COLUMNAR OBSIDIAN. 361 



this end of the cliff the columns are the same, but grow less clearly defined 

 toward the east, where a sharp bend in the lava sheet has formed gaps in 

 the rock and has destroyed the continuity of the mass. Beyond this the 

 columns incline considerably toward the west, as though the underlying 

 surface of contact sloped toward the west also. The columns in the main 

 face of the cliff are tilted 15° to the eastward, and the planes of flow which 

 cross them have an average dip of 10° E., indicating that the underlying 

 surface at this place sloped toward the east. The columns become broader 

 to the north, the largest being 20 feet in width, and with the change in the 

 character of the rock from glassy to lithoidal they grade into massive blocks 

 formed by vertical cracks farther apart. The columns have four, five, and 

 six sides, which are unequally developed, but at a distance the general effect 

 is quite regular. 



The obsidian forming the lower part of the columns is dense and black, 

 and transparent only on very thin edges. It is traversed by bands or 

 layers of small gray spherulites. In this part of the columns there are 

 almost no cavities or lithophysse, and but little contortion of the layers. 

 Higher up, the obsidian is less massive and contains large lithophysse 

 flattened parallel to the plane of flow. The tops of the columns pass into 

 obsidian, which for 10 feet is quite dense, but above this is full of large 

 cavities which honeycomb the mass. This may be seen in the photograph 

 of the columns (PI. XL). This upper portion, about 50 feet thick, is 

 divided by vertical cracks into broad quadrangular blocks. The sides of 

 the columns are comparatively straight, and are independent of the flow 

 structure within the mass, which is indicated by the spherulitic layers that 

 traverse the rock in parallel planes more or less contorted These layers 

 pass through the columns at all angles, exhibiting abrupt folds and curves, 

 which have been cut across sharply by the columnar cracks. The crystal- 

 line spherulitic layers formed planes of weakness, along which transverse 

 cracks were produced. There is another kind of parting, which took place 

 while the lava was still molten, but when it was so viscous that in places 

 where vertical layers pulled apart in flowing down the slope the gaps did 

 not close up. These are of only exceptional occurrence. 



The columnar portion of the west face of the cliff extends for only a 

 few hundred feet northward, the character of the rock also changing in this 

 direction. The spherulitic and lithoidal layers also become more frequent, 



