GNEISS AND LIMESTONE CONTACT PHENOMENA 275 



Geol. Surv., P.R.C. 171 5), contain considerable amounts of quartz 

 and microcline. They therefore must have a higher percentage of 

 sihca and potash. Assuming the plagioclase to contain 90 mol. 

 per cent Ab and a notable quantity of epidote to be present, the 

 mode given in the table was calculated from the rock analysis 

 by the writer. The pyroxene was computed as a diopside- 

 hedenbergite with 10 per cent (Mg, Fe)Si03 and 4 per cent AI2O3. 

 The clinopyroxene, in the specimen from the Peru line, is pale 

 green without any notable pleochroism. It was found to have 

 an = 1.695=1= 0.002, /3d = 1.706=^=0.002; 7d = 1-724='= 0.002, and should 



Per Cent Mol. Mode 



SiOa 60.44 1 .007 Albite 60. 26 



AI2O3 16.26 159 Anorthite 7.51 



FcaOs 0.07 Potash feldspar 2 . 78 



FeO 3.52 49 j CaSiOj 8.82 



MgO 1.75 44 Pyroxene ^^gSi03 4.40 ^^ 



CaO 7.86 140 I FeSi03 6.47' ^ 



Na20 7.13 115 IAI2O3 0.80 



K2O 0.44 5 i^r^'^T "'^ 



^^ ^ [Quartz 2.16 



H2O+ 0.38 Titanite 3 ■ 14 



H2O— o . 29 ' Apatite i • 34 



TiOa 1 . 33 16 Zircon o . 09 



P2O5 o . 58 4 Pyrite - o . 10 



MnO o . 06 



Zr02 o . 06 



FeS2 o.io 



99.80 



100.27 



therefore contain 60 mol. per cent diopside and 40 per cent heden- 

 bergite, or Di6oHe4o, while calculation from the analysis gave 

 Di48He42. Thus the specimen analyzed contained a pyroxene 

 richer in iron. As we shall see, the composition of the clinopyroxene 

 in these gneisses varies markedly from place to place. 



The epidote is highly birefracting and has jSd = 1.750=*= 0.003, 

 nearly the same value as that found in the epidote from Knappen- 

 vand, Sulzbachtal,'' which contains 2,2> per cent of the ferric epidote 

 (16 per cent Fe203) and has /3d = 1.7540. 



'M. Goldschlag, Tscherm. Min. Fetr. Mitt., Vol. XXXVIII (1917), p. 23. The 

 data given in this study, the most careful one ever carried out on the epidote group, are 

 not yet consistent enough to allow the determination of the amounts of the aluminous 

 and the ferric compounds in epidotes more accurately than with a possible error 

 of =1=5 per cent. • 



