H. S. Washington — Sblvshergite and Tinguaite. 179 



la. Ila. Ilia. 



Glaucophane 13'8 



Riebeckite 10-6 .._.. 



Hornblendes 14*7 .... 



Pyroxene (Aegirite) ~'b 15*5 



Albite 39-0 51* 50*5 



Orthoclase 29-8 30' 29'5 



Quartz . 4'3 0'7 4 '5 



Titanite.. -- 1-9 



Cordierite 0-3 



Apatite 0-3 



Accessories ro 



In this calculation the small amount of CaO is just sufficient 

 to satisfy Ti0 9 and P Q 6 to form titanite and apatite, and the 

 trace of MgO is used up in cordierite. It is evident therefore 

 that the bine hornblende is not the lime-bearing arfvedsonite, 

 and that the glaucophane molecule present is not the usual 

 magnesian one. We therefore have to assume a glaucophane 

 molecule in which RO is entirely FeO, and on this basis the 

 hornblende as a whole is represented by the formula Gl s Rb 9 . 

 It may be noted, en passant, that poverty in MgO is character- 

 istic of all the rocks of the region so far analyzed ; and in this 

 connection the occurrence of the rare, purely ferrous olivine, 

 fayalite, at Rockport* may be recalled. 



In view of the close resemblance in chemical and miner- 

 alogical composition, as well as genetic relationships, with the 

 solvsbergite of Norway, the present rock must be classed with 

 them, and may be distinguished specifically as a Glaucopha ne- 

 Solvsbergite. This group was first described as such by Brog- 

 ger in 1894 and embraces medium to fine-grained dike rocks 

 composed largely of alkali feldspar, with soda pyroxenes or 

 soda hornblendes, either without quartz or very poor in it, 

 and consequently of medium acidity. Analyses of Brogger's 

 Solvsbergites are given in Xos. II and III and their comparison 

 with I is instructive. II is of a hornblende (katoforite) solvs- 

 bergite and it is seen to be almost identical with I, the slightly 

 lower SiO a , and higher MgO being the only noteworthy dif- 

 ferences. Ill is of an aegirite-solvsbergite and this also is 

 closely similar, the chief difference here being the considerably 

 lower FeO as compared with our rock. This difference seems 

 to have determined the formation of segirite rather than a soda 

 hornblende, since we find the same features in the analysis of 

 the so-called "acmite trachyte" of the Crazy Mts. given in 

 No. IY, a rock which is referred to the Solvsbergites by 

 Rosenbusch.f The mineral composition of the Norwegian 



*Penfield and Forbes, this Journal, IV. i, 129, 1896. 

 f Rosenbusch. Mikr. Phys , ii, 476, 1896. 



