PERTHITIC FELDSPARS. 



133 



S'pccimen No. 4, from Mineral Hill, Delaivarc-Co., Pa., U. S. A. — 

 The color is slightly brownish to cream; only an indistinct lining 

 indicates the perthitic structure so that this feldspar is essentially a 

 microperthite. The albite bands are rarely over 0.2 mm. in width 

 and range commonly from 0.05 mm. to 0.1 mm. They are individu- 

 ally quite irregular as to shape, continuity and direction, though taken 

 as a whole they are uniform for this feldspar. A few deorientated 

 crystals of albite were noted closely associated with the albite bands. 

 Both feldspars are well twinned, particularly the microcline. A few 

 well grown small crystals of muscovite are present; the alteration is 

 very slight. 



Extinction angles are as follows: — 



Microcline — on 001 .... 16° to 17° on broader twin bands. 



— " 010.... 6° 



Albite — " 001 ... . 3° 



— " 010. ...19° 



Speciinen No. 5, from Pikes Peak, Colo., U. S. A. — Color a fresh 

 light green; rather short albite lamellae are easily seen although the 

 greater part of the albite is inconspicuous. The albite lamellae rarely 

 exceed 0.6 mm. in width. They are sometimes of the usual elongate 

 form, but more often they are short, irregular in outline and discon- 

 tinuous, and are usually rather widely separated from one another. 

 They often taper rapidly or may even terminate bluntly. They may 

 follow for a short distance the usual direction of intergrowth and 

 then turn off, apparently toward the direction of the prism. In 010 

 sections the lamellae appear in general to be longer but are also quite 

 irregular, particularly as to width. Sometimes the position of a 

 lamella will be occupied by a series of rather stout but small shreds. 

 A few independently orientated albite crystals are present. Twin- 

 ning is strongly developed in both members. The extinction angles 

 are: — 



Specimen No. 6, from Grafton, N. H., U. S. A.^ — White to almost 



4 The locality of this feldspar is not absolutely certain. It is from New 

 Hampshire and probably Grafton. 



