THE ARCH^AN ROCKS. 475 



19. Hornblende rock (859>^): very fine-si-aiaed, highly ciystallke, dark green- "^''''' 



ish-black; contains much of a highly lustrous black mineral (hornblende), 

 pyntiferous; adheres slightly t» the magnet in a fine powder. See also Mr. 

 Wright's microscopic examination, Appendix B 50 



20. Gneiss: resembUng that of 18; at fii-st obscurely bedded, then at 60 feet 



more distinctly bedded, the strike being N. 75° E., and the dip 53° S. E. 



At 100 feet 3 gi-anite veins, each 3 inches thick, occur 120 



21. Covered ,r, 



40 



Fig. 10. 



-2 01= 



Gbahitb Vein in Gneiss at Gband Rapids. 



22. 



S3. 

 24. 

 25. 

 26. 

 27. 



Granite^: fine-grained, light pinkish, slightly gueissoid; jointed. The three 

 ingredients are aU. perceptible with a magnifier. The mica shows some 



tendency t» a stringy arrangement 8O1 



Covered 45 



Granite: same as last described 35 



Hornblende rock: strike N. 85° E., dip 75° S 10 



Covered 150 



Gi-anite: pinkish, gneissoid 60 



Total horizontal length of section 2, 519 



Approximate thickness of rocks exposed 2, 200 



Prom the foregoing details we may construct the generalized section of Fig. 11, the red 

 granite of portions of the section being regarded as an intrusive rock. 



At the first dam below the wagon bridge at Centralia, on the west side of the river, 

 a ledge of hornblende rook, 40 feet wide, occurs. This rock (884) is moderately coarse- 

 grained, dark-colored to black, and appears to be composed of liornblende and a wliite 

 felspar with much magnetite, the latter very distinctly visible, by the aid of the magni- 

 fier, in lustrous grains. Quite coarse pieces are Ufted by the magnet. On the river 

 bank a short distance up stream, micaceous gneiss is exposed, fonning apparently the 

 next layer. These beds are higher in the series than any of those in the section at the 

 rapids. 



At the paU null, just below Centralia, is a large exposure of fine-grained, pinkisli, 

 gneissoid granite, containing much reddish felspar, and fine, glistening, greenish mica, 

 with a stringy arrangement. 



