[GNEOUS FORMATIONS. 285 



ones, which may themselves be connected at uo great distance fr the 



surface. 



Witlidni visiting these outcrops, Marvine conjectured them t.. be 

 remnants of surface flows. 



THE si BFACE FLOWS "I TABLE HOI MAIN. 



The form of Table Mountain, which i- implied by its name, may be 

 clearly understood by referring to the map and to PI. VII, its relations to 

 plains and foothills being exhibited l>\ the latter. It is caused by a cap- 

 ping sheet of basalt which lias protected soft sediments from erosion. The 

 stream of Clear Creels has cul a gorge several hundred feet deep directly 

 through the mesa, dividing it into two parts, distinguished as North and 

 South Table mountains. 



Ill I-: BASALTIC CAPPING. 



General description. — Tin- protecting basalt sheet of Table .Mountain consists 

 for the greater pari of two flows with no sedimentary rock between them. 

 Upon North Table Mountain the two are everywhere distinguishable, and 

 the total thickness of basalt is much greater than upon South Table Moun- 

 tain, where over a large area but one thin How exists. 



The structure of the capping where two flows are present is quite 

 uniform, and for North Table Mountain, al least, the following detailed de- 

 scription of the cliffs midway between the two large gulches on the south 

 side will be found to have a general application al almost any part. 



The contact with the soft tuffaceous strata is irregular on ;i small scale, 

 the lower surface of the flow appearing rudely mammillar wherever exposed 

 by the crumbling away of the tuff or l>v excavations, as in tunnels driven 

 in on the contact, for water. For 1 or "_' feet above the contact the basalt is 

 Mack and very vesicular, the cavities being small and distorted in shape 

 through the movements of the half solidified mass. The rock of the next 

 To feet upward is compact and uniform in appearance. Then begins an 

 amygdaloidal zone 1") feet in thickness, in the lower portion of which the 

 cavities are usually small, seldom exceeding 2 inches across, while in the 

 central part they sometimes reach a horizontal diameter of 6 to 8 feet by a 

 height of 2 or 3 feet, and in the upper portion are commonly small and of 



