SACRAMENTO AMPHITHEATER. 141 



springy meadows and then closes together, as it bends to the southward, 

 between gravelly ridges which are evidently the remains of former 

 moraines and which extend below the junction with Little Sacramento. 

 Owing to the dense growth of forest on these ridges, however, the actual 

 lower limits of the glacier are not easily determined. About a mile above 

 the line of the fault the narrow bottom of the present stream ends in shelf- 

 like terraces of white sandstone, above which the valley opens out into the 

 broad basin of the Sacramento amphitheater. On the face of this terraced 

 wall, and about opposite the western point of Pennsylvania Hill, are two 

 dolomitic limestone strata: the lower one, a dark-gray semicrystalline rock, 

 with clayey seams, is about ten feet in thickness; the second, sixty feet 

 above this, is only six to eight feet in thickness, of similar color and also 

 associated with clay shales, the intervening beds being of coarse Weber 

 sandstones. Among the fossils found here were identified 

 Prodnctus costatus and Athyris subtilita. 



Ascending the stream farther, successive beds of white sandstone are 

 crossed until the great body of Sacramento Porphyry is reached, which in 

 a probable thickness of twelve hundred feet forms either wall of the amphi- 

 theater. The upper extremity of the amphitheater was not explored, but 

 from information and specimens furnished by Mr. J. T. Long sufficient 

 evidence was obtained to justify the indication of an outcrop of Blue Lime- 

 stone below the Sacramento Porphyry at its deepest part. The fossils 

 obtained by him from here, besides the uncharacteristic Athyris subtilita, 

 included the new Spirifera, like Spirifera Kentuckensis, which has not yet 

 been found at a higher horizon than the Blue Limestone. Among minerals 

 small yellow crystals of pyromorphite were found with the specimens of 

 ore obtained from this horizon. 



London Hill. The line of the London fault crosses London Hill diago- 

 nally about seven hundred feet west of the summit, in such a manner that 

 the greater part of the steep northern slopes is occupied by Archean rocks, 

 with only the extreme eastern end made up of easterly dipping quartzites of 

 the Weber Grits formation, whereas on the south side the latter extend over 

 two-thirds of the lower slopes. From the saddle north of Mosquito Peak 

 the London fault runs southeast to a point in the raised basin north of the 



