'28$ GEOLOGY OF THE DE2JVEK 1!.\SIN. 



point. Within certain large and rudely spherical spaces the basalt seems 

 to have undergone a more perfect tabular jointing than in the areas between 

 these spheres. The tablets of the spheres are smaller and thinner, and 

 hence lighter and more easily removed than those of the intervening rock, 

 and the result is that basin-shaped depressions are produced, separated by 

 curved ridges that are sometimes quite narrow. None of these basins is 

 entirely surrounded by a wall, as that would naturally prevent the removal 

 of the rock fragments. Neither spherical sundering nor disintegration of 

 the basall occurs here, and the only visible reason for the " crater " forms 

 lies in the variable jointing within and without these imperfectly spherical 

 areas. 



The basalt at this point is quite compact and contains no more small 

 vesicles than may he found in the most massive parts of the lower How. 

 From analogy .with the lower sheet this highest point must he at least 50 

 to To feel below what was formerly the upper surface of the flow, and it is 



probably _<><» feet above the bottom. 



The cliff at the southwest point of North Table Mountain shows fully 

 1 in feet of the rock belonging to the upper flow. Ascending this cliff, 

 through a crevice near the point, it will he noticed that the rude columnar 

 structure prevailing in the lower parts passes by a horizontal fissuring of the 

 columns into a tabular jointing which is most pronounced at the highest 

 horizon remaining. The rock of the tablets is also much lighter in color 

 and apparently coarser in grain than below, and thus approaches in char- 

 acter that found some .">!> to 7"> feet higher in the flow, at the points shown 

 in PI. XV. 



south Table Mountain. — While the hasaltic capping of South Table Mountain 

 is, at some points along its northern face, resolvable into two flows, like 

 those of North Table .Mountain, this is by no means commonly the case. 

 Instead of the regular structure described, there are many places along the 

 cliffs of the northern, western, and eastern faces where the basalt from the 

 tuff upward consists of solid and compact rock mixed most irregularly with 

 \vr\ porous matter, producing a breccia-like structure. Sometimes uni- 

 formly compact rock appears above or below such hrecciated material. 



Castile Rock, the projecting point above the town of Grolden, exhibits 



