290 GEOLOGY OF THE DENVEK BASIN. 



THE EARLIEST BASALTIC OUTFLOW, 



The capping sheets of Table Mountain do not represent the earliest sur- 

 face flows of basalt in this region, for at various points on all sides of North 

 Table Mountain, at a horizon which is quite uniformly 100 feet below the 

 capping sheet, there appear typical streams of basalt. The present outcrops 

 of these bodies form little cliffs on the edges of the benches occasioned by the 

 presence of the eruptive rock, and all exposures seem to be sections of small 

 streams. None of them reach as far south as South Table Mountain, and 

 no dikes of exactly corresponding rock are known, for although a thorough 

 examination of this type shows it to be petrographically identical with that 

 of the more recent sheets, still it can always be recognized at once by its 

 numerous augite crystals, which lie in a much darker and denser ground- 

 mass than is presented by the other type. The map shows the position and 

 relative size of the six outcrops of this earlier rock that have been examined. 

 By far the largest of these bodies appears below the high cliffs at the north- 

 east extremity of North Table Mountain, forming itself a bench and minor 

 cliff, distinguishable even from Denver. The southern end of this outcrop 

 is well exposed In a gully which cuts across it. On the northern side of 

 this small ravine the mass is shown as it rapidly thins out. The whole 

 lace here exposed is rough and very irregularly porous, with an almost 

 SCOriaceous crust of the same structure that is shown in the upper sheets. 

 South of the gully a thin arm, 2 or 3 feet thick, consisting entirely of very 

 vesicular lava, extends tor some distance. To the north the body thickens 

 rapidly and soon reaches a vertical thickness of 50 to CO feet, which is 

 maintained nearly to the extremity shown by the map on the north side of 

 the mountain. When it reaches this thickness the mass lias a structure 

 much like that of the higher sheets. A porous zone adjoins the strata 

 below, succeeded by massive rock in the center, and a thicker porous zone 

 comes at the top. There are, however, no large cavities in these bodies, 

 and they seldom contain other minerals than calcite and a green substance 

 allied to delessite. There is a rude, vertical, columnar structure on the 

 cliffs, and locally a spherical sundering. 



On the western slope of the northeastern portion of the mountain is a 

 small outcrop of similar character, whose extremities are covered by debris. 



