.Ill 



GEOLOG v OF THE MCm'Ki; BASIN. 



fringe about magnetite. A.patito has a phenocrystic development in large, 

 short, more or less rounded prisms wiili :i\i:il inclusions, and also in clear 

 needles < if later formation. 



TABLE M.n STAIN BASALT; CAPPING SHEETS. 



omui The basaltic Bows of the capping sheet of Table Mountain 



ivsi ii i >• hi Denver strata, and they are believed to have been poured oul 

 upon the sen bottom al n time in the Denver epoch corresponding to their 

 preseul horizon in that formation. The reasons for this conclusion have 

 been given in full, the mosl weighty of the considerations being the 

 existence of the earlier smaller basall bodies which are undeniably of 

 Denver age. The capping is made up of two flows, the lower of which 

 has a maximum thickness of about 125 feet, while the upper one has been 

 so subjecl i" erosion thai its former thickness can not now be estimated, 



though fr the remnanl on North Table Mountain ii can safelj be 



considered to have been much greater than thai of the lower How. In the 

 lower Bheel ni;i\ be found almosl all the structural phases of basaltic 

 lava flows, and the upper bodj corresponds to the same standard as far us 

 can be seen. The upper Burface oi the lower How is very porous, or even 

 scoriaceous, with a cracked and broken appearance in places. For In feel 

 below the surface the rock is vesicular, some cavities being several feel in 

 diameter, The central portion is uniformly massive and compact, while a 

 narrow porous zone is found adjoining the lower contact. 



It is nut deemed of great importance to the 

 present discussion to know whether these basaltic lavas were poured out 

 into shallow seas or not. The water could not have had greal influence 

 upon the course of consolidation after ;i firm crust was formed, though the 

 pressure of confined steam assists in explaining the occurrence of very 

 vesicles al rare intervals among the smaller ones, Whether cooling 

 in water or not, the earlier flow was certainly covered by the lava from a 

 second eruption before its jagged surface w;is appreciably modified l>v 

 erosion and before either sediments or otherwise-formed deposits could 

 lodge upon it. The identity of lavas and their juxtaposition suggesl thai 

 \ er\ possibly the firsl lava was covered l>\ the second before complete 

 consolidation, In such a case the rate of cooling in the two bodies was 



