258 THE CRYSTAL FALLS IRON-BEAEING DISTRICT. 



of the hornblende subsequent to and in such intimate association with the 

 pyroxene which it surrounds in zonal growth. An explanation of such 

 occurrences has been attempted hj Becke in a recent article, ^ in which the 

 conclusion is reached that the formation of the hoi'ublende and pyroxene 

 depends upon changes in temperature and pressure. His explanation is 

 based upon the facts of occurrence of pyroxene and hornblende in plutonic 

 and effusive rocks, and also upon the well-known fact that under high 

 temperature and atmospheric pressure they can not exist, but when fused 

 recrystallize as pyroxene; and in addition to this, upon the experiments of 

 Von Chrustschoff,^ who has obtained hornblende at a temperature of 550 C. 

 with the presence of water, under which conditions a high pressure must be 

 developed. However, attention should be called to the fact that his explana- 

 tion does not take into accoiuit other important factors which certainly 

 influence the crystallization of minerals — for example, the chemical composi- 

 tion of the magma and the fusing point and specific gravity of the minerals. 



Whatever the factors are which determine its crystallization, the fact is 

 that hornblende began to crystallize from this peridotite magma in the 

 place of pyroxene. 



The biotite appears to have been formed at the same time with the 

 hornblende. The production of these two minerals, hornblende and biotite, 

 then continued until the remaining magma had reached the composition of 

 basic feldspar, which then crystallized and now forms the mesostasis. 



A zone of orthorhombic pyroxene, succeeded by one of hornblende, 

 has been described as surrounding the olivine in this peridotite. The term 

 reaction rim has been applied to similar zones by various observers, but it 

 seems to me that this term is inapplicable to such zones. It is not probable 

 in such a case as this that there is a reaction between the magma and the 

 olivine. Moreover, the zones should not be compared to the resorption rims 

 found so commonly in certain effusive rocks, where from the fusion of the 

 hornblende crystals pyroxene has been produced. 



Such a zonal growth around the olivine seems to me comparable to 

 such a case as that described by Washington,' where colorless diopside 



' Gesteine der Columbretes ; Anhang : Einiges iiber die Beziehung von Pyroxen und Amphibol 

 in Gesteinen, by F. Becke : Tschermaks mineral. Mittheil., Vol. XVI, 1896, pp. 327-336. 



2 Bull. Acad. imp. sci. St.-P6tersboing, 1890, p. 13. Cf. Becke, Op. cit., p. 337. 



'Italian petrological sketches; 4. The Rocca Monfina region, by H. S. Washington : Jour. Geol., 

 Vol. V, 1897, p. 254. 



