632 GEOLOGICAL SUEVEY OF THE TEREITORIES. 



canal projiles are not so mucli due to imperfect instrumental icorTc as to 

 hasty comjnitation and careless combination of the results. 



The difficulties of making connection between the end of one profile 

 and the beginning of another have been very great. Most cities have 

 now a datum or base-point from which all the city-levels date, often 

 called the city directrix. The United States Signal-Office has taken 

 great pains to get reports from most of the city-engineers of the heights 

 of the railroad-depots above these city -directrices ; but the difficulties 

 of using this connection between railroads is that in many cases the 

 present depots are not the ones referred to on the profiles of the roads; 

 and even when the present depots are the same as the old ones, the 

 grade at the depot has been changed since the railroad-profile was 

 made, and no note of the present grade made on the profile. This seems 

 to be the case in Chicago, where the railroad-profiles almost all indicate 

 a lower grade for the depots than those reported by the city-engineer. 



By visiting the ground, and making connections with old benches, 

 I have gotten rid of many of these errors, and, fortunately, in many 

 cities the railroad-engineers have connected their datum with that of 

 the city. If the engineers of this country will adopt this as a rule, the 

 value of their work for general and scientific purposes will ' be very 

 much increased. 



The railroad-lines from Philadelphia, and the railroad and canal lines 

 from Albany, had reported the elevations along their lines above tides 

 of various stages at these points, and the G. T. E. W. of Ca. had 

 reported their elevations as referred to tide at Three Rivers, the head of 

 tide-water in the Saint Lawrence. These datum-points differ from each 

 other, and from the mean surface of the ocean, which is the only proper 

 plane of reference for our elevations. The errors due to this cause have 

 entered into all previous reports of elevations in Pennsylvania and the 

 regions about the great lakes. By the assistance of the United States 

 Coast Survey, and of Mr. Smedley, city engineer and surveyor of Phil- 

 adelphia, the datum-planes of the Erie Canal and E". Y. C. E. E. levels 

 and of the Pa. E. E. have been connected with the mean surface of the 

 ocean. 



Important changes are made as the result of this investigation. The 

 elevation of the great lakes and surrounding country is found to be 

 about 9 feet more than previously reported by the State geologist of 

 Ohio, and that of Saint Louis about 23 feet higher than reported by 

 Humphreys and Abbot. While Kansas City, and all the surrounding 

 country for many hundred miles south and west, has heretofore been 

 reported more than 100 feet too low, Omaha is raised about 31 feet, and 

 Indianapolis about 100 feet. The fall of the Mississippi above Memphis, 

 and of the Ohio, and of the Missouri Eiver, is also changed. The 

 amounts of these changes are so great, and the accuracy of the results 

 of such importance to science and to our work of internal improvements, 

 that I publish the evidence upon which they rest, and a statement 

 of the evidence upon which previous reports were made where such 

 could be found. 



The checking at Denver of the levels brought through by the U. P. 

 and D. P. E. E., and by the K. P. E. E. is so close that I believe the 

 error of elevation of this point cannot exceed 10 feet, exclusive of that 

 due to deflection of the i)lumb-liue by attraction of mountain-masses. 

 The result by the K. P. E. E. is 5198.97 ; and by the U. P. and D. P. E. 

 E.s, 5194.20 feet above mean sea. My determinations of the elevation 

 of Ogden, above the Atlantic Ocean, by the U. P. E. E., and above the 

 Pacific Ocean, by the C. P. E. E., differ only 25 feet. When it is con- 



