292 GEOLOGY OP THE DENVER BASIN. 



Streams were poured out upon a shallow sea bottom near the shore-line. 

 As for the larger streams above, their structure is also emphatically that of 

 surface flows, and no reason is known for objecting to the view that their 

 time of eruption followed that of the lower streams l>v the period occupied 

 in the deposition of the intervening LOO feel of Denver strata. It is true 

 that there is nothing in the present position and appearance of the upper 

 sheets of Table Mountain which might not be satisfactorily explained upon 

 the supposition that the surface upon which the basalts were poured out 

 corresponded to a comparatively recent surface of erosion. But when a 

 lava of almost identical composition and so related in occurrence can he 

 shown to be of the Denver epoch, there would seem to be no reason for 

 claiming two widely separated periods of eruption of the same rock in the 

 same small area. At least the burden of proof would seem to rest with 

 any supposed advocates of the latter view. As a matter of fact, nothing is 

 known incompatible with the former. The fact that the basalt sheets are 

 not known in Green Mountain at a horizon corresponding to that of Table 

 Mountain is simply explained by assuming the sheets to have been of very 

 limited extent, an assumption also necessary in case the second view is 

 adopted. 



THE ZEOLITIC MINERALS OF TABLE MOUNTAIN. 



General occurrence. — The minerals occurring in the amygdaloidal zone of 

 the I'alile Mountain basalt have already been described in some detail 

 by Dr. W. F. Hillebrand and the writer, 1 chiefly from the purely mineral- 

 ogical standpoint. In this place some further facts of the mode of occur- 

 rence will We given, with suggestions as to the genesis of certain unusual 

 varieties. 



The list, of species identified embraces the following: Analcite, 

 apophyllite, chabazite, laumontite, levynite, mesolite, natrolite, scolecite, 

 stilhitc, thomsonite, calcite, and hole. All of these species except the 

 bole appear in white or colorless crystals lining the various cavities in 

 the basall in more or less distinctly concentric layers, representing different 

 periods of formation. Several of the species are often found together in 



Bull. CJ. S. Geol. Surv. No. 20, Contributions t<> the geology of the Rocky Mountains, p. 13, 

 k lB0,Am. Jour. Sei. (3), Vol. XXIII, pp. 452-458, Vol. XXIV, pp. 129-138, June and August, 1882. 



