THE PETROGRAPHICAL PROVINCE OF ESSEX 

 COUNTY, MASS. II 



Essexite. — The term essexite was first introduced by Sears 

 who applied it to a group of basic rocks composed essentially of 

 augite, hornblende, biotite, and plagioclase, with subordinate 

 orthoclase, nepheline. or sodalite, which he regarded as the 

 earliest crystallized and most basic portion of the nepheline- 

 syenite magma of Salem Neck. 1 Rosenbusch 2 enlarges the 

 mineral list to include olivine and apatite, and erects the essexite 

 into a group of igneous rocks of the same order as the granites 

 or diorites. He says of them that they are related to the 

 gabbro family as the monzonites are to the normal syenites. 

 They may therefore be considered as essentially basic monzonitic 

 rocks, in which both lime-soda and alkali-feldspars and feld- 

 spathoids are present, and which are usually, if not always, 

 derived from an alkali, and especially a soda-rich, magma. In 

 Essex county they are confined to the immediate vicinity of 

 Salem Neck, where they occur in large masses, accompanying 

 and cut by the nepheline-syenite. They are quite distinct from 

 this, and, so far as I know, few transition forms into this rock have 

 been seen. On the other hand, they grade into the diorites of 

 the neighborhood, so that in this direction it is difficult to draw 

 a hard and fast line. These rocks have been described by Sears 3 

 and by Rosenbusch. 4 



They are dark gray or almost black rocks, of a granitic 

 structure, and usually fine-grained, though varying to some 

 extent in this respect. Biotite and feldspar phenocrysts and 

 small round spots of augite and hornblende are seen in most 

 specimens, but are not prominent. Specimens from one locality 



'Sears, Bull. Essex Inst. Vol., XXIII, 1891. 



2 Rosenbusch, Elem. d. Gesteinslehre, 1898, p. 171. 



3 Sears, loc. cit. 



4 Rosenbusch, Mikr. Phys., p. 247, 1896. 



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