428 



University of California . 



[Vol 3. 



DETAILED DESCRIPTION OP TERRACE REMNANTS. 



The figures of the heights of terraces given in the succeeding 

 pages were derived largely by a very rough method of level- 

 ing, but are sufficiently accurate for the purpose of this paper. 

 Through the courtesy of the superintendent of the hydraulic 

 mine of the Orleans Bar Gold Mining Company, Mr. Fred Hale, 

 I have been able to avail myself of information contained in a 

 blueprint topographic map of the company's property, com- 

 prising all of the Middle Basin north of the river. This map 

 bears evidence of being based on careful leveling, and by means 

 of it I am able to accurately determine the altitude and height 

 above the river of the different terraces in the Middle Basin. 



The following table will introduce the reader to the terrace 

 system : 



TERRACES OF THE ORLEANS BASIN. 



Height Depth to Bed-Roek, Height- 

 Above Kiver. Average. of Bed-Kock. 



S50 feet. 125 feet. 725 feet. 



675 feet, 70 feet. 605 feet. 



475 feet. 70 feet. 405 feet, 



120 feet. 30 feet.* 90 feet. 



70 feet. 25 feet. 45 feet, 



45 feet. 35 feet. 10 feet. 



The 850-Foot Terrace. — West of Sims Gulch (on the northern 

 border of the Middle Basin) and north of the ditch of the 

 Orleans Bar Gold Mining Company, is Donahue Upper Plat, 

 about 125 feet above the ditch level, and clearly the highest river 

 terrace in the basin. It has an area of eleven and one-half acres. 

 The altitude of the central portion is 1,326 feet. The surface 

 slopes very gently toward the southwest, but is quite regular. A 

 small creek has cut a ditch into it near the western end, and this 

 ditch, just above the company's ditch, shows ten feet of coarse 

 river gravel, containing thickly packed cobbles and boulders up 

 to a foot in diameter. The rock species are mostly greenstone 

 phases from up the river, but also a considerable percentage of 

 granite and dioritic rocks. The bottom of this gravel bed must 

 be at about the ditch level. The gravel is gray in color, but the 



*This figure of thirty feet thickness of alluvium applies only to the 

 outer border of the terrace; the thickness of the alluvium farther back 

 is much greater. In this respect this terrace differs decidedly from 

 the others. 



