THE ARGH^AN ROCKS. 495 



N. W. This rock is exposed in a low lodge on the east side of the river, Feet. 

 almost immediately succeeding the granite. It shows also, on a small 

 rock in the middle of the stream, where a distinct contorted lamination is 



observable. Horizontal width 60 



III. Magnenimi slate (516, 522) ; pale-gray, Ught-greenish-grey, dark-greeri, oc- 

 casionally pink, or even bright brick- red, from presence of iron sesquioxide; 

 sometimes quite soft, at others quite hard and gritty, from the presence of 

 fine granulax- quartz, which appears never to be entirely absent; highly 

 schistose, the laminae striking N. 60° W. and dipping N. E. 70°. The 

 magnesian mineral appears to be talc in the light-green kinds, and chlo- 

 rite in the dark green, the former kind much predominating, and nover- 

 sharply defined from the other. The hght-gray to nearly wliitish kinds are 

 the most silicious, and most firm, the others showing much tendency to 

 crumble and decompose. This is especially so with those that are stained 

 bright-red, their contained oxide of iron arising from the oxidation of py- 

 rite, which sometimes is to be seen stiU unchanged, in minute cubes. These 

 schists are exposed on the east side of ' tlie river, on a nearly perpendicular 

 bank, 100 feet in height, which forms the western end of "Tilden's Iron 

 Mound." About 75 feet along the river bank from the lower end of the 

 exposure, a bright-red layer, 30 feet thick, occurs, in which hematite forms 

 a prominent constituent, the sm-faces of some of the laminae having even a 

 bright specular lustre (522) and in which nests and seams of porous, iron- 

 stained quartz are quite abundant. At one time this ferruginous schist 

 was mined as an iron ore. Averaged specimens from it jdelded respectively 

 9.81 . 28. 13 and 31 .27 per cent, of metallic iron; the first representing a thick- 

 ness of 24 feet, near the water's edge, the second the same thickness at an 

 elevation of 30 feet above the river, the third, 6 feet, more ferruginous 

 than the rest, immediately next below (stratigraphicaUy) the preceding 

 layer. The six feet layer does not continue the whole height of the bank. 

 The length of the exposure of tliis rock along the river bank is about 700 



feet, its horizontal width about 200 



I"V Covered: on Tilden's Iron Mound ■ • 550 



\. Ferruginous quattz- schist: finely laminated, varying from a hght gray, 

 somewhat ferruginous quartz-schist, to a dark-colored higlily magnetic 

 rock; in many places weathered brownish, iron-stained, partly crumbling. 

 Tins rock is exposed in a low outcrop on the water's edge on the east bank 

 of the river; in a similar low outcrop on the west.bank; and again (appar- 

 ently the same layer) some distance east from the river on the north flank 

 of the Iron Mound, at points indicated on the map. The first exposure 

 shows a very much decomposed, crumbly, brown-stained, non-magnetic 

 rock, containing 42.32 per cent, of metaUic iron. The outcrop on the west 

 bank of the river is somewhat larger, showing a material similar to that 

 on the east side, with a very plain N. 50° W. strike, and N. E. dip. A 

 sample across the whole width of 30 feet yielded 35.96 per cent, of iron. 

 The exposures on the Iron Mound are partly artificial, a considerable quan- 

 tity of the rock having been removed for smelting purposes. In one of the 

 two main openings the rock or ore is brown-stained and magnetic, contain- 

 ing 34.22 per cent, of metallic iron; m the other, somewhat deeper in the 

 hill, a much less oxidized material is seen. Of this, the innermost portions 

 present a dark-gray to neariy black appearance, and exceedingly fine- 

 grained texture, being composad of alternating darker and hghter colored 

 (more quartzose) bands, but having this bandmg much less prominent than 



