54 HENRY S. WASHINGTON 



on Salem Neck show a marked subschistose or platy structure. 

 Rosenbusch has described the essexites of Salem Neck in con- 

 siderable detail, and in general my observations agree with his 

 descriptions. He has, however, included among the essexites 

 proper certain rocks with what he calls a hyperitic structure, 

 which, it seems to me, are not essexites proper, inasmuch as 

 they contain neither alkali-feldspar nor nepheline, but constitute 

 a dioritic facies of it. 



In thin section the structure of the essexites proper is gran- 

 itic, though the plagioclase shows a tendency to tabular devel- 

 opment. The feldspar is mostly a plagioclase, showing clear 

 twinning lamellae, whose extinctions vary, but which correspond 

 to compositions ranging from Ab 1 An 1 to AbjAng. Rosen- 

 busch speaks of it as "hoch idiomorph," but for my specimens 

 this is rather strong. It is certainly much more so in the hyper- 

 itic facies, while in the more normal essexites (such as the one 

 analyzed) , it is only rarely so. An alkali-feldspar is not uncom- 

 mon, generally anhedral, and often microperthitic. This, and a 

 microcline which is occasionally met with, are apparently rich in 

 soda. Nepheline is fairly abundant, generally interstitial, but 

 occasionally in well shaped crystals. I could not identify with 

 certainty any of the sodalite seen by Sears. 



Rosenbusch speaks of two remarkable peculiarities of the 

 feldspars. The first consists of the presence, in gray dusty 

 crystals or portions of crystals, of minute biotites, or horn- 

 blendes, about which there is a dust-free zone ; the other is the 

 presence of specks and veins of a colorless substance of low 

 refrangibility, and either isotropic or faintly birefringent, which 

 he thinks might be either glass or nepheline. Of the first of 

 these I could find no example in my sections, and of the second 

 only a little here and there which did not allow me to answer 

 the question which Rosenbusch raises as to its nature. 



In the typical essexite of Sears the most common ferro- 

 magnesian mineral is a deep green or greenish-brown, highly 

 pleochroic hornblende, basal sections of which often show 

 prismatic planes. This occurs scattered through the mass in 



