234 GEOLOGICAL SUEVEY OF THE TEKEITOEIES. 



At the head of Four-Mile Creek are two branches which head in 

 Horseshoe Mountain. Each of these small streams heads in an amphi- 

 theater, the center of which is granite, uj)on which rest the quartzite 

 and limestone given in the section above at the lower part. They form 

 an arch, as is seen in the illustration, and it is this that gives the nam© 

 to the mountain. Between the two amphitheaters is a point which is 

 capped in the same manner. The limestone and quartzite extend up 

 on the divide, and it is in them that the silver-bearing galena is found. 

 They are very much metamorphosed and somewhat broken up by veins 

 of volcanic material. In Empire Gulch, which, with Iowa Gulch, heads 

 directly opposite Four-Mile Creek, these beds outcrop again. Iowa 

 Gulch affords the best exposure, however. Fig. 2, Plate IX, repre- 

 sents the outline along the south side of Iowa Gulch, continuing the 

 section that we made on Four-Mile Creek. We see, then, that at the 

 head of the gulch another fault, as great as the one noticed on the divide 

 as the head of Mosquito Creek. It is in fact a continuation of the same 

 fault. A section through the hill, marked B in Fig. 2, Plate IX, is 

 represented in Fig. 3, Plate XI, the figures corresponding with those 

 in the following section : 



Section A. 



1. Volcanic, capping the hill. 



2. Black limestone, very much metamorphosed. 



3. Thin layer of quartzite. 



4. Volcanic rock, columnar. 



5. Bed of quartzite. 



6. Volcanic, columnar. 



7. Quartzite. 



8. Volcanic, columnar. 



9. Fragment of quartzite bed. 



10. Quartzite. 



11. Volcanic, columnar. 



12. Fragment of quartzite. 



The quartzite from a distance has a yellow color. The base of the hill 

 is covered with debris, but underneath I think we would find gneiss. 

 The beds of volcanic rock (which is a variety of trachyte) are beautifully 

 columnar. Beds 7, 9, 10, and 12 are fragments that seem to have been 

 caught in the flow of volcanic material. Figure 2, Plate XI, represents 

 a section through a hill about half a mile west of the hill B, on the north 

 side of the creek, between the main creek and a small branch. Here we 

 have the following section : 



Section B. 



1. Volcanic. 



2. Blackish limestone, very much metamorphosed. 



3. Yellowish quartzite. 



4. Volcanic, in fine columns. 



5. Gneiss. 



6. Volcanic. 



The rest of the hill is covered to the base. In the gneiss, layer 5, 

 there are seams of the volcanic rock. A section through the hill A, 

 which is the same hill as A in Fig. 1, Plate IX, is shown in Fig. 1, Plate 

 XI, corresponds to the following : 



