50 



PROTOZOIC ROCKS 



SECTION II. 



ITS PALAEONTOLOGY. 



It had been usually believed, up to the date of my Annual Report of 1848, that 

 the lowest members of the sandstone formation of which I am now speaking, were 

 devoid of fossils. The geologists of our own country had set down the Lingula beds 

 of the New York Potsdam Sandstone as the oldest fossiliferous rocks in the United 

 States. And, in Europe, with the exception of the Obolus Appolinis of Eichwald, 

 abundantly found in the inferior sandstones of the protozoic strata of Russia, no 

 fossils whatever (according to any established system) had been described or dis- 

 covered beneath what has been usually regarded as the ecmivalent of the above- 

 named Lingula beds. I am now able to exhibit a new and interesting geological 

 feature with regard to this formation. 



The present Survey has brought to light the fact, that in Western America are 

 found strata underlying coarse Lingula grits, and at a depth of seventy-five to one 

 hundred feet beneath them, which are highly fossiliferous, and contain not the 

 Lingula and Obolus alone, but Orbiculas, Trilobites, and compressed subconical 

 bodies, resembling some forms of Cephalopoda, but probably not actually of that 

 order. The sedimentary strata in which, on the Mississippi and most of its tributa- 

 ries, these fossils occur, either rest immediately on the igneous rocks of Wisconsin, 

 or are separated from them by an inconsiderable thickness of chloritic and ferrugi- 

 nous slates ; and are, in all probability, the oldest fossil-bearing rocks yet brought to 

 light in any part of this continent, if not of the world.* 



Without assuming to determine the much-disputed question of an absolute 

 palceozoic base, it may be safely asserted that the fossiliferous strata above referred 

 to, exhibit the true base of the zoological series in the Mississippi Valley. 



It was in August of 1847, while descending the St. Croix, that I first observed 

 multitudes of Lingulas and Orbiculas disseminated in strata abutting against the 

 protruding trap ranges which cross that stream at its falls. In tracing out the 

 geological position of these strata, during the succeeding months of the same year, 

 I usually found them either, as in the above instance, in almost immediate contact 

 with the trap or granite, or else separated from these only by the lowest member of 

 F. 1. Their depth below the base of the Lower Magnesian Limestone, I found to 

 be not less than five hundred feet. 



I observed, also, that besides Lingulas and Orbiculas, there occurred in the sand- 



* In the chloritic and ferruginous slates here mentioned no organic remains have been discovered. In 

 these rocks I am unable to trace any analogy, in lithological character, to Professor Emmons's "Taeonic 

 System." Whatever may occur elsewhere, it does not appear that in the Valley of the Upper Mississippi, 

 any fossil-bearing rocks, deserving the name of a distinct system, occur intervening between the igneous 

 rocks and the base of the sandstones belonging to the Silurian period. 



