THE LOWER SILURIAN ROCKS. 601 



very even-bedded, close-textured, yellowish, limestone, which is much marked by dend- 

 ritic manganese oxide, and contains the following fossils: Petra'm comic-iiluM, Strophe- 

 mena alternate!, a small Ortlils, a EhyiicoiieUa, Cypricardifes ventricosa, Trodionona 

 umbilicata, Hclicotonta planulata, and fragments of small orthoceratites. The fossils 

 are casts only. 



The quarries on Sec. 23, Sun Prairie, show in all a thickness of about 20 fec-t, the 

 upper layers of which are thin, shaly, and bluish in color, and appear to belong to th-3 

 "Blue" beds, whilst below there are heavy regular layers of buff-colored limestone. 

 Immediately beneath the shaly layers are found layers of a very close-textured, purplish 

 brown, chonchoidal fracturing rock (698), carrying Columnaria alreolata. From the 

 buff layers, the following fossils were obtained (697,698): Petra'm cornicuJion, Orthis 

 trieenaria, Strophomena camemta, Cypricardites ventricosa, very large casts of the ex- 

 terior of Trochonema umbilicata, Eaphistotna lenticularis, a Muchisonia, Orthoceras 

 rertebrale, Orthoceras anelhun, Gyroccras duplicostatum, n. sp., Oncoceras pandion, 

 and cr'moidal stems. A strong lime, hard to slack, is made hero from the buff beds, but 

 the stone is usod chiefly for building purposes. 



At the large quarries on the prairie, in the north half of Sec. 34, Bristol, are exposed 

 of the Blue limestone about 8 feet, of the Buff about 10 feet. The Blue beds show a 

 dark bluish-gray rock (703). with a flinty-textured matrix, in which are scattered numer- 

 ous minute strings and patches of calcite. The layers of this rock are about one-half to 

 there-fourths inch in thickness, very rough-surfaced, and show numerous obscure im- 

 pressions of fossils, of which two brachiopods, Rhynconella and Strophornena, appear to 

 be most abundant. Between these layers are very thin, regular, fragile, dark brown 

 shaly layers, on which are fine, black, graptolitc-like markings. The rough-surfaced 

 blue layers contain: silica, 7.08; alumina, 2.21; carbonate of lime, 84.02; carbonate of 

 magnesia, 5.33; iron peroxide, 0'83; iron protoxide, 0.39; water, 0.61 = 100.42. The 

 Buff beds below are regular heavy layers of yellowish close-textured limestone, includ- 

 ing some of a dark purplish brown, chonchoidal-fracturing rock, like that already men- 

 tioned as seen on Sec. 14 of the town of Sun Prairie. The buff-colored rock (704), from 

 directly below the junction with the Blue, contains: silica, 4.45; alumina, &G8; carbon- 

 ate of lime, 56.07; carbonate of magnesia, 35.32; iron peroxide, 0.69; iron protoxide, 

 0.58; water, 0.46 99i65. The usual fossil casts are found in the Buff beds, including 

 fine ones of Cypricardites ventricosa and Gyrocems duplicostatum. 



Fig. 50. 



[;/j:^;y!77^'.i--^ i - - : . ; ; \^vj /;'V-. : : J- .'- --'--.-:. :'. ;''-. ^f^^^^^^^^{^^^<-- : ^^:~-y~-'l :.''-" ^\&-. 

 SECTION ON A LINE RUNNING N. 25= E., FROM THE S. W. QB. OF THE S. W. QR. OF SEC. 2, NEARLY 



TO THE NOKTH LlNE OF SEC. O, JJURKK. 



Horizontal scale 4 inches, 1 mile ; vortical scale 1 inch, 20D feet. 



Fig. 50 represents a section obtained in the northern part of the town of Burke, 

 showing the small thickness reached by the St. Peters and Lower Magnesian in this 

 part of the county. 



In the small quarry on the N. W. qr. Sec. 35, in the same town, a 2 inch layer is 

 crowded with impressions of the exterior, and casts of the interior, of the following fos- 

 sils (694): Petraia Corniculum, StropJwmena camerata, S. incrassata, Steptorhynchus 



