286 GEOLOGY OF THE DENVER BASIN. 



very distorted shapes. At about 115 feet above the tuff the upper limit of 

 the lower sheet is reached. The rock is here exceedingly porous, or even 

 scoriaceous, and the surface of the flow is rough and jagged, resembling 

 markedly that of many modern lava. streams. Resting immediately upon 

 this rough surface is a second sheet of basalt, identical with the first in all 

 respects so far as can be known. There could have been but a short 

 interval between the two flows, as no foreign substance appears between 

 the sheets, and the jagged surface of the lower body shows no marks of 

 erosion. The second flow was undoubtedly thicker and probably of greater 

 lateral extent than the first, for, although as much as 140 feet of the second 

 sheet may be seen in some portions of the cliffs, as at the southwest point 

 of North Table Mountain, yet nothing corresponding to the amygdaloidal 

 zone of the lower one remains. 



The line between the two sheets is usually marked by more or less of 

 a bench or break in the cliffs, specially noticeable along the southern face of 

 North Table Mountain, a feature which makes the study of this zone practi- 

 cable at numerous points. One of the best of these spots for the observation 

 of the amygdaloidal zone is on the south face of the mountain, only a few 

 yards west of the projecting point on the west side of the large gulch 

 which here cuts into the table. The top of the debris slope here reaches 

 up to a zone containing many large cavities partially filled with zeolites, 

 and a small indentation in the cliff above makes it easy to examine the 

 entire thickness of basalt here exposed. The greater number of the 

 zeolites which have been described from this locality were obtained here, 

 as earlier blasting had already opened many cavities in fresh rock. 



On the little bench indicating the contact zone the upper portion of 

 the lower sheet is seen to be a somewhat confused mixture of massive and 

 porous material, similar to that shown on a much larger scale in Castle 

 Rock, South Table Mountain. There are many angular cavities as well as 

 the normal vesicles. The actual surface of the lower sheet is here quite 

 scoriaceous. Resting upon this rough surface is the upper flow, which at 

 the contact is also very porous and contains a tew small cavities for 10 feet 

 upward. A common feature of the porous semibrecciated portion of the 

 lower sheet is the appearance in the cavities or fissures of a red zeolite in 



