Pirsson — Petrography of Tripyramid Mountain. 419 



I. Olivine gabbro (hessose) "ossipite," Black Cascade, Slide 



Brook, Waterville, N. H. C. J. Monahan, analyst. 

 II. Gabbro (hessose), Split Rock Mine, Westport, Essex Co., 

 N. Y., (Kemp, 19th Ann. Rep. U. S. Geol. Surv., Pt. 

 Ill, p. 402, 1899). W. F. Hillebrand, analyst. 



III. Gabbro near anorthosite, Whiteface Mt., Adirondacks, 



N. Y. (Bull. 228, U. S. Geol. Surv.), G. Steiger, analyst. 



IV. Light band of Gabbro, Druim an Eidne, Isle of Skye, 



Hebrides (Geikie & Teall, Quart. Jour. Geol. Soc, 

 vol. 1, p. 653, 1894). J. H. Player, analyst. 

 V. Gabbro with predominant labradorite, Baste, Harzburg, 



Streng, Neues Jahrb., 1862, p. 963. 

 VI. Norite (andose). Above the gabbro, Slide Brook, Water- 

 ville, N. H. C. J. Monahan, analyst. 

 VII. Molecular proportions of No. I. 



Gabbros, as is well known, are very variable rocks in the 

 relative proportions of the feldspathic as compared with the 

 ferromagnesian minerals ; they can, therefore, be roughly 

 divided into two groups, the feldspathic and the ferromagne- 

 sian ones. In the latter the iron and magnesia are very large 

 in amount, accounting for the predominance of ferromagnesian 

 minerals. The analysis shows the rock under discussion to 

 belong to the former class and this accounts for the agree- 

 ment of the analyses with well known feldspathic types, as 

 illustrated in the above table. Of the banded gabbros of Skye 

 it has nearly the same composition as the light feldspathic 

 bands. If all of the potash were present in biotite there 

 would be some 8 per cent of this mineral in the rock ; the 

 actual amount estimated is not so large as this, and if correct, 

 a small amount (1-8 per cent) of orthoclase must also be in the 

 rock. It has not been seen and may well be lost in submicro- 

 scopic intermixtures in the other feldspars. The adjustment 

 of molecules in the calculation of the following norm also pro- 

 duces in it a little nephelite, which could not be found present 

 in the rock. 



Classification. — The position of this rock in the quantitative 

 system is seen in the following calculation of its place and 



norm 



Or.... 



3-89 



Ab.... 



25-68 



An.... 



36-97 



Ne ... 



1-99 



Di.... 



14-03 



01 .... 



8-93 



Mt.... 



3-02 



11 



3-80 



Ap.... 



1-34 



H.0 .. 



•28 



Total 



99-93 



Sal 68-53 ^ , 



:2-2 = 2, Dosalane 



Fern 31-12 



L 1-99 



68-53 = =0-03=:5, Germanare 



F 66-54 ' 



K„0' + Na„0' 63 



:0*49 = 4, Hessase 



CaO' 

 Na„0'~56 : 



K O' 7 

 31-12 ±±ill.< =0 -i2 = 3, Hessose 



