164 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



which weathers in large shaly blocks, with a remarkably beautiful con- 

 choidal fracture and the peculiar sherdy habit which is common among 

 volcanic rocks. The rock is white, slightly tinged with reddish yellow, due 

 to minutely disseminated particles of hydrated oxide of iron. In the fresh 

 fracture it shows a white granular homogeneous mass, with occasional grains 

 of feldspar. It was first thought to be a later eruptive rock, probably a 

 rhyolite, but careful microscopical study shows it to be a true White Por- 

 phyry, differing in no essential from the normal type. On Lamb Mountain, 

 as shown in the sketch, Plate XVIII, this body has a maximum thickness 

 of about four hundred feet at the summit of the hill, its lower limit corre- 

 sponding in general with the bedding plane of the underlying sandstone. 

 This correspondence, however, on close examination, is not absolute, inas- 

 much as it occupies a slightly lower horizon to the eastward, and on the 

 north face of the ridge just west of the ravine between Lamb and Sheep 

 Mountains it can be seen to cross the beds nearly at right angles, in the 

 form of a dike. On the steep east side of Lamb Mountain toward the 

 saddle are beds of slate and micaceous sandstone, curving up at an angle of 

 50 against the eruptive mass. In these slates were found abundant im- 

 pressions of Equisetce, or Horsetails, a plant characteristic of the Coal Meas- 

 ures. Sandstone outcrops can be traced on this saddle and across it to the 

 base of the steep western slope of Sheep Mountain, where they soon dis- 

 appear beneath the plentiful ddbris of White Porphyry. The White Por- 

 phyry from which they come is, as will be shown later, the body which 

 belongs above the Blue Limestone; therefore the fault line must run very 

 nearly at the foot of this steep western slope. That the Lamb Mountain 

 body is itself a small laccolite, with a separate vent or channel, is evident 

 from the fact that it ends abruptly on the east and that, while the steeply- 

 dipping beds rest against it on the saddle east of the peak, lower down the 

 slope of the hill the dike-like channel, which extends downward from the 

 main mass of porphyry, is found to cross the shallow-dipping sandstone strata 

 without perceptibly changing their angle. The steepness of the beds on the 

 saddle might be explained by the expansion of the intrusive body of por- 

 phyrv, which would push them up, but this explanation is rendered unnec* 

 essary, since, as we have already seen, the beds immediately adjoining the 



