THE LOWER SILURIAN ROCKS. 575 



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

 containing nmnerous 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 incli in width, and often show an interior cavity or " vug," lined with black, 

 duU metaUic-lustred, crystalline plates, which have a distinct cubical cleavage and red- 

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

 J^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 pyiite. 



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. 2.5° W. direction. The liase is 

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

 or about 555 feet above Lake Michigan. On the east face the dift's 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 bluft', 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 Ught yellowish smooth surface; thin lamina- 

 tion indicated by fitne 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 Ptychasjns (n. sp.), Conocepha- 

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

 lying m fragments thickly strewn over the surface of the blufl:', which is 

 grassed, the rock being thus concealed; lymg 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 

 tossU contents, and pecuUar hthologic'al characters, and occupying the 

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

 Roche a Cris section, v/here, however, it might easily have been over- 

 looked on some of tlie less accessible portions of the ohff (1400) 6? 



2. Very coarse, tiiable, reddish brown; weathering into thm layers 9 . . 



3 VeiT coai-se and frialile, white-and-brown banded; carrying large ripple- 



marks at top ^" •• 



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 " • • 



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



which on exposed edges are vitrified; neartlio base including some pink 



layers ^^ ^ 



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



8. Not seen in detail . . . '• ^0 . . 



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; coai'ser ones (1401) very plainly banded, and containing rolled 

 grains up to ^5 to tV inch in diameter; all showing very marked surface 

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

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

 of the north end of the clifi' 20 .. 



Height of the bluff. 



155 



