THE LOWER SILURIAN ROCKS. 575 



below the fossil horizon, the rock (1,390) is somewhat the same, but often brownish and 

 containing numerous iron-stained cavities. It is traversed also in every direction by 

 films and veins of dark- brown and reddish black hematite. Some of the veins are as 

 much as an inch in width, and often show an interior cavity or " vug," lined with black, 

 dull nietallic-lustred, crystalline plates, which have a distinct cubical cleavage and red- 

 dish streak. Amongst the plates are concretionary balls, chiefly of the browner oxide, 

 %th inch in diameter, and made up of concentric shells. The structure of the crystal- 

 line plates indicates that the hematite has resulted from an oxidation of pyrite. 



One mile southwest from Rattlesnake Rock is another quite remarkable pile of rock, 

 lying in the midst of a large marsh. The summit is a flat, oval-shaped area, about 300 

 by 1,500 feet in size, the greatest length being in a N. 25 W. direction. The base is 

 about 35 feet lower than that of Rattlesnake Rock, and the summit is 155 feet higher, 

 or about 555 feet above Lake Michigan. On the east face the cliffs are 50 to 75 feet in 

 height; on the west, over 100 feet. At the northern end the rock is exposed for most 

 of the height of the bluff, being worn into towers partly separated from the main rock. 



A section of the bluff, taken chiefly on the east side, is as follows : 



Ft. In. 



1. Very fine-grained, non-friable but porous, yellowish, non-calcareous; dotted 



with fine shining scales of mica; made up of very sharply angular quartz 

 grains; weathering with a Light yellowish smooth surface; thin lamina- 

 tion indicated by fine lines, parallel to which there is a tendency to split- 

 ting; filled with minute fragmentary fossil impressions, chiefly of trilo- 

 bites; among these were determined Ptt/cJuispis (n. sp.), Conocepha- 

 lites minor, and Orthis Barabuensij; not found definitely exposed, but 

 lying in fragments thickly strewn over the surface of the bluff, which is 

 grassed, the rock being thus concealed; lying about 100 feet lower than 

 the fossil horizon on Rattlesnake Rock, and the same horizon on Roche 

 a Cris; not a mere local layer, because found again with exactly the same 

 fossil contents, and peculiar lithological characters, and occupying the 

 same position, 10 miles eastward on Pilot Knob; not appearing in the 

 Roche a Cris section, where, however, it might easily have been over- 

 looked on some of the less accessible portions of the cliff (1400) . . . . 6? 



2. Very coarse, triable, reddish brown; weathering into thin layers 9 



3. Very coarse and friable, white- and-brown banded; carrying large ripple- 



marks at top 1G .. 



4. Moderately coarse, friable; uniformly white in color, except on weathered 



surfaces, which show brown-stained layers 2 inches to 4 inches in thickness, 18 . . 



5. Finer-grained, very friable, pink-and-white banded, white predominating 



below 14 .. 



6. Coarse-grained, very friable; whitish with dirty colored ridgy projections, 



which on exposed edges are viUified; near the base including some pink 



layers 19 6 



7. Fine-grained, very friable, brown- and- white-banded; thin-laminated 8 



8. Not seen in detail 50 



9. Alternating very coarse and finer sand layers, all very friable; alternating 



also in color, being banded white and brown ; finer layers cross-lamin- 

 ated; coarser ones (1401) very plainly banded, and containing rolled 

 grains up to ^ to T \j inch in diameter; all showing very marked surface 

 vitrification, which in the coarser parts extends in much further than in 

 the finer, the grains being glassy and closely adherent; seen at the foot 

 of the north end of the cliff 20 



Height of the bluff 155 . . 



