THE ARCH^AN ROCKS. 49I 



beneatli may bring them up through to tho surface at any point. The same is true to 

 some extent of the region between Yellow river and the Wisconsin, but here the sand- 

 stone does not extend nearly so far north. 



In Hemlwk Creek, at the crossing of the wagon road from Grand Rapids to Dexter- 

 ville, N. E. qr. of the S. S. qr. of Sec. 5, T. 22, R. 4 E., are ledges of rather fine-grained, 

 flesh-colored, gneissoid granite (967). Translucent, wine-colored quartz, and pinldsh 

 orthoclase in small, brilliant facets, make up most of the rook; the mica is sparse, in 

 fine, green-black flakes, which have a distinct linear arrangement. This rock is a hand- 

 some one, and would probably dress well, though showing some tendency to weather 

 and iron-stain. The bedding directions appear to show a strike of N. 60' E., and a dip 

 of 70" S. E. 



On Yellow river itself, the southernmost Archsean exposure is to be seen about two 

 miles north of Dexterville, in the N. hf . of Sec. 14, T. 22, R. 3 E. The rock here is 

 medium-gi-ained, pinkish, quartzosc, gneissoid granite (973), composed chiefly of hmpid 

 quartz and orthoclase felspar, the former the most abundant. Mica is present in fine 

 black scales arranged in parallel lines. The strike appears to N. 55° W., and the dip 

 60° S. W. Near the top of the river bank, which rises directly from the granite, thin- 

 bedded, friable, horizontal sandstone is exposed. 



On Sec. 3, T. 22, R. 3 E., three miles north of Dexterville, there are large flat 

 ledges in the bed of the river, of gneiss, bounded on the north by quartz-porphyry. The 

 gneiss (969 and 971) is very fine-grained, laminated, dark gray to black in color, and 

 consists of a black mineral (mica, hornblende, or both), in small brilliant flakes; and 

 whitish quartz and felspar. Its weathered surface is earthy and of a dirty white color, 

 but shows the fine lamination even more distinctly than the interior. The quartz- 

 porphjTy (970) consists of a hght greenish-grey, aphanitic matrix, having the peculiar 

 flaky appearance that is characteristic of the quartz-porphyries of the various isolated 

 Archaean patches of Wisconsin, in which are imbedded somewhat sparsely scattered 

 facets of pinkish orthoclase felspar up to one sixteenth of an inch in diameter. It is a 

 very tough, compact, rock; and is worn by the running water into smoothed and 

 polished surfaces. This porphyry appears to penetrate the adjacent laminated rock in a 

 very irregular manner. In one place amass of the gneissoid rock, some 50 feet in di- 

 ameter, is nearly surrounded by the porphyry, the lines of jimction between the two be- 

 ing very sharp, and rendered especially noticeable by the different appearances of their 

 weathered surfaces. The lines of junction are not curved, but straight, beavmg, re- 

 spectively, N. 70° W., N. 30° B., and N. 70° W.; tlie first and last on opposite sides of 

 tlie enclosed mass. The strike of tlie gneiss Ls N. 25° W., its dip 60° N. E. The. 

 porphyry is 20 to 30 paces wide, and appears to be bounded on the north by the some, 

 gneiss as before, with the same bedding. Beyond, porphyry agam comes in. 



At Pitts' Mill, five miles north of DexterviUe, on Sec. 34, T. 23, R. 3 E., are very large 

 exposures on Yellow river, wliich passes here through a naiTOw rocliy gorge, of a very- 

 beautifully and coarsely banded gneiss (993), tlie bands being alternately dark-gray to 

 black, and bnght pink, and having a nortlwest chrection. Tlie dark-colored bands 

 predominate, and run from % inch to one or two feet in breadth, but when so broad, are 

 rarely free from fine lines resembHng the material of the pinlc bands, which i-un in 

 width from these fine lines up to 6 or 10 inches. Tlie dark-colored portions are fine- 

 grained, with an intimate paraUel structure, and consist predommatingly of fine green- 

 Lsh-black mica, with which are seen fine wliite and red felspar facets, and some fine, 

 quartz. In places, greenish black, cleavable hornblende appears to partly replace the 

 mica. The red bands consist chiefly of coarse-grainod oriihodase felspar, with some 

 Umpid granular quartz and occasional blotches of mica, and show numerous whitish 

 kaohnized patches, the whole rock having a tendency to decompose. Several folds in 

 the strata occur, and are rendered especiaUy striking by the veiy marked red and blaxik 



