342 THE JOURNAL OF GEOLOGY. 



spends in structure and composition with the other specimens, 

 except for the absence of melilite and perovskite, and so they 

 may be described together. 



All the rocks are porphyritic, since they bear large phenocrysts 

 of olivine. Under the microscope the olivine is colorless and trans- 

 parent, and only shows indications of serpentinization along the 

 edges and fissures. It contains rounded inclusions of glass, 

 abundant in some sections, besides octahedrons of magnetite, and 

 others that are transparent with a brownish violet color. Whether 

 the latter are a mineral of the spinel-group or belong to perov- 

 skite, with which they accord in color, could not be decided. 



Augite occurs in only one generation; phenocrysts of augite 

 are wanting. In the rather coarse-grained groundmass, it be- 

 comes the most abundant constituent. The mineral shows a 

 grayish-brown color, common to basaltic augite, sometimes with 

 a tint of violet. It generally forms well-shaped crystals, rarely 

 irregular grains, and bears inclusions of magnetite and glass. 



Melilite occurs in the groundmass in large and well-shaped 

 crystals, its dimensions never becoming as small as those of 

 many of the augite crystals. They may be designated as micro- 

 porphyritical phenocrysts. Cross sections parallel to (ooi) 

 reach a diameter of 0.5 mm. The shape of the melilite is the 

 common one, tabular parallel to (001). The diameter of the 

 tables generally exceeds their thickness from four to six times. 

 Sections parallel to the prism-zone, therefore, are lath-shaped and 

 the vertical axis lies perpendicular to their length; the axis of 

 greatest elasticity coincides with the vertical axis. Between 

 crossed nicols these sections show the particular blue interference 

 colors characteristic of melilite and zoisite. Sections perpen- 

 dicular to the prism-zone are eight-sided by reason of the planes 

 (iio) and (100), but frequently the outlines are rounded. In 

 some of the sections examined the melilite incloses minute 

 opaque grains arranged zonally, which present very sharply the 

 prismatic outlines of their host. Besides the two prismatic faces 

 above mentioned, there is also a ditetragonal prism, the angle of 

 which upon the adjoining faces of (iio) and (100) was found 



