17 



Such a chemical constitution belongs to no known species or 

 variety of feldspar, though coming very near orthoclase ; and, 

 fearing there had been an error in the first analysis, a second 

 was made from the same piece, — a clear, apparently un- 

 weathered specimen, — with a like result. A careful inspection 

 of a large number of specimens seemed to explain the anomaly, 

 by showing that the feldspar, although coarsely and perfectly 

 crystalline, is probably a mixture ; the crystals appearing to be 

 formed of thin interlaminated plates of at least two different 

 feldspars. This can be seen only on slightly weathered sur- 

 faces, one of the feldspars decomposing more readily than the 

 other. The constituent layers were not analyzed, as it seemed 

 impracticable to separate them. But if, as appearances indicate, 

 it is a mixture, and a mixture of two species only, they are 

 probably orthoclase, and albite or oligoclase, the former pre- 

 dominating. The color of this feldspar is bluish-gray, weather- 

 ing white, and its sp. gr. varies from 2.55 to 2.60. Dissemi- 

 nated through it are numerous small crystalline grains of 

 magnetite. The coarse feldspar of the Beverly shore, similar 

 in physical characters to the preceding, was analyzed by Mrs. 

 Crosby, and found to have substantially the same chemical con- 

 stitution, containing one-half of one per cent, of calcium oxide. 

 The specimens just described, from the coarsely crystalline ,. 

 nearly pure, feldspar rock, are certainly not labradorite, and yet 

 that this species exists in some of the rocks of this series does not, 

 in my mind, admit of reasonable doubt. Much of the labradorite 

 in the norite of Essex County, New York, and probably of 

 other localities, is of a pale-green color and waxy lustre, and 

 seemingly compact. In the coarse pyroxenic rock of Nahant, 

 the feldspar is frequently physically identical with this, and 

 yields by analysis a larger percentage of lime. The silica and 

 calcium oxide only were determined, with the following 

 result : — 





Si0 2 



CaO 



Labradorite from Westport, N.Y. 



. 54.60 



4.85 



Feldspar from Nahant 



. 48.71 



8.70 



OCCAS. PAPERS B. 8. N. H. — III. 



