386 /. MORGAN CLEMENTS 



applied to similar zones by various observers. It seems to me 

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

 that in such a case as this there is a reaction in a strict sense 

 between the magma and the olivine. Moreover, the zones 

 should not be compared to the " resorption-rims" found so com- 

 monly in certain effusive rocks, where, from the fusion of horn- 

 blende crystals, pyroxene has been produced. Such a zonal 

 growth around the olivine seems to me comparable to the case 

 described by Washington, 1 where colorless diopside phenocrysts 

 are surrounded by a narrow border of yellowish-green augite, 

 which corresponds to. the small augites in the groundmass, or to 

 those cases which are so common in plutonic rocks, even in this 

 rock described, where hornblende is found surrounding the 

 pyroxene. 



A general explanation which would account for the succes- 

 sive crystallization of pyroxene and hornblende in this rock 

 should be applicable to such a zonal growth as occurs around 

 the olivine, taking into consideration, of course, the probability 

 that a factor of slight importance in the one case may be the 

 controlling factor in the other. Such occurrences seem clearly 

 to indicate a change in the chemical composition of the magma 

 as the chief factor, but influenced more or less strongly by the 

 pressure, the temperature, and also by other factors. 



An analysis of this peridotite is given below : 



ANALYSIS OF PERIDOTITE BY DR. H. N. STOKES. 

 Si0 2 



Ti0 2 - 



A1 2 3 - 



Cr 2 3 - 



Fe 2 3 - 



FeO - 



MnO 



NiO - 



CaO - - - 8.79 99-17 



'Italian Petrological Sketches, 4, The Rocca Monfina Region, by H. S. Wash- 

 ington, Jour. Geol., Vol. V, 1897, p. 254. 



