310 Scientific Intelligence. 



up on these streams the Rampart rocks come to the surface, and 

 the Fortymile rocks are supposed to be uncovered at their very 

 heads. Between the two areas of Tertiary rocks the Rampart 

 rocks occupy a belt ] 5 to 20 miles wide along the river, and are 

 cut by great dikes of intrusive granite. 



From Fort Hamlin up to near Circle City, a distance, neglect- 

 ing curves, of about 200 miles, the river flows through a perfectly 

 flat region covered by fine silts and gravels, known as the Yukon 

 Flats, in which no outcrops of solid rock have been observed. In 

 the Birch Creek district, around the headwaters of Birch Creek 

 and southwest of Circle City, the Birch Creek series occupy a 

 broad area; their general strike is east and west, curving at 

 either end to the northward, and the prevailing dip is between 5° 

 and 30° to the south. There is, however, evidence of a northern 

 dip as well, and the Fortymile schists and marbles rest upon 

 them along the trail to Circle City. Marbles, probably belonging 

 to the Fortymile series, are also reported in the hills between 

 Birch Creek and the Tanana to the southward. 



At the crossing of Birch Creek by the trail from Circle City, 

 and along the Yukon River for 30 or 40 miles above the Yukon 

 Flats, rocks with the characteristic dark coloring of the Rampart 

 series are exposed. From these up to the mouth of Mission Creek 

 rocks of the Tahkandit, Mission Creek, and Kenai series occupy 

 the banks of the river. On Mission Creek itself only these later 

 formations are found, but the gold in the gravels is supposed to 

 come from the conglomerates of the Mission Creek series, which 

 contain pebbles ot the older rocks. On American Creek, the 

 main branch of Mission Creek which comes in Jrorn the south, 

 the dark rocks, shales, limestones, and tuffaceous beds which form 

 the bed-rock are supposed to belong to the Rampart series, which 

 also occur along the Yukon River from 5 to 10 miles above 

 Mission Creek to within 25 miles of the mouth of Fortymile Creek. 

 Above this to some distance above Fortymile Creek the river 

 runs in beds of the Mission Creek series. 



It is in the Fortymile district and the adjoining mining district 

 on tributaries of Sixtymile Creek that the relations of the differ- 

 ent gold-bearing series are best seen. Here there is an east-west 

 axis or backbone running parallel to the upper part of Fortymile 

 Creek and along the divide between it and Sixtymile Creek, with 

 quartzite-schists of the Birch Creek series resting immediately on 

 it both to the north and to the south. Above these on either 

 side are the marbles and alternating schists of the Fortymile 

 series. Fortymile Creek below the forks runs for a considerable 

 part of its course along the junction between these two series, on 

 the northern flank of the anticline. Dikes of various eruptive 

 rocks, including intrusive granite, are very abundant, especially 

 on the South Fork. On the upper part of this fork are green 

 tuffs and slates of the Rampart series, overlain unconformably by 

 conglomerates, sandstones, and coaly shales of the Mission Creek 

 series. Both the South Fork and Sixtymile Creek are supposed 



