AKT. 17 MINERALS OF OBSIDIAN CLIFF FOSHAG 5 



Under the microscope the lithoidite has a texture and habit 

 strono^ly individual and peculiar. It is made up of radiated masses 

 of feldspar needles arranged haphazardly or aligned in rows. The 

 interstices between these masses are in turn lined or completely 

 filled with crystals of tridymite, which is more coarsely crj^stalline 

 than the feldspar in which it occurs in the form of nests or lenses. 

 Scattered through the rock are well formed crystals of fayalite. The 

 hornblende always forms very minute crystals, which are scattered 

 through the mass or arranged along the flow lines and apparently 

 represent the microlites in the glassy phaise of the rock. 



Very little of the excess silica shown in the norm is present as 

 quartz, but occurs almost entirely in the form of tridymite, both in 

 the cavities and in the groundmass. The normative albite and ortho- 

 clase are largely combined to form sanidine, or, more properly, natro- 

 sanidine, while the ferrous iron is contained for the most part in the 

 fayalite. The other constituents are largely in the minute crystals 

 of what is probably hornblende. 



LITHOPHYSAE 



Both the obsidian and the lithoidite carry abundant hollow cavities 

 lined with crystalline matter. These, for the reason that they sug- 

 gested bubbles formed by expanding steam, were called lithophysae 

 by Von Richthofen. In their simplest form they are hollow cavities 

 in the glassy obsidian, lined with a thin coating of crystalline matter 

 which is shown by close inspection to be made up of minute rods 

 of white or gray feldspar with small pellets of white, waxy cris- 

 tobalite (pi. 4, fig. 2). The boundary between the glass and the 

 crystalline lining of the cavities is sharply defined and the hollow 

 spheres often break easily from the glass, leaving a smooth, clean 

 cavity. The simple forms become more complex when the crust is 

 made up of partially concentric leaves, or when fibers of feldspar 

 almost completely fill the cavities. In the latter case they approach 

 the solid spherulites in character, although in this type of lithophysae 

 the radial fibrous structure is decidedly more pronounced than in 

 the solid bodies (pi. 4, fig. 3). When the rock becomes more lithoidal, 

 especially in those facies having a banding of glassy and stony mat- 

 ter, the lithophysae increase in size, are irregular in outline, and 

 complex in structure (see pi. 1). It is apparent that the larger 

 lithophysae, like the simpler forms, are intimately connected with 

 the stony spherulites, since they are usually aligned with them and 

 in many cases appear to be but special instances of spherulitic crys- 

 tallization. Frequently the lithophysae form on only one side of 

 a line of spherulites, but less often on both sides, so that the stony 

 matter passes through the cavity as a median plane without other 



