of southern Berkshire. Bull. Geol, Soc. Am., vol. 4, 

 [1892], 1893, p. 167. 



LXXV— Bull. Geot.. Soc. Am., Vot.. 13, 1901 



