OF THE NOETHWEST. 



49 



b. But on the west branch of the Chippewa, or Menominie River, on the Mis- 

 sissippi River between Prairie a la Crosse and Lake Pepin, and especially in the 

 northern part of the district explored, to wit, on the St. Croix, at and below its falls, 

 the above-named varieties are not seen ; at these points the lowest strata visible, and 

 those immediately in juxtaposition with the trap of the falls, and at various points 

 lower down, are highly fossiliferous, schistose, siliceo-calcareous layers, interlami- 

 nated with argillaceous marly beds, charged with pyrites ; they are exposed some 

 fifty feet in thickness just below the falls. A few feet above low water of the Mis- 

 sissippi, near the mouth of Black River and Mountain Island, there are layers of a 

 similar description, but more intermixed with schistose sandstones and gritstones 

 towards the upper part, occupying in all some twenty-five feet. Here, in addition 

 to some species of the genera Lingula and Orbicida, which occur at the Falls of St. 

 Croix, there are some remarkable forms of Trilobites, associated with numerous 

 specimens of Obolus. 



c. Superimposed on these is found usually a rather coarse sandstone, varying in 

 colour from a white to brown and green, containing Lingidas and Orbiaidas, and 

 hence designated " Lingula sandstone." Its thickness at Lawrence Creek is ten or 

 twelve to fifteen feet ; but at Mountain Island and other places farther south it is 

 much thicker. 



d. This division comprises many subordinate layers ; it is characterized especially 

 by many of its beds being impregnated with green particles of silicate of iron, 

 disseminated either through soft sandstone, incoherent sandy deposits, or in subordi- 

 nate argillaceous beds, which, as at Marine Mills, are unctuous, and stain green 

 everything with which they come in contact. It includes also three Trilobite beds, 

 the two lower of which are gritstones : one found near the mouth of Miniskah River, 

 the other at the top of the Marine Mill section ; the third is a gray argillo-calca- 

 reous bed, found near high-water mark of Lake St. Croix, above Stillwater. It 

 embraces also two dolomitic bands : one near its base, the other towards its top ; the 

 latter of which contains the remains of Ortliis and Crinoidea. This division 

 includes also fucoidal beds, occurring under the fourth Trilobite bed. 



e. Soft, light-yellow gritstones, with hard, botryoidal, magnesio-calcareous concre- 

 tions. At some localities this subdivision includes, near its base, beds of soft and 

 fine-grained freestones, approaching in their character to tripoli. 



/. Alternations of sandstone and magnesian limestone, with botryoidal layers of 

 concretionary sandstone, and siliceous and calcareous oolite. The upper beds of this 

 subdivision are usually nearly white, and either soft or indurated ; some contain 

 imperfect casts of Trilobites. The six principal divisions are usually persistent ; 

 some of the subordinate subdivisions are more local, or at least only obscurely 

 marked at some localities. These latter vary too in their lithological characters, at 

 distant points. The above arrangement is, however, their predominating character 

 on the east side of the Mississippi, from Prairie a la Crosse to the igneous ranges on 

 the St. Croix. Sections 1, 2, 3, and 4, give a connected view of the principal 

 divisions. Section 6, at the Great Slide, presents most of the members, from c to/, 

 inclusive. 



