35G HALL AND SARDESON — PALEOZOIC FORMATIONS OF MINNESOTA. 



for these states at the very base of that column. This relation is shown 

 in figure 4. 



It may further he said that the Saint Peter was involved with the re- 

 mainder of the Lower Silurian in the movements which brought about 

 the gentle minor undulations seen in the latter at many places in south- 

 eastern Minnesota, and in the major wave whose crest is shown on the 

 profile drawn at the bottom of the map (plate 10) accompanying this 

 paper. 



THE TRES TOX LIMESTONES AND SHALES. 



Localities. — In many different townships of Fillmore and Olmsted 

 counties, at Saint Charles and Clinton tails, around Faribault, near 

 Elgin, at Cannon falls and southward to Kenyon, at Berne, Old Con- 

 cord, Belle creek, Farmington and Mendota, in several outliers in Wash- 

 ington county, and at numerous places in the cities of Minneapolis and 

 Saint Paul, the Trenton rocks occur. 



For convenience in description, the foregoing localities will he grouped 

 in two areas, viz, the Saint Anthony area and the Southern area. The 

 former comprises those exposures of Lower Silurian rocks within twenty 

 miles or so of Saint Anthony falls, where the Mississippi breaks over the 

 shelf of Trenton limestone almost at the northern limit of the formation ; 

 while the latter includes all those exposures within the state south of 

 Hastings and Farmington. This is, in the area underlain by its rocks, 

 by tar the more important of the two. 



Structural Characters. — These characters are extremely varied. There 

 is almost every phase of a stratified rock from a compact massive lime- 

 stone to a thinly laminated, fissile, carbonaceous shale. They will he 

 chiefly considered in connection with the paleontologic characters of the 

 different beds into which the representative fossils appear to divide the 

 formations. Here, however, it may be stated that, resting upon the green 

 and somewhat shaly top of the Saint Peter, there lies in a stratum of some 

 inches in thickness, hut with no well defined upper boundary, a blue- 

 green-gray finely textured rock which lacks adhesion to such an extent 

 as to crumble and become worthless. The limestone above contains 

 numerous interrupted layers of this crumbling material. These layers 

 cause the rock to separate easily on exposure, thus becoming an inferior 

 building stone unless laid in the same horizontal position as they occupy 

 in the quarry. Many joints occur, and sometimes they can be traced 

 hundreds of feet. Only one or two cases of faulting are known. 



Lithologic Characters. — A discussion of these will be restricted largely 

 to the more compact lower layers, since the shales are very difficult to 



