19^ SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



similar changes have occurred in the peripheral portions. Accom- 

 panying the separation of prochlorite there is much light-brown 

 iron oxide, limonitic matter, resulting from the ferruginous portions 

 of the olivine. Often these ferritic portions are oriented in fairly 

 parallel layers about the prochlorite nuclei. Immediately adjacent 

 occurs a layer of limonite, bounded in turn by a rim of serpentine, 

 while this in turn is surrounded by a border or zone of magnetite, 

 compact and black with jagged border, the points projecting normal 

 to the surfaces of alteration. At times the limonite is absent ; in 

 other cases limonite alone is present with no magnetite. 



Alteration is not at all confined to the olivine, for whole areas of 

 pyroxene have made way for it and, curiously enough, in those 

 spots where the changes in the pyroxene has been most profound, 

 the olivine retains its original perfection of form and composition. 



Liquid inclusions are abundant and are arranged in zones or 

 clouds, nearly always concentrically and in juxtaposition to a layer 

 of iron ocher. Often these inclusions are so numerous as to cause 

 these particular areas to become nearly opaque. 



The ground mass of the rock is formed by a violet-tinted augite, 

 faintly green, however, in spots. It is not pleochroic. Sections 

 from the two pinacoids are well represented. The augite, like the 

 olivine, exhibits the micropoecilitic structure, with a particularly inter- 

 esting development represented in the accompanying plate liv, 4, to 

 the right. It will be noted that there are two automorphic olivines 

 adjacent ; that to the left has its interior filled with augite, optically 

 continuous with that which partially surrounds it. That the olivine 

 formed first is assured ; that the augite filtered in after the forma- 

 tion of the olivine was complete forces the assumption of a primary 

 cavity in the olivine. It seems to the writer that the phenomenon 

 is an excellent illustration of the power of crystallization even under 

 the most adverse circumstances, the olivine assuming its perfect 

 form, even in spite of the intimate admixture of foreign augite mole- 

 cules ; that with the formation of the olivine phenocrysts the augite 

 molecules were thrust apart to the interior and here segregated, 

 optically continuous, however, with the main augitic mass without. 

 The phenomenon is unique, only this instance having been observed 

 in the sections studied in this series. In spite of this, I have dwelt 

 at some length upon it with the hope that my explanation might be 

 the means of bringing to light a more plausible solution of the 

 phenomenon. 



The iron oxides occur in the usual forms of magnetite, hematite, 

 and limonite, included in the olivine,, augite, chlorite, etc. Most 



