THE ARCH.EAN ROCKS. 



19. Hornblende rock (859%); very fine-grained, highly crystalline, dark green- 



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

 pyritiferous; adheres slightly to the magnet in a fine powder. See also Mr. 

 Wright's microscopic examination, Appendix B 



20. Gneiss: resembling that of 18; at first 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 granite veins, each 3 inches thick, occur 



21. Covered 



FIG. 10. 



475 



Feet. 



120 

 40 



GRANITE VEIN IN GNEISS AT GRAND RAPIDS. 



22. Granite: fine-grained, light pinkish, slightly gneissoid; jointed. The three 

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



tendency to a stringy arrangement 80 



S3. Covered 45 



24. Granite : same as last described 35 



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



26. Covered 150 



27. Granite : pinkish, gneissoid 50 



Total horizontal length of section 2, 519 



Approximate thickness of rocks exposed 2, 200 



From 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 rock, 40 feet wide, occurs. This rock (884) is moderately coarse- 

 grained, dark-colored to black, and appears to be composed of hornblende and a white 

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

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

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

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

 rapids. 



At the pail mill, just below Centralia, is a large exposure of fine-grained, pinkish, 

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

 with a stringy arrangement. 



