PE TR O GRA PHICA L PRO VINCE OF ESSEX CO UNTY 5 7 



99.60 99-92 



II. Essexite. Salem Neck. M. Dittrich anal. (Rosenbusch. Elem. d. Gesteins- 

 lehre, 1898, p. 172. No. 1.) 



The resemblance between the two analyses is close, the 

 greatest differences being in ferric oxide, titanium oxide, and 

 magnesia. It is possible that the low titanium oxide in Dittrich's 

 analysis is due to the fact that it represents only the residue left 

 after evaporation of the silica with hydrofluoric acid, while in 

 mine, where the similar residue amounted to only 0.72 per cent., 

 the titanium oxide was determined directly. The low silica and 

 high lime and alkalies will be noticed, showing the basic raon- 

 zonitic character of these rocks. Magnesia is rather lower than 

 might be expected, a point which will be discussed later on. 



Diorite. — This group is quite extensively represented in 

 Essex county, the main occurrence being a long area with a gen- 

 eral northeast-southwest trend in the western part, in Danvers, 

 Topsfield, and Ipswich, a smaller area occurring about Salem 

 and Marblehead and extending north into Beverly. From the 

 large western area I have no specimens, all of mine coming 

 from localities in the smaller areas about Salem and Marble- 

 head. These rocks have been partially described by Sears 1 and 

 are quite diversified in character. 



Megascopically these are very dark, almost black, rocks, 

 though a few are quite light, especially the main rock at Fort 

 Sewall, Marblehead, which is a mottled light gray. This mass, 

 by the way, is notable for the great number of "schlieren " and 

 rounded masses of a dark, more basic diorite which it contains. 



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



