360 HALL AND SARDESON — PALEOZOIC FORMATIONS OF MINNESOTA. 



The Buff Limestone. — This is 15 feet thick. It has the constant char- 

 acter of being made up of somewhat irregular laminae, usually composed 

 of alternating hard, firm limestone and softer, darker colored argillaceous 

 1 lands. The action of percolating waters may render these strata porous 

 by removing the more soluble parts. On the other hand, it may render 

 them more crystalline by their metamorphosing effect on those constit- 

 uents remaining behind. In the former case fossils are reduced to mere 

 casts and cavities; in the latter they are entirely destroyed. At Minne- 

 apolis this layer preserves more fossils than it does further southward. 



The following fossils occur in this bed : 



Cm a in trentonensis, Hall. 

 Leptsena sericea, Hall. 

 Orthis deflecta, Conrad. 

 0. j" /'■' ta, Conrad. 

 0. tricenaria, Conrad. 

 Rhynchonella orientalis, Billings. 

 Skenedium anthonensis, Sardeson. 



Streptorhynchus filitextum. Hall. 

 Strophomena minnesotensis, N. 



Winchell. 

 Zygospira aquila, Sardeson. 

 Cypricarditis rotundatus (?'), Hall. 

 ModiolopsU meyeri, Billings. 



H. 



The Blue Limestone. — This layer is 12 feet thick. It lies directly 

 upon the Buff limesh me just described, save at Faribault. The two beds 



■ M-f.v ^.i sn 



xtrr- ^..iQll W,4-. j.; .. 





tt r T^,? T .r V> , ■ y 



Figuke 6. — Lenticular Segregations of Fossils in the Blue Limestone, Minneapolis. 



The lenses represent the deposition of vast numbers of fossils within restricted areas. The 

 shells have totally disappeared, leaving only easts of the interiors. 



are separated by a, distinct change in rock texture and usually, though 

 not always, by a carbonaceous seam. The Butt' separates along lamina' 

 determined by the argillaceous bands; the Blue lies in heavy strata 

 which break in all directions with a conchoidal fracture. The lower 

 half is more crumbling when exposed and presents few fossils save in 

 lenticular horizontal seams. These seams show how the fauna dwelt in 

 colonies. For one or three inches in depth and stretching out over 100 

 or 200 square feet, the rock is wholly made up of casts of fossils whose 

 surfaces carry coatings of calcite and pyrite crystals, while the rock for 

 some distance above and below shows scarcely a trace of fossils (see 

 figure 6). From the very uppermost stratum a few well preserved shells 



