PENNSYLVANIA HILL. 147 



The correctness of this assumption, so iar as tlie horizon of the beds goes, 

 has been proved by analogy in other localities, as will be described later, 

 notably in the case of Weston fault on Empire Hill, where similar structural 

 conditions exist, but with less intense alteration of the beds adjoining the 

 fault, and where, moreover, the strata of the Upper Coal Measure formation 

 were recognized definitely not only by their lithological characteristics but 

 by abundant fossil remains found in them. The dividing line between the 

 great silicious series of Weber Grits and the Upper Coal Measure formation 

 having been arbitrfrily assumed at the first development of calcareous 

 beds, this line has been drawn on the map at the base of the lower bed of 

 limestone mentioned in the above section. A thickness of something over 

 one hundred and fifty feet of Upper Coal Measure beds is thus assumed to 

 have escaped erosion on the western side of the fault. 



On the south wall of Pennsylvania Hill, facing Big Sacramento gulch, 

 the beds Avhich outcrop are practically identical with those on the north 

 wall. They preserve the same strike of N. 20° W., with a dip of 20" to the 

 east. The steepening of the dip as they approach the fault line is, however, 

 not so apparent on the north wall of the hill, the surface being to a still 

 greater extent obscured by dtibris. Near the line of the fault the wall, as 

 on the north side, is scored by a shallow ravine, on whose steep slopes fivag- 

 ments of White Porphyry are mingled with those of almost equally white 

 quartzite. The former belongs evidently to the same body mentioned 

 already as occurring on the north wall to the west of the assumed line of 

 fault. Owing to the uncertainty which exists with regard to the structural 

 relations of this body of White Porphyry, it has not been indicated either 

 on the map or section. 



Sacramento arch. — The soutli Wall of Sacrameuto gulch, a sketch of which 

 is given on Plate XV, presents an even more interesting study of the great 

 London fault-fold than that of Pennsylvania Hill. The cliff" section, as the 

 sketch shows, presents a broad and rather flat arch, which has but little 

 resemblance to the sharp S-fold already indicated on London Hill or to that 

 which can l)e distinguished in the background of the sketch on the north face 

 of Sheep Mountain. At first glance the curve on either side of the arch seems 

 to be nearly equal in degree; but a more searching examination discloses on 



