130 GEOLOGICAL SUEVEY OF THE TEREITORIES. 



tliey still suffice to show from whence the lava came. From near this 

 point the lava flowed toward the then lowest portions of the country, 

 and, in virtue of its hardness, formed a protective cap to the softer beds 

 beneath, which could withstand erosion better than the surrounding 

 unprotected hills. The latter have, therefore, wasted away the faster, 

 and the former valley has become the mountain. The. thin edges of the 

 lava-sheets have also been gradually worked backward, and broken 

 away, and the limits of the tables are constantly becoming more and 

 more circumscribed. 



The mare compact varieties of the lava vary from brown to black in 

 color, and from a very compact to a fine-grained texture, being rather 

 dull in luster, (basalt.) In this as a matrix there may or may not be 

 porphyretically imbedded numerous black and well-formed crystals 

 of augite, giving a doleritic character to the basalt, while yellow decom- 

 posing spots of an undetermined mineral, probably olivine, are often 

 present. These varieties are columnar in structure. Very vesicular or 

 scoriaceous lava also occurs, as well as amygdaloidal material. At one 

 point the latter had a light gray or j^Lirplish base, granular, with im- 

 bedded augite crystals, the numerous irregular cavities being filled 

 with zeolitic minerals, Ghahazite, leucite, natrotile, and others are said 

 to occur. 



IsTear the north end of the north mountain the edges of several of the 

 harder lava-flows show well as lines of cliffs or j^alisades running along 

 the hill-side, separated by the slopes of softer rock between. A sec- 

 tion was here observed as follows : 



Feet. 



Cliff at top. — Dark columnar doleritic basalt 40 



Slope. — Scoriaceous basalt and amygdaloidal dolerite 30 



Palisade. — Columnar basalt , 60 



Slope. — Debris of scoria and volcanic sand 100 



Palisade. — Columnar basalt 30 



Slope. — Covered to base. 



These lava-flows are not widely continuous, some extending much 

 farther than others. The northern end of South Table Mountain rises 

 about 600 feet above Clear. Creek, the volcanic products reaching an 

 average thickness of about 125 feet. Immediately below the lava the 

 lignitic beds at several points yield many fossil-leaves. Besides the two 

 tables, and extending from near them in a line trending a little west of 

 north to the bend of Ealston Creek, near the Murphy mine, are four 

 rounded buttes topped with the same lava ; whether an extension of 

 the dike through which the lava has been erupted, or remnants of a 

 lava stream from the Korth Table, was not ascertained, though it is ap- 

 parently the latter. 



Green Mountain, or Mount Hendricks, is a bulky hill situated a few 

 miles south of South Table Mountain, and, like it, isolated from the 

 main range by the zone of hog-backs. Below it also is made up of lig- 

 nitic strata, while above occur large bowlder-beds of rolled volcanic 

 rocks, showing, in its northeast side, that it is apparently an extension 

 of the same lava that caps the South Table Mountain. 



Yalmont. — The only remaining instance of the occurrence of volcanic 

 rock east of the mountains is the dike at Valmont. This dike was 

 examined by Dr. Hayden, and a description and illustration will be 

 found in his report. Being within my district, however, I will mention 

 it briefly, the facts being gathered from the notes of Dr. Hayden and 

 Mr. Holmes. 



The dike is vertical, and appears as a wall-like ridge rising abruptly 



