180 II. S. Washington — Solvsbergite and Tinguaite. 



solvsbergites, as calculated by Brogger, is given in Ilaand Ilia, 

 and they are seen to be poorer in dark minerals and richer in 

 the albite molecule. The mineral composition of the rock rep- 

 resented in IV could not be satisfactorily calculated. An 

 analysis by Bischof of the "aegi rite-trachyte " of Kuhlsbrunnen 

 in the Siebengebirge is given in Y, and is seen to be closely 

 similar to the others, though the lack of separate determina- 

 tions of Fe 2 3 and FeO and its date (1852) render it less valu- 

 able than one might desire. It is chiefly of interest as an 

 instance of rocks of practically the same chemical and miner 

 alogical composition occurring in connection with magmas of 

 quite diverse characters. 



In connection with the above rock it may be mentioned that 

 closely similar rocks occur in Shaler's dike 182 (south of 184), 

 near Pigeon Cove, and from a dike near Bass Rocks, east of 

 Gloucester. The rock of dike 3, at Magnolia, is of similar 

 character, but is remarkable for the abundance of well crystal- 

 lized phenocrysts of feldspar and quartz. Specimens were 

 collected from these localities this summer, but too late for 

 description in this paper. 



The rocks described above are of interest in view of the 

 increase in localities of blue hornblendes identified in recent 

 years, and call attention to the fact that such hornblendes are by 

 no means uncommon in the igneous rocks of Eastern Massachu- 

 setts, just as they are frequent in similar rocks of the ^Norway 

 region.* 



Mr. T. G. White has recently describedf the well known 

 granite of Quincy, Mass., and has found that it contains a blue 

 hornblende which he refers to glaucophane. This occurrence 

 seemed of such importance that an analysis was deemed desir- 

 able, and through his kindness, for which I express my hearty 

 thanks, I was put in possession of a specimen collected by him- 

 self. The results of an analysis made by me are given below 

 in ~No. I. It will be seen that, as compared with the analysis 

 of the solvsbergite the differences are chiefly those depending 

 on the higher acidity, with the exception of the relative 

 amounts of Fe 2 3 and FeO, which determined the character of 

 the hornblende. 



The character of the rock admits of a very satisfactory cal- 

 culation of the mineral composition, with the result given in 

 la below. From this it appears that while the composition in 

 general is analogous to that of the solvsbergite, yet that there 

 are one or two differences of note. While the absolute and 

 relative amounts of albite and orthoclase do not greatly differ, 

 quartz, instead of being quite subordinate in amount, takes 



*Gf. Broker, op. cit. 186, and Zeit. Krvst., xvi. 411, 1890. 

 f T. G. White, Prcc. Bost. Soc. Nat. Hist., xxviii, 128, 1897. 



