60 HENRY S. WASHINGTON 



olivine entirely altered to serpentine were seen, but they are too 

 rare to be of any importance. Magnetite is abundant in all the 

 specimens, usually in large rounded grains. There seems to be 

 a tendency for biotite to be produced from it when included in 

 feldspar and hornblende when in augite, but this rule is not con- 

 stant. Apatite is abundant in fair-sized stout crystals, more so 

 in the basic than in the acid varieties. 



It is evident that the rocks which are grouped under the 

 heading of diorite are highly varied and that they represent tran- 

 sition forms from the essexites to the akerites. This is true at 

 least for the area under examination ; of the larger Danvers- 

 Ipswich area I can say nothing. For the satisfactory study of 

 these rocks several analyses will be necessary, but at present 

 only one is available. The rock chosen for analysis was a speci- 

 men from the south side of Peach's Neck, a coarse-grained dark 

 rock which shows under the microscope plagioclase, less ortho- 

 clase, no quartz, diopside, diallage, magnetite, apatite, and sec- 

 ondary hornblende and biotite. It is not quite fresh, but not 

 altered enough to affect the result seriously. 



Si0 2 - - - 51.82 CaO - - 8.59 



Ti0 2 



AI0O3 - 



Fe 2 3 - 



FeO - 



MnO 



MgO - - - - 



This is evidently the analysis of a diorite, though a basic 

 one, the silica and alkalies being too high for a gabbro. It will 

 be discussed later. 



Quartz- augite- diorite .- — The rocks which Sears calls by this 

 name occupy a narrow area west of the rocks described in the 

 preceding pages, which stretches through the county in a north- 

 east-southwest direction from Andover to Newburyport and the 

 New Hampshire line. Representing them I have only three 

 specimens from Newburyport, given me by Mr. Sears, by whom 

 they have been briefly described. 1 They are light gray, medium- 



1 Sears, Bull. Essex Inst., Vol. XXVII, p. 7, 1895. 



