226 ANNUAL OF SCIENTIFIC DISCOVERY. 



peculiar curvature prevails, the great anticlinal exposing invariably, or, 

 with rare exceptions, a much steeper dip upon the side which faces the 

 Alps, than upon the opposite side. The average dip of the northwestern 

 abutments does not amount to 40, while that of the southeastern even 

 exceeds 70. In regard to the chain of the Alps, Prof. Rogers 

 proved that it consists of two principal zones of closely-plicated strata. 

 The entire belt of the Bernese Oberland displays folds which dip 

 inwardly toward the high central peaks, with a parallel or south-dipping 

 system of cleavage. The southern chain of the Monte Rosa exhibits 

 a similar system of flexures, but of an opposite order of dips, these being 

 directed toward the north, and, therefore, also inclining inwards, toward 

 the high central summits. This opposite direction of the folds in the 

 two opposite flanks of the chain at once explains the hitherto unsolved 

 phenomenon of the inward dipping or fan-like position of the planes 

 of stratification. The cleavage dips on each flank of the chain, as in 

 every other district, are parallel with the average dips of the anticlinal 

 folds. 



LYELL'S VIEW OF THE GEOLOGY OF THE MISSISSIPPI VALLEY. 



THE delta of the Mississippi may be defined as that part of the 

 great alluvial slope which lies below, or to the south of the branch- 

 ing off of the highest arm, or that called the Atchafalaya. Above 

 this point, which is the head of the delta, the Mississippi receives 

 waters from its various tributaries ; below, it gives out again, through 

 numerous arms or channels, the waters which it conveys to the sea. 

 The delta, so defined, is about 14,000 square miles in area, and elevated 

 from a few inches to ten feet above the level of the sea. The greater 

 part of it protrudes into the Gulf of Mexico, beyond the general coast- 

 line. The level plane to the north, as far as Cape Girardeau in Mis- 

 souri, above the junction of the Ohio, is of the same character, including 

 an area of about 16,000 square miles, and is, therefore, larger than the 

 delta. It is very variable in width from east to west, being near its 

 northern extremity 50 miles wide, at Memphis, 30, at the mouth of the 

 White River, 80, and contracting again farther south, as at Grand Gulf, 

 to 33 miles. The delta and alluvial plain rise by so gradual a slope 

 from the sea, as to attain at the junction of the Ohio (a distance of 800 

 miles by the river), an elevation of only 200 feet above the Gulf of 

 Mexico. 



Finding it impossible to calculate the age of the delta from the 

 observed rate of the advance of the land on the Gulf in each centu- 

 ry, I endeavoured to approximate, by a different method, to a mini- 

 mum of the time required for bringing down from the upper country 

 that large quantity of earthy matter which is now deposited within 

 the area of the delta. Dr. Riddell communicated to me the result of 

 a series of experiments which he had made, to ascertain the propor- 

 tion of sediment contained in the waters of the Mississippi. He 

 concluded that the mean annual amount of solid matter was to the 

 water as T ^' J in weight, or about ^Vc- m v l ume - Since then he 



