396 C. R. Stauffer—The Minnesota Devonian. 



Aet. XXXV. — The Minnesota Devonian and its Relation- 

 ship to the General Devonian Problem of North 

 America; by Clinton R. Stauffeb. 



The Devonian of Minnesota has long been known and 

 roughly mapped, but a detailed study of the rocks belong- 

 ing to this system has been very much neglected. This 

 is probably because the area in which it occurs is a drift- 

 covered plain that has been only partially dissected by 

 erosion and the outcrops that may be found are neither 

 frequent nor very satisfactory. But the country has now 

 been settled for a longer period than when the early 

 surveys of Minnesota counties were made, hence more 

 wells have been drilled and more quarries have been 

 opened in the region so that the rocks of this system are 

 now very much better known than they were thirty or 

 forty years ago. Moreover certain parts of the Devonian 

 have been found to be filled with fossils thereby making 

 the age determination a certainty. 



In all ^bout 1,200 square miles of southern Minnesota 

 are covered by Devonian rocks. This area lies in Fillmore, 

 Mower and Free Born counties* (1). In the northern 

 and western parts of this region, much of the surface is 

 comparatively level and well covered by drift so that it is 

 not always possible to trace the Devonian border in those 

 directions. On the east side it approaches the driftless 

 area and the mantle of glacial debris is reduced to a thin 

 film, often insufficient to conceal the bed rock, and the 

 possibilities for satisfactory stratigraphic work are much 

 improved. Enough outcrops can be found to make it 

 certain that the Devonian is slightly more extensive in 

 this region than it is indicated to be on the present geo- 

 logical maps. Numerous masses and fragments of fos- 

 siliferous Devonian rock are known or have been picked 

 up in the drift of central Minnesota, even as far north as 

 Todd and St. Louis counties. Some of these masses are 

 quite large (2). This has suggested that possibly there are 

 other areas of Devonian, existing as outliers, which have 

 not yet been recognized or which may be entirely drift 

 covered. It is noticeable, however, that the larger frag- 



* For references see the end of this article. 



