XXXV111 THE PALEONTOLOGY OF MINNESOTA. 



R. P. Wliitfield. 



1878. Geology of Wisconsin. Survey of 1873-1877. Vol. n, of the final report; 

 Madison, 1878. T. C. CHAMBEKLIN, chief geologist. 



In this volume are numerous references to the preliminary identification of fossils by 

 Prof. R. P. Whitfield, paleontologist of the survey. Of these the following refer to the 

 Lower Silurian as defined by the M innesota Survey. 



Page 561 gives a list of Trenton fossils, without specification of their geographical 

 localities; and after the discussion of the Cincinnati shales and limestones is given a full 

 tabulation of the fossils of the Trenton period (p. 320), for which the identification of the 

 species was by Whitfield. It appears, however, in Prof. Chamberlin's general chapter on 

 the "Lower Silurian" and will be mentioned more fully under his name. 



T. C. Chamberlin. 



1878. Geology of Wisconsin. Survey of 1873-1877. Vol. n, T. C. CHAMBERLIN, 

 chief geologist, Madison, 1878. 



Part n of this volume, entitled "Geology of Eastern Wisconsin," is by Prof. Chamber- 

 lin. Of this, chapter vn is devoted to the Lower Silurian, which by the author is con- 

 sidered to include all the rocks of this district from the Archean formations to the Clinton 

 in the Upper Silurian. The Trenton group is said to consist of three main divisions, viz. , 

 in ascending order, Trenton limestone, Galena limestone and the Cincinnati shales and 

 limestone. The Trenton proper is given a thickness in southeastern Wisconsin of 120 

 feet, divided as follows: 



Upper Blue beds, 15 feet. 

 Upper Buff beds, 55 feet. 

 Lower Blue beds, 25 feet. 

 Lower Buff beds, 25 feet. 



It was found that the designations "Buff" and "Blue" of former reports had been 

 used indiscriminately for either the upper or lower, and that the strata are all strongly 

 dolomitic. It was learned that they cannot be separated on paleontological grounds. 

 That which is above named Lower Buff is what has been known generally simply as Buff. 

 The color which has given it its name is wholly a superficial character due to weathering, 

 the interior of the rock being blue. The Buff beds, upper and lower, are less intermixed 

 with argillaceous matter than the Blue beds, and for that reason are more readily changed 

 in color. The Buff beds are particularly marked by the preponderance of lamellibranchs, 

 gasteropods and cephalopods, and the Blue beds by corals, bryozoans and small brachi- 

 opods, especially the Orthidas. Murchisonia gracilis occurs abundantly near the base of 

 the Lower Blue, which also contains sometimes a notable amount of carbonaceous material. 



"It appears from all the facts that there was an alternation of conditions in the depositing Trenton 

 seas, and that when the conditions were such as to favor the formation of limestones simply, the life 

 above characterized predominated, and that whenever the conditions changed so as to cause a deposit of 

 shale interleaved with layers of limestone, the brachiopodous and coralline fauna prevailed. These sub- 

 divisions than signify rather physicial mutations of a more or less local nature than wide-spread changes 

 in the life-character of the period." (P. 294.) 



