132 EDSON S. BASTIN 



plagioclase and olivine, the bands of minute magnetite inclusions 

 so characteristic of the latter frequently continue through the amphi- 

 bole band nearest the olivine, but are not present in the band nearest 

 the feldspar, a relation which indicates that both these minerals have 

 been involved in the replacement. The layer nearest the olivine 

 is usually nearly colorless while that nearest the feldspar usually 

 shows a pale-greenish tint. Usually, though not invariably, the 

 rim next the olivine is narrower than that bordering the feldspar. 

 Between feldspar and pyrrhotite only a single reaction rim is developed ; 

 it frequently is pale brown in color especially near the pyrrhotite. 



The general appearance of the reaction rims is shown in Fig. 3. 

 In the more altered portions of the rock, the reaction zones become 

 somewhat broader and in some cases lose their fibrous character 

 probably as a result of recrystallization. Secondary hornblende of 

 this type in some places grades with perfect continuity into original 

 hornblende. Larger crystals of hornblende associated with chlorite 

 and angular areas of pyrrhotite in certain narrow zones traversing 

 the rock are probably of secondary origin. The hornblende of 

 the reaction rims occasionally grades into hornblende of this type. 



In seeking to determine what original minerals were the source, 

 partial or complete, of the hornblende of the reaction rims, it is evident 

 that the best results are likely to be obtained from a study of the 

 freshest portions of the rock, where the alterations are in their initial 

 stages. In these parts of the rock hornblende is uniformly absent 

 between the olivine and the pyrrhotite, serpentine being the secondary 

 mineral developed along these contacts. Between the feldspar and 

 olivine and between feldspar and pyrrhotite, however, a reaction rim 

 of hornblende is always present. The darker color exhibited by 

 the hornblende formed along the feldspar-pyrrhotite contact presum- 

 ably indicates that the mineral here is richer in iron and proportion- 

 ately poorer in magnesia than that developed next to the olivine. These 

 relations clearly indicate that the hornblende is developed as a result 

 of chemical interchange between plagioclase and the femic minerals, 

 olivine or pyrrhotite. 



It is hardly possible, however, to regard this change as taking 

 place without accession of material from outside or loss to other 

 parts of the rock. This follows from a consideration of the chemical 



