IGNEOUS FORMATIONS. 289 



this irregular structure very well, as will !><• seen from the illustration, PI. 

 XVI, taken from ;i photograph, which represents the composition <>t' the 

 southern face of this point. The high cliffs running cast from Castle Rock 



arc of quite massive ruck until near the indentation at about the center of 

 the northern face. For some distance east of this place the cliffs show a 

 very irregular relation between compact and porous rock, anil the fully 

 normal status is not again shown on the northern face. 'Phis prominent 

 northeast point of the mountain is likewise more or less irregular in its 

 presentation of massive and porous rock. 



'The explanation of this structure consists in assuming that the earlier 

 basalt sheet did not extend so far to the south as the northern line of South 

 Table Mountain, but that it sent off arms or tongues of the common lava- 

 like character, witli high walls of rough, scoriaceous fragments cemented 

 by parts of more compact rock, (her these arms and walls, filling the 

 spaces between them, came the second sheet, itself broken and irregular in 

 structure from the unevenness of the surface. The present cliff lines of 

 the mountain give profiles across or parallel to these arms. The confused 

 nmigling of structures is most marked along the northern face of South 

 Table Mountain. 



The northern cliffs correspond nearly in height to the opposite ones of 

 North Table Mountain, but the thickness of the capping decreases southward, 

 until scarcely more than 10 feet remains in some places on the south line. 

 This thinning is evidently in part due to erosion — for there is no amygda- 

 loidal zone left when the basalt is \ erv thin — but as this may be regarded 

 as having been approximately equal over the whole mountain, it is plain 

 that the lava itself thinned out. < >n the north slope of ( ireen Mountain, 2 

 miles south from the southeast point of Table Mountain, there is a knoll, 

 upon the summit of which loose, angular fragments of pomus basalt lie in 

 such quantity as to suggest that they represent a remnant of the sheet 

 broken up in place. As no outcrops of basalt appear to the south, on the 

 rising slopes of (ireen Mountain, it must lie assumed that the lava did not 

 extend so far in connected sheet form, but angular fragments strewn over 

 the surface of several little benches of about the same elevation indicate 

 the former presence of a thin sheet or of outlying anus of basalt 

 MON XXVII ID 



