PEEIDOTITE INTRUSIVES. 255 



constituents are the same in the two rocks, and with some few modifications 

 his description would answer. 



Augite is the chief constituent, and following it, in order of importance, 

 come olivine, hornblende, biotite, and feldspar. The diallagic augite is more 

 automorphic (see fig. B, PI. XLV) as the feldspar increases in quantity. 

 It is the only one of the minerals which shows any marked degree of 

 automorphism. The augite present in the sections which I have studied has 

 a light-brownish color, differing fi-om that described by Patton, which is 

 green to colorless. The augite contains the inclusions occurring in hyper- 

 sthene, as well as the green (hornblende!) ones already described. It is 

 invariably surrounded by a narrow rim of light-brown hornblende, and 

 includes in places on the edges irregular patches of the same brownish 

 hornblende. 



J. Romberg describes the augite in Argentinian gabbros,^ both with 

 and without olivine, as being almost always surrounded by a rim of green 

 hornblende. In one case, however — that of the olivine gabbro from the 

 island of Martin Garcia, in the La Plata River^ — both brown and green 

 hornblende is present around the augite. The brown hornblende forms 

 part of the periphery of a crystal; the green the remaining portion. Of 

 the green hornblende some is fibrous, and is considered by Romberg to be 

 certainly secondary. 



The olivine possesses its usual properties. It is in annedra, with the 

 exception of three or four individuals, which show a fair degree of auto- 

 morphism. The olivine includes rounded grains of a brown spinel, and is 

 traversed by anastomosing veins of the iron oxide. It shows the usual alter- 

 ation to serpentine, and the iron oxide is the result of this serpentinization. 

 The olivine is of exceptional interest on account of the fact that it is sur- 

 rounded by certain zones where it is close to the feldspar (figs. A and B, 

 PI. XLVI). The characters of zones observed in sections from this same 

 locality, and which are almost, if not quite, identical with these which I 

 shall proceed to describe, have already been described by Patton.^ There 

 are two of these zones. An inner one is composed of a mineral which is 

 probably an orthorhombic pyroxene. It was so determined by Patton in 



' Untersuchungen an Diorit-Gabbro-und Ampliibolitgeateinen aus dem Gebiete der Argentini- 

 Bcaen Republik, by J. Romberg: Neues Jahrbuch fiir Mineral., BB. IX, 1894, pp. 320-321. 

 - Op. cit., p. 322. ' Op. cit., p. 168. 



