292 HENRY S. WASHINGTON 



Si0 2 ■ 



Ti0 2 



A1 2 3 



Fe 2 ; 



FeO - 



MnO 



For the analysis a typical specimen was chosen from the 

 northeast coast of Marblehead Neck. As will be seen, these 

 rocks are rather acid, and resemble the quartz-syenite-porphyry 

 more than they do the aplite. The only point to be mentioned 

 here is that soda is considerably higher than potash. 



Keratophyr. — The last rock to be described is that by which 

 this region is, perhaps, best known, which Rosenbusch 1 has taken 

 as the type of his bostonites, and which Sears 2 has described as 

 keratophyre. Accepting provisionally Rosenbusch 's system of 

 classification the choice of names depends on whether the rock 

 occurs as a dike or a flow. Owing partly to the fact that the 

 exposure is only visible at low tide the relations are somewhat 

 difficult to make out. My observations were confirmatory of the 

 views expressed by Wadsworth 3 and Sears 4 that the rock forms 

 a flow and not a dike, overlying rhyolite and conglomerates. 

 This being so, I think that the name bostonite is not justified in 

 this case, and I prefer to retain Sear's name, keratophyr (rather 

 than trachyte), on account of the large content of anorthoclase, 

 even though this name is in several respects a very bad one. 



My specimens come from Boden's Point, below Mr. Foster's 

 house, and from below the Corinthian Yacht Club House. 

 Although the rock has been described by Sears and Rosenbusch, 

 a few words may be devoted to it. The freshest specimens are 



'Rosenbusch, Tsch. Min. Pet. Mitth., Vol. XI, p. 447, 1890; Mikr. Phys., Vol. 

 II, p. 467, 1896. 



2 Sears, Bull. Mus., Comp. Z00L, Vol. XVI. p. 167, 1890. 



3 Wadsworth, Proc. Boston Soc. Nat. Hist., Vol. XXI, p. 288, 1881. 



4 Sears, op. cit. 



