GEOLOGY. 257 



hundred miles; making the total difference of level 21.1 feet, from a 

 point ten miles above Natchez to the Balize. As to depth, extreme 

 depths of 188 feet have been found in some places, but, when the chan- 

 nel is distant from both shores, 125 feet is the usual maximum. A 

 section made in front of Vidalia gave a mean depth of 80.5 feet, with 

 a maximum of about 130, and a like section of high-water channel at 

 Carrollton gave a mean depth of 71.C feet. The low-water depth may 

 always be obtained by subtracting the range from the high-water. At 

 Vidalia, the range is 51 feet, and at Carrollton 15 feet. The mean 

 depth of the low-water sections would then be 29.5 feet and 56.6 feet. 

 And the sectional areas at low water give 133,010 square feet for Car- 

 rollton, and 108,000 at Vidalia. 



The uniformity of breadth in the channel is a remarkable feature of 

 the Mississippi River. A great number of measurements from the 

 Balize to Galena, 1,700 miles up the river, give a mean width, includ- 

 ing wide places in the bends, of about 1,000 yards ; excluding these, 

 800 yards ; and the upper portion of this is wider, including expanses 

 produced by bends and islands, than 1,000 yards ; but excluding them, 

 it is the same. The addition of the four great rivers below makes 

 no increase in the breadth of the river. The Missouri, 300 miles 

 above its mouth, is half a mile wide. The result of observations made 

 on drift-wood shows a mean surface velocity at high water of 2.61 

 miles per hour at Carrollton, and 2.60 miles per hour at Vidalia ; 

 respectively, 3.80 and 3.82 feet per second of time. The one was de- 

 rived from 176 observations, and the other from 70 observations. At 

 low water, Carrollton. 1.45 miles per hour, or 2.11 feet per second ; 

 at Vidalia, 1.54 miles per hour, or 2.25 feet per second. The mean 

 velocity for mean water, as derived from thirty years' observation, is 

 2.26 miles per hour, or 2.95 feet per second. 



From these and other data, Mr. Forshey deduces a mean discharge 

 for the last thirty years of 12,250,000,000,000 cubic feet of water per 

 minute. According to the estimate of Prof. Riddell, the cubic con- 

 tents of sediments would be by these measurements 4,083,333,333 

 cubic feet, so that the sediment would cover twelve square miles one 

 foot deep. 



RATE OF FALL OF THE MISSOURI RIVER. 



Appletoii's Mechanics' Magazine, for January, contains an article on 

 the Missouri River, apparently written by one of the engineers who 

 had been engaged in surveying the route of the proposed railroad from 

 St. Louis to the western line of Missouri. It is found that the descent 

 of the Missouri River for the 219 miles between Kansas and Jefferson 

 City is 179 feet or 9.8 inches per mile, and for the 150 miles between 

 Jefferson City and the mouth 124 feet or 9.9 inches per mile, giving 

 for the whole 369 miles a fall of 303 feet, which is at the rate of 9.85 

 inches per mile. The descent of the Mississippi is but 2.8 inches 

 per mile. The rapidity of the current of the Missouri is judged to 

 average from two to four miles an hour. 



Tt is mentioned that, in Crawford County, at the head of one of the 



22* 



