THE LIFE OF THE TRENTON SEA. 



301 



weather out. Rarely the Blue and the Buff beds become somewhat 

 alike lithologically through the effect of destroyed fossils. 



The fossils of the Blue limestone are — 



Cm a itr trentonensis, Hall. 

 Discina concordensis, Sardesou. 

 TAngula elderi, Whitfield. 

 Lingulella iowensis, Owen. 

 Orthis bellarugosa, Conrad. 

 0. deflecta, Conrad. 

 0. pervi ta, ( Jonrad. 

 0. tricenaria, ( lonrad. 

 Rhynchonella minnesotensis, Sarde- 



son. 

 Strt'j>t<>rlii/itrli/is filifi .rtn in. Hall. 

 S. minnesotensis, N. H. Winched. 

 Zygospira recurvirostris, Hall. 

 Z. aquila, Sardeson. 

 Bucania bidorsata. Hall. 



Helicotoma planulata, Salter. 

 Maclurea bigsbyi. Hall. 

 Murchisonia gracilis, Hall. 

 M. milleri, Hall. 

 ,1/. tricarinata, Hall. 

 Pleurotomaria subconica, Hall. 

 Raphistoma lenticulare, Emmons. 



B. nasoni, Hall. 



Subulites elongatus, Emmons. 

 Trochonema beloitense, Whitfield. 

 < 'ypricardites rectirostris, Hall. 



C. niota, Hall. 

 Tellenomya nasuta, Hall. 

 Modiolopsis plana, Hall. 



The Stictoporella Bed. — The Buff and Blue limestones described above 

 constitute the true Trenton limestone in Minnesota. The 10 feet here 

 described as the Stictoporella bed is, however, partly composed of lime- 

 stone strata from two to sixteen inches thick. But they are crystalline, 

 very firm and compact strata, often called marble in the west. They 

 contain few fossils except at their surfaces, hut alternate with richly fos- 

 siliferous strata of shale. 



In the Saint Anthony area, particularly within the cities of Minneap- 

 olis and Saint Paul, the proportion of crystalline limestone to the shale 

 is about one to two, with the former predominating at the bottom. The 

 junction with the Blue bed is defined either by a granular seam or a car- 

 bonaceous hand, or less frequently by a sudden transition to " marble." 

 While the succession of strata is somewhat variable, it is broadly stated 

 as follows: A stratum of purple crystalline stone (> to 8 inches thick : a 

 thin layer of shale: a gray crystalline stratum is to 24 inches thick: 

 shale : bluish limestone 6 to s inches : and shale with thin strata of lime- 

 stone and carbonaceous laminae to the top of t he series. In the southern 

 area limestone layers predominate over the shale. 



The name given to this series of layers is suggested by the abundance 

 of remains of Stictopon lla.* 



• The names Stietopori lift and St i given to two of these Lower Silurian beds are from two 

 genera of bryozoa abundant in them and ii the same time s< whal restricted (■> them, ;c- deter- 

 mined and described by E. Ulrich, Geol and Na< His! Mir. Minn., Ann. Rep for 1885, pp. Ci 



