1882.] 



393 



IGenth. 



iudivicliials, joined together. In other places the albite gradually pushes 

 itself, as it were, into the orthoclase, leaving in many instances only a 

 small nucleus of the flesh-colored feldspar in the white. 



In other instances there is between the orthoclase and the crystals of 

 albite a grayish or grayish-white cleavable feldspar with deep striation. 

 The analysis (3) shows it to be a mixture of albite with oligoclase, the 

 oxygen ratio between R^ (RO) : AljOg : SiO.^ being — 1 : 3.1 : 10.6. 



Although not in connection with the alteration of orthoclase into albite 

 and muscovite, I will mention that at the same locality orthoclase has also 

 been found in colorless crystals (4), and white cleavage masses (5) asso- 

 ciated with imperfect crystal of muscovite. 



The analyses gave the following results : 



100.03 — 100.68 



100.03 



100.26 



100.6-; 



Such alterations of orthoclase into albite occur not only in the quarries 

 of Upper Avondale ; in the lower quarries at Leiperville similar facts can 

 be observed, although not so striking, many of tlie large orthoclase crys- 

 tals showing small patches of a thin coating of a white feldspar, albite or 

 oligoclase, in many places penetrating into the orthoclase to a consider- 

 able extent. 



Oligoclase is very common in our gneissic rocks, often associated with 

 orthoclase, and very probably it is the result of the alteration of the 

 latter. 



III. Alteration of Talc into Antliophyllite. 



The suggestion which I made over 20 years ago (Am. Journ. Sc. [2] 

 XXX, 200), that the chrome and nickel-bearing serpentines have resulted 

 from the alteration of chrysolite, is, at present, I believe, generally ad- 

 mitted, since the numerous investigations of Tschermak, von Drasclie, 



