SOME EXAMPLES OF ROCK VARIATION 38$ 



not surrounded by it. Finally, however, the condition of 

 the magma was such that in the place of the monoclinic 

 and orthorhombic pyroxene the crystallization of hornblende 

 began. 



I do not know what the conditions were which caused the 

 formation of the hornblende subsequent to and in such intimate 

 association with the pyroxene, surrounding it in zonal growth. 

 An explanation of such occurrences has been attempted by 

 Becke 1 in a recent article in which the conclusion is reached 

 that the formation of hornblende 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 at high temperature and atmospheric pressure hornblende 

 cannot exist, but when fused recrystallizes as pyroxene, and 

 upon the experiments of von Chrustschoff, 2 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 explanation does not take into account other important 

 factors which certainly influence the crystallization of minerals, 

 for example, the chemical composition of the magma, and the 

 fusion point and specific gravity of the minerals. 



Whatever the factors are which determine its crystallization, 

 the fact is that hornblende began to crystallize in the place of 

 pyroxene. The biotite appears to have been formed at this time 

 with the hornblende. The production of the hornblende and 

 biotite continued until the remaining magma had reached the com- 

 position 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-rims" has been 



1 Gesteine der Columbretes ; Anhang : Einiges iiber die Beziehung von Pyroxen 

 und Amphibol in den Gesteinen. By F. Becke : Tsch. Mitt., Vol. 16, 1896, pp. 



2 Bull, de l'Academie des Sciences, St. Petersburg, 1890, 13. Cf. Becke, loc. cit., 

 P- 337- 



