THE CINCINNATI SHALES AND LIMESTONES. 319 



fley's Bluff is underlaid by about 65 feet of this formation, consisting, so far as ex- 

 posed at the time of examination, mainly of blue shale, but including, also, some beds 

 of limestone. A portion of the shale consists of a bluish, drab-colored, scarcely indurated, 

 clay, of very fine texture, nearly free from sand and similar impurities, and is, in this 

 respect, much superior to most drift clays, and is worthy of attention, as an excellent 

 clay. Crystals of gypsum, of the variety Selenite, are found at this locality. 



North of this, the formation is quite changeable in nature. This may be well ob- 

 served along the clitf that lines the shore of Green Bay, south of Little Sturgeon Bay. 

 On the western curve of the point, at the mouth of the latter bay, where the formation 

 last displays itself in force, the entire exposure, 15 feet, is of a hard, compact, fine 

 grained, laminated limestone, showing mud cracks. 



Following along the shore southward, a most interesting series of minor changes and 

 fine exhibitions of the manner in which this class of rocks are deposited, is presented for 

 study, and is worthy of note here for the benefit of students of geology, since nowhere 

 else in the state are equally good opportunities for the study of shale deposits afforded. 

 Degrees of induration, from that of ordinary clay, to rocks of almost flint-like hardness, 

 varieties of lamination, from that which is so delicate and uniform as to indicate the 

 most quiet depositing waters, up through various degrees of undulation and irregularity, 

 to ripple marks, eight inches from crest to crest, and an inch high ; together with mud 

 cracks, so pronounced and regular as to sometimes cause the surface to resemble a pave- 

 ment of octagonal bricks, may be observed, while the cliffs are banded and variegated 

 with various shades of blue, green, gray and purple, the whole indicating great variety 

 of conditions and of material, within a limited area. 



The formation is here much more calcareous and more firm and resisting in its nature, 

 and, at the same time, less fossiliferous than at any point observed to the southward. In 

 some of the lower layers exposed, there is much chert in large nodular or lenticular 

 masses. 



The following section is perhaps as nearly typical as any that could be selected to rep- 

 resent the formation at this extremity of its area : 



1. Hard, fine grained, compact, argillaceous limestone, in beds 10 inches 



or less in thickness, some of them weathering rough and irregular. 

 About 9 ft. 



2. Similar, but harder limestone, marked with undulating and contracted 



laminations; some shaly layers interstratified; bluish gray in color, 



lined with red 7 ft. 6 in. 



3. A group of shales of varying color and texture, the most prominent of 



which are finely laminated and slaty, splitting up on exposure into 

 scales and plates, while some are more clay-like. The group in- 

 cludes limestone layers 16 ft. 8 in. 



4. Thick bedded, blue, cherty limestone, with some argillaceous impurities 



in the form of laminse and partings 5 ft. 8 in. 



5. Very similar to the above, but harder 1 ft. 1| in. 



6. Thin bedded, broken, irregular, nodular, quite eherty, calcareous layers. 



Exposed 3^ ft. 



Total, about 44 ft. 



At the point west of Little Sturgeon Bay, the formation sinks beneath the waters of 

 Green Bay. 



