164 GEOLOGY OF THE DENVER BASIN. 



tridymite. Compact porphyritic structural types prevail, although even- 

 grained and porous rocks arc also abundant. 



This stratum, while the most persistent known in Table Mountain, 

 may, nevertheless, be also used as an excellent illustration of the lateral 

 variation in composition common to all or nearly all the conglomeratic and 

 sandy beds of the formation. On following this conglomerate toward 

 Castle hock, it is found to decrease rapidly in thickness, and at the same 

 time the size of the pebbles becomes less. The horizon can he traced 

 for sonic distance uninterruptedly, is then covered for an interval, and it 

 can scarcely be identified with the equivalent sandy layers shown on the 

 south face of Castle Rock. There are in the latter outcrops sonic sandy 

 beds containing small pebbles, but not in sufficient quantity to constitute 

 a conglomerate, nor in so great a quantity as may he found in many 

 other horizons where no conglomerate proper is ever developed so far as 

 known. In spite of this described variation the horizon is exceptionally 

 well marked as a conglomerate, and at many points on both Table 

 Mountains, and on all sides, the clear equivalent of this stratum is to be 

 found at or within a few feet of the contact with the basalt sheet. 



Section at the northeastern point of South Table Mountain. "While tllC CXpOSlU'CS ahead V 



described give a clear idea of much that is characteristic in the series under 

 discussion, the lack of continuity in outcrops tails to bring out other impor- 

 tant peculiarities of the formation. This lack is in a measure supplied by 

 the succession of strata shown on the eastern slope of the northeastern 

 extremity of the mountain. There is here a continuous section of 170 feet 

 of strata, from the basalt downward. No other outcrop exhibits so great a 

 thickness of the lower beds of the formation. 



The succession of beds here seen gives a very good idea of the 

 constitution of all the Denver beds, except the heavier conglomerates. 

 Below this section there are several small outcrops at points down to the 

 ditch level. They are either of clayey beds, with hut little sand, or they 

 represent layers corresponding to 1 or 2 of the section. In the sand rock 

 included under 1 of the section, black biotite leaves are very prominent; 

 pebbles arc very rare here, and most of the mineral grains are angular. 

 The section is as follows: 



