I 90 SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



Magnetite occurs rather plentifully, especially in the region of the 

 altered mica. It has been produced as an alteration product of the 

 ferro-magnesian minerals and simultaneously with the secondary 

 mica. Some original magnetite was also noted. 



The lower dike material, with its finer grain and homogeneous 

 structure, represents perhaps more fairly than do any of the upper 

 sheets the typical dioritic magma, whence the sheets have come. Its 

 homogeneity renders it a safer criterion to judge of the constitution 

 of the dioritic magma — hence its analysis is given, as follows : 



Analysis of Diorite. (W. C. Phalen, Analyst.) 



SiO. 47.80 



ALO3 18.24 



FcOs 35 



FeO 9.27 



MgO 8.08 



CaO 11-44 



NasO 2.24 



K.0 45 



H2O above 100° 58 



H2O below 100° — 



Ti02 1.46 



P.O5 24 



MnO ^ 



100.70 



In addition to the rocks already described from Umanak island, 

 there occur others of sufficient interest to warrant description. Un- 

 fortunately the relationships of these rocks are entirely unknown, 

 hence their value as petrographic factors in this province is largely 

 curtailed. The one is a syenite, the other a diorite of more than 

 passing interest, owing to the occurrence in it of an amphibole with 

 peculiar parting. The description of these rocks with an analysis of 

 the separated amphibole follows. 



Syenite. — This rock (Cat. No. 75,485) is salmon pink in color, with 

 holo-crystalline texture, consisting for. the greater part of feldspar 

 with an occasional vitreous quartz. The structure tends to the por- 

 phyritic, the largest feldspars frequently having diameters of one 

 centimeter or more and sinking from this size to microscopic dimen- 

 sions. Occupying fissures between the feldspathic particles may 

 be seen a green mineral, closely resembling epidote in color, while 

 lustrous specks of pyrite are not uncommon. Cavities are scattered 

 throughout the mass of the rock. 



