Washington — Pyroxenite and Hornblendite in Brazil. 87 



of Lacroix,* which are composed of augite and hornblende but 

 with much more magnetite, ilmenite and apatite, so that they 

 •are lower in silica and magnesia, and higher in lime and iron 

 oxides. 



General Remarks. 



Petrographically, these Bahian rocks are of interest because 

 of their bearing on certain features of classification. As has 

 been seen, both the hornblende and pyroxenite are absolutely 

 free from feldspars, though their norms show respectively 

 about 30 and 18 per cent. Closely allied chemically to each 

 are feldspar-bearing types represented respectively by the 

 kylite of Tyrrell, which shows considerable plagioclase along 

 with augite, olivine, and a little hornblende ; and the Radau 

 rock, which also shows considerable bytownite with much 

 bronzite. These types are therefore distinctly more normative 

 in their modes than the Brazil rocks. 



A similar shifting of the mode from a less to a more norma- 

 tive character through the appearance of feldspar and the non- 

 formation of hornblende, is shown in some rocks from Garabal 

 Hill, recently studied by Wyllie and Scott.f They show that 

 bordering an intrusion of tonalite is a mass which forms a dio- 

 rite near the contact, with about 70 per cent of sodi-calcic feld- 

 spar and 7 per cent of hornblende, and which gradually changes 

 to an almost pure hornblende (with pyroxene), with about 84 

 per cent of hornblende and 5 per cent of feldspar. These two 

 extremes closely resemble each other chemically as shown by 

 the analyses here given. 



A B 



SiO, 52-83 53-29 



A1 2 3 11-74 8-81 



Fe 2 3 6-66 4-68 



FeO 6-13 6-66 



MgO 6-41 9-07 



CaO 8-05 8*99 



Na 2 2-67 3-21 



K 2 G 2-06 1-87 



H 2 + 1-20 1-51 



H 2 — 0-20 0-17 



C0 2 _ _ none none 



Ti0 3 1-82 1-41 



P 2 5 0-08 trace 



99-85 99-67 



A. Diorite, Garabal Hill, Scotland. Analyst not stated. Wyllie and Scott, 

 Geol. Mag. (V), x, p. 540, 1913. 



B. Hornblendite, Garabal Hill, Scotland. Analyst not stated. Wyllie and 

 Scott, Geol. Mag. (V), x, p. 540, 1913. 



* A. Lacroix, VIII Cong. G. Int., C. E., p. 832, 1901. 

 f Wyllie and Scott, Geol. Mag. (V), x, p. 539, 1913. 



