ANNIVERSARY ADDRESS OF THE PRESIDENT. xU 



fossil remains. The terms we employ to designate formations can 

 only be considered as expressing the general predominance of certain 

 characters, to be used provisionally, as a convenient mode of classi- 

 fying the facts we collect, whilst that knowledge is accumulating 

 which, in after ages, will unravel the complicated changes that be- 

 long to the successive periods into which the history of the structure 

 of the whole earth may be divided. 



GEOLOGICAL CHANGES NOW IN PROGRESS. 



Among the most remarkable of those recorded during the past year, 

 none is more instructive, from the magnitude of its operations, than 

 the formation of the alluvial plain of the Mississippi, of which we have 

 received an account from Mr. Lyell, the result of inquiries and per- 

 sonal observation made by him during the spring of last year, along 

 a considerable part of the course of that river from its mouth to the 

 junction of the Ohio. He brought before the British Association last 

 September the principal facts he had collected, and the conclusions 

 he had been led to deduce from them, respecting the progress of that 

 vast accumulation of sedimentary matter, and he has referred to them 

 in greater detail in the seventh edition of his * Principles of Geology,' 

 just published. 



The alluvial matter brought down by this river and its tributaries 

 has formed a tract of level land, which extends from the embouchure 

 in the 29th degree of latitude to Cape Girardeau in the state of Mis- 

 souri, fifty miles above the junction of the Ohio, in latitude 37° 20', 

 a distance, in a direct line from S. to N., of 576 miles. The width of 

 the plain varies considerably, between thirty and eighty miles, which 

 last dimension it attains in lat. 34° ; and it has been estimated by 

 Mr. Forshey to occupy an area of 31,200 square miles, with a cir- 

 cumference of about 3000 miles ; thus exceeding the area of Ireland. 

 That part of the plain which, according to the usual language of geogra- 

 phers, would more properly be called the delta, viz. all that lies below 

 the point where the highest arm, or the Atchafalaya, branches off 

 from the Mississippi, is estimated at nearly one-half of the whole area, 

 or 13,600 square miles. This delta, which spreads out into a vast 

 level region, extending beyond the general coast, is in form very un- 

 like the delta of the Nile, for the main stream does not divide into 

 two separate branches to form the two sides of a triangle, nor is there 

 the curvilinear base towards the sea ; a tongue of land protruding fifty 

 miles into the Gulf of Mexico, through which the main river flows 

 until near its extremity, where the water is discharged into the sea 

 by four main channels, or Passes as they are called. The plain at 

 Cape Girardeau is not more than 200 feet above the sea-level, so that 

 the rise is only about four inches in a mile. Mr. Lyell states the 

 rise at three inches in a mile, but he takes the distance at 800 miles, 

 following the sinuosities of the river. Small as this is, it is much 

 greater than the rise in the valley of the Nile from the sea. That 

 alluvial plain is 420 miles in extent from the first cataract to the apex 

 of the delta ; from that last point to the Mediterranean, in a direct 

 line, is about 102 miles ; the base between the eastern and western 



