CENTURY OF IOWA GEOLOGY 413 



the country. This work was intrusted to Joseph N. Nicollet, a 

 French geographer. 



Along with his geographic and engineering abilities Nicollet pos- 

 sessed a keen appreciation for matters geological. His practical 

 familiarity with fossils was quite extraordinary. He was acquainted 

 with Murchison's then new classification of rock-terranes. Notwith- 

 standing the fact that his strictly geological observations in Iowa- 

 land were incidental only to his special work in hand, he presented 

 a wealth of interesting items concerning the geology of the region. 

 He was the first to announce that in the lead district of Dubuque 

 the main mineral-bearing formation should be correlated with 

 Locke's Clifif limestone of Ohio, and Hall's Trenton limestone of 

 New York. Calcareous rocks outcropping near the mouth of the 

 Big Sioux river w^ere shown, by microscopical examination, to be 

 composed largely of minute shells like those occurring in typical 

 chalk and to be of Cretacic age. 



Nicollet's physiographic descriptions are notable productions. His 

 finished map was a real marvel.^*' According to the high authority 

 of Warren this map was "One of the greatest contributions ever 

 made to American geography." 



PRIMAL CLASSIFICATION OF lOWAN GEOLOGICAL FORMATIONS 



First efiforts to arrange the rock terranes of Iowa in orderly suc- 

 cession according to modern criteria was by Dr. D. D. Owen, in 

 1840.^^ This scheme grew out of his examination of the mineral 

 lands of the Dubuque district, as a part of a comprehensive plan 

 adopted by the Federal government to separate the mining properties 

 from those which were not ore-producing. 



Owen's subdivision of the Clifif limestone into Upper, Middle and 

 Lower sections proves to be valid. This scheme having passed 

 every test stands today essentially as originally proposed. These 

 subdivisions are respectively the Devonic, Siluric and Ordovicic suc- 

 cessions of later nomenclature. In the second and revised edition 

 of the report these names actually appear.^- Thus four major 

 subdivisions of the Paleozoic sequence are upon strictly faunal 

 grounds firmly established in the West. The rocks of the Cambric 

 system, as it is now called, could not very well have attracted Owen's 



"Rept. Intended to Illustrate a Map of the Hydrographic Basin of the Upper 

 Mississippi River : Twenty-sixth Cong., 2nd Sess., Sen. Doc. Vol. V, pt. ii, No. 

 237, 177 pp., 1843. 



"Rept. Geol. Expl. Iowa, Wise, and 111. ; Twenty-sixth Cong., 1st Sess., House 

 Doc. No. 239, 161 pp., 1840. 



'^Thirtieth Cong., 1st Sess., Sen. Ex. Doc. No. 57, 1848. 



