No. 136.] 37 



way, by movements of those waters under which were deposited 

 the sands which we see hardened into the Potsdam sandstone. 

 Similar markings are frequent in almost all rocks of sandy texture. 



The Potsdam sandstone, though not seen distinctly in the 

 Mohawk valley (where its place betAveen the gneiss and the Cal- 

 ciferous sandstone appears to be vacant), is a thick mass in Penn- 

 sylvania, and is known northeastward and northwestward over a 

 great area. It is seen on the Lower St. LaAvrence, and can be 

 traced westward by the north shore of Lake Huron, and the south 

 shore of Lake Superior, through Wisconsin, Minnesota and North- 

 ern Iowa, and has even been recognized in the " Black Hills " 

 near the Rocky mountains. In this extension it often varies in 

 color and hardness, being in some places so soft as to crumble 

 with very little difficulty ; but it is always mainly a siliceous 

 rock, though with some bands which contain a portion of lime. 

 The little Lingula is found in the far northwest as here (speci- 

 mens being in the cases from the Falls of the St. Croix, Wis- 

 consin), and in that region it contains a few remains of trilobites. 



Next above this sandstone lies another, which, however, con- 

 tains a considerable portion of lime mingled with it, and thence 

 has received the name of the 



CALCIFEROUS SANDSTONE. 



It may be described as a siliceous or gritty limestone, gene- 

 rally of a brownish color, lying in straight thin layers, and attain- 

 ing an entire thickness of two* or three hundred feet. It is well 

 seen at the "Noses" about Fonda on the Mohawk, and also at 

 Little-falls; in each of which places it has been raised to light 

 by an uplift which has brought it from its originally lower posi- 

 tion. It may also be examined near Middleville on the West- 

 Canada, creek (where it contains in its cavities many beautiful 

 quartz crystals), and in many places in the vicinity of Lake 

 Champlain and the St. Lawrence river, in which latter region it 

 has some layers so purely calcareous as to be. profitably burnt for 

 lime. Toward the west and northwest it extends about equally 

 with the Potsdam sandstone, seeming in some localities so much 

 like it as not to be easily distinguished from it, though in most 

 places it is highly calcareous, forming the " Lower Magnesian 

 limestone " of the Upper Mississippi as described by Dr. Owen ; 

 and it is seen in moderate thickness in Pennsylvania. (In Mis- 

 souri, a limestone belonging at the junction of this rock with 



