Genth.] *>^^ [August 18, 



• 



iag to their hardness are more or less water-worn and rounded, whilst the 

 corundum Avhich they enclose is quite sharp and angular, Avhich fact 

 proves that, since the great gravel deposits were formed no alteration of the 

 corundum has taken place in these deposits. 



11. Alteration of Orthoclase into Albite. 



Orthoclase changed into albite is undoubtedly one of the most interest- 

 ing alteration of one mineral into another. Numerous occurrences of it 

 have been observed in Europe, but I am not aware that it ever was no- 

 ticed in this country ; I will therefore give the description of an occur- 

 rence fi"om the neighborhood of Philadelphia. 



At the gneiss quarries of Upper Avondale, in Delaware county. Pa., 

 druses have some time ago been found, which are lined with crystals of 

 albite, associated with those of muscovite, and rarely with beautiful, but very 

 minute, crystals of white beryl in hexagonal prisms and many pyramids, 

 small crystals and groups of black tourmaline and calcite in cleavage 

 masses and small scalenohedra, 1^, and thin hexagonal plates, which had 

 so much the form of muscovite crystals that, at first, they were thought to 

 be pseudomorphs. Mr. Lewis Palmer, of Media, presented me with a 

 number of specimens. 



The albite appears in short, stout -colorless or white crystals, mostly in 

 twins, showing principally the planes I. 0. i-l. 2-1. i-3 and l-l. and 1; the 

 latter plane very small and indistinct. Many of the crystals are very 

 small and imperfect, and form a crystalline coating upon the cleavage 

 masses, either directly upon a flesh-colored orthoclase or a grayish-Avhite 

 plagioclase intervening. I have analyzed perfectly colorless crystals (1). 



Some of the specimens show conclusively that the albite is more recent 

 than the orthoclase, and results from the decomposition of the latter, 

 sometimes with the intermediate development of a plagioclase, and that 

 the crysttils and crystalline masses of muscovite have resulted at the same 

 time, and contain the potassium oxide of the former orthoclase. The or- 

 thoclase wlilch is associated with these albite crystals forms flesh-colored 

 cleavage masses, which on the cleavage planes are bright and lustrous. 

 The purest which with a strong lens appeared to be without admixture, 

 was examined by my son, Mr. F. A. Genth, Jr. In their sections under 

 the microscope it shows the rectangular reticulation characteristic of ortho- 

 clase, but disseminated through it, minute particles of plagioclase, giving 

 proof of an incipient alteration (2). 



One specimen, particularly is quite interesting. It is a mass of coarse 

 cleavage particles of flesh-colored and white feldspars, with colorless 

 albite crystals in cavities and crystals and scaly aggregations of muscovite 

 and a little quartz. A cleavage cr3-stal of flesh-colored orthoclase, espe- 

 cially on one side shows a rotten appearance, as if in part eaten away and 

 one of the edges and planes is replaced by a lining of albite in the 

 form of an imperfect crystal made up by un aggregation of many small 



