MEXICAN BOUNDARY LINE. 123 



The limestones in the lower part of the section are those usually termed " carhoniferous 

 limestone." The group consists of distinct members, each marked by numerous characteristic 

 fossils, and the whole together representing the phases of a calcareous formation, going on in an 

 ocean, where the conditions of its bed and its limits were subjected to change. 



The limestones of this period are well developed in the valley of the Mississippi, from above 

 Burlington, in Iowa, and Oquaka, in Illinois, as far south as below the towns of Kaskaskia 

 and Chester, on the Illinois side, and St. Genevieve and St. Mary's, on the Missouri side. 

 Several of the members are known in Indiana, and one or more in Kentucky, Tennessee, and 

 Alabama, and also in Arkansas. Throughout all this region these limestones, whether 

 developed as the full series or in a single member, underlie the coal measures proper. 



In all the collections which I have examined from Texas and New Mexico, and from points 

 further north in the same line, and particularly in the collections made by Captain Stansbury, 

 on his route from the Missouri to the Great Salt Lake, and in that region, I have never observed 

 fossils which are characteristic of any member of the lower carboniferous limestone. We have, 

 thus far, no evidence of the occurrence of lower carboniferous strata among the Rocky mountains ; 

 while at intervals from the northern limits of the United States along the range of the Rocky 

 Mountains, and both east and west of the principal range, we have the upper carboniferous 

 limestone everywhere more or less perfectly indicated by its characteristic fossils. Among these 

 are Spiri/er cameratus, S. Kneafus, Terebratula subtilita, Produdus Bogersi, P. semireticulatus, 

 Zaplirentis Stanshuryi, and others. From a recent comparison of specimens from Ohio, Illinois, 

 Iowa, Missouri, Kansas, Texas, and New Mexico, I find the same association of species from 

 numerous localities.* 



In the eastern and northern part of the State of Ohio, (and perhaps extending into Pennsyl- 

 vania,) there are thin bands of limestone associated with the coal measures. These beds are 

 usually shaly in character, often separated by wide vertical joints, and weather to a brown 

 color. Although recognized at numerous points, I am not aware that these beds hare been 

 regarded as continuous, though they are doubtless indications of a continuous formation. A 

 comparison of fossils from numerous points in the coal measures of the West, shows very con- 

 clusively that one or more of these beds of limestone are continuous, or at least that the same 

 association of fossil species occur at so many points as to leave no doubt of a similarity in the 

 conditions of the ocean bed over a wide area now occupied by the coal measures. 



One of these bands of shaly limestone, containing throughout the same species of fossils, may 

 be traced from northeastern Ohio, or even from Pennsylvania, through Indiana, Illinois, Iowa, 

 and Missouri, becoming in the latter State, and in the adjoining jiarts of Nebraska and Kansas, 

 an important limestone formation, and constantly increasing in a westerly direction until it 

 becomes the i^rominent limestone of the Rocky Mountain range. 



According to the report of Captain Stansbury, and from specimens brought by him from the 

 Salt Lake region, we learn that it there forms extensive mountain ranges or at least that it is 



• It was from a limestone in the coal measures of Ohio that Dr. Flildrcth procured soTcral species of fossils which are 

 descrihed by Dr. Morton in the American Journal of Science, vol. xxi.x. The Spiriftr cameralus of that paper appears to be 

 identical with one described by Dr. Roemer as S. .yieusebaclianus, and by the writer as S. Iriplicatui, in Stansbury 's report, and 

 the same is referred, with doubt, by Dr. Owen to S;nri/er/ascig-er of Keyserling. The species presents much variety in different 

 localities, but a comparison of specimens from Ohio, Iowa, Missouri, Te.tas, and New Mexico, leads me to infer that they are 

 all of a single species first described by Dr. Morton. It is possible that further comparisons may slmw the occurrence of two 

 closely allied species, but more extensive collections are required for this purpose. 



