THE LOWER SILURIAN ROCKS. 601 



very even-bedded, close-toxtured, yelloTvish, limestone, which is much marked by dend- 

 ritic manganese oxide, and contains the followiug fossils : Petraia cormculum, Stropho- 

 mena alternata, a small Orthis, a Ekynoonella, Cijpricardites ventricosa, Trochoncma 

 umbilicata, Helicotoma planulata, and fragments of small ortliocoratitos. The fossils 

 are casts only. 



The quarries on Sec. 23, Sun Prairie, sho\v in all a thickness of about 20 feet, the 

 upper layers of which are tliin, shaly, and bluish m color, and appear to belong to the 

 "Blue" beds, whQst below there are heavy regular layers of buff-colored Uniestone. 

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

 brown, chonohbidal fracturing rock (698), carrying Columnaria alveolata. From the 

 bufi' layers, tlie following fossils were obtained (697, 698) : Petraia cormculum, Orthis 

 trieenaria, Strophoinena camerata, Cypricardites ventricosa, very large casts of the ex- 

 terior oi Trochoneina umbilicata, Baphistoma lenticularis, s, Muchisonia, Orthoceras 

 vertebrale, Ortko.eras anellum, Gijrocoras duplicostatum, n. sp., Oncoceras pandion, 

 and crinoidal stems. A strong hme, hard to slsick, is made hero from the buff beds, but 

 the stone is us'.d chiefly for building purposes. 



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

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

 dark bluish-gray rock (70S), 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 wliich two brachiopods, Bhynconella and Strophomena, appear to 

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

 shaly layers, on which are fine, black, graptolitc-Uke 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 hmestone, includ- 

 ing some of a dark purpHsh brown, chonehoidal-fractm-ing rook, hke that ah-eady men- 

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

 directly below the junction with the Blue, contains: sihca, 4.45; alumina, 2..08; 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 fossd casts are found in the Buff beds, including 

 fine ones of Cypricardites ventricosa and Chjroceras duplicostatum. 



Fig. 50. 



WaaJnranfj' pit 

 4s' 



Jtrj^m^MEJS. 



Seciiok ok a Line Runnino N. 25= E., feom the S. W. qb. off the S. W. qb. op Seo. 3, Ne.iex,t 



TO THE NOKTU LlNE 01" SeO. 3, BUBKE. 



Horizontal scale 4 inclies, 1 mile; vortical scale 1 inch, 2O0 feet. 



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

 sho^g the .mall thickness reached by the St. Peters and Lower Magnesian mtlus 



nart of the county. , , c ■ ^ i 



In the smaU quarry on the N. W. qr. Sec. 35, in the same_ town, a 2 inch layer i. 

 crowded with impressions of the exterior, and ca^ts of the mtenor, of the follo^™lg fos- 

 liils (694): Petraia Cormculum, Strophomena camerata, S. incrassata, Steptorhynchus 



