Vol. 57.] OE THE YELLOWSTONE AND GKEAT BRITAIN. 215 



its spherulite by a parallelism at the ends of the latter, or by its 

 general concentric form. On the flat surface of a specimen we 

 often see an arc-like disposition of cavities subtending angles up 

 to 180° ; two such arcs are often concentric one to the other. 



One fine pink spherulite measuring about 1| inches by 1 inch, 

 clearly traversed by the flow-lines which run through the rock, 

 possesses an open and porous texture, preserving at the same time 

 its spherulitic habit. On the upper side of a flow-band, and, as it 

 were, springing from it, the spherulitic growth is distinctly visible to 

 the naked eye, and by the aid of a low-power lens is resolved into 

 bundles of branching rays, the free ends of which project into a 

 crescent-shaped cavity. Much tridymite adheres to these fibres, 

 but above the rift the spherulite is more solid in texture and the 

 radial structure less readily recognizable. Through this part fine 

 flow-lines can be traced. Eelow the band coarse tufts again appear, 

 also connected with a cavity, and extend for the entire length of 

 the spherulite. Tliese fibres pass into the more compact portion of 

 the spherulite, and apparently differ in no respect from it except in 

 habit. 



In this instance the cavities of the spherulite are related (a) to a 

 flow-band, and (6) to the periphery of the spherulite. 



The lithophysse of Obsidian Cliff graduate into another common 

 variety, distinguished by a porous structure and by the absence of a 

 definite cavity. An entire absence of a cavity is, perhaps, rare, but 

 it is not structurally important. Such forms may be conceived of 

 as lithophysae in which the cavities have been distributed through 

 the body of the spherulite and not localized or arranged in a definite 

 way. These porous spherulites are irregular, lobate, or circular in 

 outline : often the outer zone is harder and less friable than the 

 central portions, and shows traces of a radial growth. , 



III. Statement of the Problem. 



The preceding description demonstrates that no distinction can 

 be drawn between the various kinds of spherulites described. 

 Even the bands of hard bluish-grey spherulites are found frequently 

 to possess small central cavities, and by gradation to merge into 

 those in which the cavity is the dominant feature. Between such 

 and the typical lithophysae no distinction can be made that would 

 indicate a difference in mode of formation, since all stages can be 

 observed connecting the two. 



It remains, then, to see what explanation best accords with the 

 varying forms of structure which the rock presents, and if possible 

 to bring to light the fundamental property of the original magma 

 to which they owe their birth ; or, on the other hand, to see whether 

 decomposition by heated waters may not be responsible for the whole 

 or, for part of the observed facts. To this end, let us investigate 

 first the effect of solfataric action as revealed in the rocks of the 

 Yellowstone Canon. 



