porphyritic constituents of the schists. 169 



Porphyritic Minerals of the Schists resulting from metamorphic 



Action. 



The Minerals and their Association. — It is my object in this paper espe- 

 cially to describe the so-called porphyritic constituents of the schists that 

 have been developed or modified by metamorphic agencies. Under this 

 head are included feldspar (largely an acid plagioclase), garnet, stauro- 

 lite, tourmaline, biotite and ottrelite. Other Constituents present in 

 greater or less quantity are sericite. quartz, graphite, chlorite, magnetite, 

 ilmenite, pyrite, nbrolite, calcite, rutile. sphene (and leucoxene), zircon 

 and apatite. In nearly all specimens there is a matrix made up of vary- 

 ing amounts of feldspar, quartz and a micaceous mineral, which is in 

 some cases a silvery mica (sericite) ; in other instances sericite with bio- 

 tite or chlorite. Associated with these minerals are accessory graphite, 

 ore material, etc. Almost without exception porphyritic feldspars occur 

 in the matrix, usually many times larger than the feldspar grains com- 

 posing it.* In addition to the modifications of the rock arising from the 

 micaceous constituents present, petrographic variations consist mainly in 

 the character of the porphyritic feldspar and in the presence or absence 

 of the other porphyritic constituents, viz: garnet, staurolite, tourmaline, 

 biotite and ottrelite. The central schist bed has furnished most of the 

 specimens in which the structures I shall describe were observed, though 

 such structures do not seem in all cases to be restricted to that bed. 



Porphyritic Feldspar. — The porphyritic feldspars appear under a num- 

 ber of modifications. In certain facies of the rock they are more or less 

 oval in shape and inclose with more or less uniformity blades of seri- 

 cite, particles of graphite, ore material or tourmaline. They are com- 

 monly either simple individuals or simple twins. They quite resemble in 

 sections the beautiful photomicrograph which is figure 8 of the paper 

 by AVolff on "The Metamorphism of Clastic Feldspar in Conglomerate 

 Schist." f Though these feldspars may show no marked evidences of 

 strain, many instances of polysynthetic twinning have been observed, and 

 the occasional localization of the lamella? about cracks in the individual 

 would indicate that the twinning is the result of internal mechanical 

 movement. The twin lamella 1 allow of the determination of the feldspar 

 as an acid plagioclase. 



Dynamic metamorphism lias at other localities been more intense, as 

 evidenced in sections where the granulation of the feldspars may be 



* See descriptions of similar feldspars in the schistose and conglomeratic rocks of lloosao moun- 

 tain and elsewhere. (Pumpelly: The Relation of Secular Rock-Disintegration to Certain Transi- 

 tional Crystalline Schists. Bull. Geol. Soc. Am., vol. ii, 1891, pp 209-224. Wolff: Metamorphism of 

 Clastic Feldspar in Conglomerate Schist. Bull. Mus. Comp. Zool., vol. xvi,1891, pp. 173-183, pis. i-ii.) 



t Bull. Mus. Comp. Zool., vol. xvi, 1891, pp. 173-183, pis. i-ii. 



