Photomicrographs of Unusual and Common Feldspars 

 PLATE II 



A. Microcline-microperthite(hypoperthite), (approx. Mieo Abi, An^Heirii). 

 Sunstone, Delaware County, Pennsylvania. White areas, albite (MijAbsvAni) . 

 Dark areas, soda microcline (MisoAbiyAnj) , showing a coarse type of inter- 

 growth. (974) 



B. Nephelite bearing anorthoclase, chesterlite (Or42Ab45An3Neio) . Poor 

 House Quarry, Chester County, Pennsylvania, showing queer type of wavy 

 extinction. 



C. Microcline-microperthite near Unionville, Chester County, Pennsyl- 

 vania (approx. Mi57Ab34An5Qz4), showing irregularly bordered albite spindle 

 (bleb) in microcline. (996) 



D. Microcline-microperthite (hypoperthite) from pegmatite of Yonkers 

 gneissoid granite, Valhalla, New York, a typical pegmatitic feldspar. 



E. Cassinite, Blue Hill, 2 miles north of Medina, Delaware County, Penn- 

 sylvania. Long, thin spindles of albitic plagioclase (approx. Mi4Ab9oAn3Cn3) 

 in hyalophane (approx. OrTeAbnAniCnia) . Note the small blebs of "second- 

 ary" origin giving a "grained" appearance to the whole. (Kindness of Dr. 

 S. G. Gordon.) (1437) 



F. Bytownite (Mi2.oAb24.7An73.3), Crystal Bay, Minnesota. The white 

 bar is a twinning striation, other vvise the slide is devoid of twinning of any 

 kind. Unless extinction angles or indices of refraction are noted it may well 

 be mistaken for "orthoclase." (969) 



G. Albite (approx. Or7Ab83An2Ne8) from Kragero, Norway. Possibly 

 the so-called "barbierite," showing coarse albite twinning. (Kindness of 

 Dr. Olaf Andersen.) (1123) 



H. Potash oligoclase, Risor, Norway (Mi7.30Ab73.6sAn19.05), showing bleb 

 of soda microcline (center) in slightly weathered untwinned oligoclase. (Kind- 

 ness of Dr. Olaf Andersen.) (1122 b) 



I. Granite feldspar, Adirondacks, Ausable Quadrangle near Ausable 

 Forks, Essex County, New York (quarry near Stickney Bridge), consisting of 

 potash feldspar blebs (approximately OrssAbjAnio), dark areas, in Labradorite 

 (Mi5Ab4oAns5), striated plagioclase. (1096) 



