62 HENRY S. WASHINGTON 



This may be calculated roughly to represent : 



Orthoclase, - - - 6.5 Diopside, - - - - 2.1 



Albite, ----- 42.0 Hornblende, - - - 3.2 



Anorthite, - 44.2 Magnetite, - - - -2.1 



Here the albite and anorthite molecules are in the ratio of 

 1 : I, but since the microscope shows that the plagioclase has 

 the composition of about Ab 2 An 3 , it is evident that the alkali- 

 feldspar is rich in soda, and has approximately the composition 

 OrjAbg. The analysis, however, does not represent the compo- 

 sition of the rock as a whole, and for most purposes is of little 

 or no use. 



Gabbro. — Rocks which belong to this group are found in 

 typical development only at Nahant, and are called norites by 

 Sears, who has briefly noticed them. 1 According to Wads- 

 worth 2 and Sears, gabbros also occur at various localities in 

 Essex county, especially at Davis' Neck, Cape Ann, and Wood- 

 bury Point, Beverly. These, however, judging from the some- 

 what unsatisfactory specimens in my possession, are rather 

 diorites in Brogger's sense, but will not be described further. 



The gabbro of Nahant, as represented by the few specimens 

 collected by myself, are dark, coarse-grained rocks composed 

 of plagioclase, which even in the freshest specimens are dull or 

 waxy and greenish through epidotization and black augite, 

 besides titaniferous magnetite grains. They show both mega- 

 scopically and in thin section a typically granitic structure. 



The abundant plagioclase, although rather decomposed, 

 shows twinning lamellae whose extinctions correspond to those 

 of a basic labradorite about AbjAng. A little orthoclase is 

 also present. A pale gray augite is abundant, which is often 

 automorphic and shows constantly high extinction angles. In 

 my specimens I could find none of the hypersthene mentioned 

 by Sears. Large titaniferous magnetite grains are common and 

 are often surrounded by borders oi leucoxene. With the excep- 

 tion of limonite, epidote, chlorite, and a few other decomposi- 



1 Sears, Bull. Essex Inst., Vol. XXVI, 1894. 



2 Wadsworth, Geol. Mag., 1895, p. 208. 



