EXPLAXATIG^r OF SECTIONS. 265 



theoretical prolongations of observed dips. The depth of the Blue Lime- 

 stone horizon at the east end of the section is probabl}^ a maximum. 

 Analogy renders it probable that the eastward dip shallows, and it is pos- 

 sible even that the beds rise somewhat towards the Mosquito fault. This 

 remark applies equally to the corresponding points in the next four sections. 

 The sheet of Sacramento Porphyry is represented here between the Weber 

 Shales and Weber Grits, the horizon at which it occurs on the crest of the 

 range east of the Mosquito fault, since it is fair to suppose that the sheet 

 extended as far west as indicated on the sections. The thickness of the 

 Mount Zion Porphyry on the west slope of Prospect Mountain can only 

 be a matter of conjecture; it is fair to infer that it reaches at least 700 feet 

 in its maximum development. The extension of Lake beds as far north as 

 the line of this section in the Arkansas Valley is proved by excavations on 

 the north bank of the stream. At the western end of the section the shore 

 line against the Ai'chean is shown in the abrupt termination of the Lower 

 Quartzite. This would have been more striking had the section line been 

 placed a little farther north, when the White Limestone would have been 

 found to come in actual contact with the Archean. 



Section B. — Scctiou B follows in its western course the bed of Evans 

 gulch ; then, cutting across the southern spur of Prospect Mountain below 

 the Prospect amphitheater, follows approximately the line of Little Evans 

 gulch, and, ci'ossing the two gulches diagonally just above their junction,^ 

 runs out on to the mesa at the intersection of the railroad line with Evans 

 gulch. It thus shows portions of the Evans north moraine and, at its 

 crossing of Evans gulch, the supposed shore-line of Arkansas lake. The 

 eastern half of the section shows practically the same structure as Sec- 

 tion A, except that the Weston fault comes in in the axis of the broad 

 anticline. In its western half it cuts across the northern extension of the 

 Yankee Hill and the Big Evans anticlines, and of the included Little Stray 

 Horse syncline; and west of this shows the probable slope of the beds in 

 the S3mcline beneath the mesa, as proved by the explorations of the Oolite 

 and Sequa shafts; also, the White Porphyry, cutting across the Blue Lime- 



1 On the section its intersection with Little Evans gulch is wrongly marked " ditch." 



