IGNEOUS FORMATIONS. 287 



small spheres, with a radiate structure, which arc either Loosely aggregated 

 or form walls. They are sometimes embedded in another zeolite of similar 

 color and granular structure. The former mineral proves to be thomsonite, 

 the latter laumontite, with stilhite as a common associate. These zeolitic 

 deposits are noteworthy because they frequently color this zone so that it 

 is distinguishable at a considerable distance. 



Debris of basalt accumulates at the base of the cliffs so that the lower 

 contact is usually hidden, ami the loose material may rise so that the foot 

 of the bare cliff is at or near the top of the lower sheet. The debris piles 

 consist of large blocks formed by the jointing, which is very pronounced. 

 A good columnar structure characterizes the tditils in many places, while 

 the spherical sundering and other forms produced by weathering are also 

 locally well developed. The photograph reproduced in PI. XIV shows the 

 perfection of the spherical sundering as exhibited on the south face of 

 North Table Mountain near the main zeolite locality. The larger spheres 

 are '_' or o feet in diameter. 



North Table Mountain. Tile Upper SUl'taCC of Xoi'tll Table MolUlfaill is 1>Y 



no means so flat as it appears at a distance. Numerous shallow drains lead 

 into the two large gulches and toward the minor indentations. There are, 

 too, a few knolls rising to greater elevation than any point upon the cliff 

 line. In one or two depressions are stagnant pools. 



The highest point of the mountain. 6,599 feet, near its western edge, 

 is a rather sharp knoll surrounded by minor points with connecting ridges, 

 producing a series of senhbasin-shaped areas, to which the fanciful name of 

 "The ('raters" has been locally given. Marvine, too, lends his support to 

 the idea that they indicate the points at which the lava welled up and 

 whence it spread out as a sheet. 



In point of fact the area about this knoll represents the basalt of a 

 higher horizon in the upper sheet than is now left at any other spot. The 

 rock, which is here much bleached, is more coarsely crystalline than 

 elsewhere, and the surface configuration, which alone suggested the term 

 "crater," is due entirely to a peculiar development of jointing and to the 

 manner in which a rock thus jointed is naturally affected by erosion. The 

 accompanying illustration, PI. XV, shows the structure about the highest 



