_Western Extension of the New York system._—Before Hall finished his final report, we find him in 1841 on “a tour of exploration through the states of Ohio, Indiana, Illinois, a part of Michigan, Kentucky, and Missouri, and the territories of Iowa and Wisconsin.” This tour is described in the Journal (=42=, 51, 1842) under the caption “Notes upon the Geology of the Western States.” His object was to ascertain how far the New York system as the standard of reference “was applicable in the western extension of the series.” In a general way he was very successful in extending the system to the Mississippi River, and he clearly saw “a great diminution, first of sandy matter, and next of shale, as we go westward, and in the whole, a great increase of calcareous matter in the same direction.” He also clearly noted the warped nature of the strata, the “anticlinal axis,” since known as the Cincinnati and Wabash uplifts and the Ozark dome. Hall, however, fell into a number of flagrant errors because of a too great reliance on lithologic correlation and supposedly similar sequence. For instance, the Coal Measures of Pennsylvania were said to directly overlap the Chemung group of southern New York, and now he finds the same condition in Ohio, Indiana, and Illinois, failing to see that in most places between the top of the New York system and the Coal Measures lay the extensive Mississippian series, one that he generally confounded with the Chemung, or included in the “Carboniferous group.” He states that the Portage of New York is the same as the Waverly of Ohio, and at Louisville the Middle Devonian waterlime is correlated with the similar rock of the New York Silurian. Hall was especially desirous of fixing the horizon of the Middle Ordovician lead-bearing rocks of Illinois, Wisconsin, and Iowa, but unfortunately correlated them with the Niagaran, while the Middle Devonian about Columbus, Ohio, and Louisville, Kentucky, he referred to the same horizon. The Galena-Niagaran error was corrected in 1855, but the Devonian and Mississippian ones remained unadjusted for a long time, and in Iowa until toward the close of the nineteenth century. _The Geological Column of the New York Geologists of 1842–1843, according to W. W. Mather 1842._