7(5 



ON CHANGES OP THE SEA LEVEL. 



[1854 



and a slightly ferruginous mixture gives it a pale yellow colour. 



The strong calcareous quality of the clay which would give 

 it value as a manure, renders it unfit for bricks or pottery. 

 But clays suited for such purposes are found in abundance in 

 some parts of the interior, such as in the vicinity of London 

 and of Thorold, where it is supposed to overlie the calcareous 

 clay. 



Such brooks and rivulets as issued from marshes or swamps, 

 often gave indications of iron ochre or bog ii'on ore by ferrugi- 

 nous incrustations on the banks or on the bottom, and in my 

 excursion up the Grand River, numerous loose masses of bog 

 iron ore were found strewed over the surface in the Township 

 of Dumfries, near Gait, where, if it should be found in avail- 

 able quantity, it cannot fail to be of considerable importance 

 to this thriving town, in which an extensive iron foundry is 

 already established. 



On Changes of the Sea-Level effected by existing Physical 

 Causes during stated periods of time. 



By Alfred Tylor, F. G. S. 



Concluded from page 60. 

 Part II 

 Allusions have already been made to the difficulty of prov- 

 ing whether or not the sea-level had been gradually elevated, 

 because the rise of the waters would conceal the evidence of 

 their former height except just at the mouths of rivers, where 

 deposits of fluviatile alluvium might raise the land from time 

 to time and keep it above the waves. The recent strata for- 

 Pig. 1. — Diagram showing depth of the Delta 



med at a few such localities have been described by the best 

 observers ; and while there are appearances in several cases 

 which might be to some extent explained by the supposition 

 of a gradual rise of the sea-level, yet no proof could be obtained 

 without the concurrent testimony of a much greater number 

 of instances than have yet been brought forward. Sufficient 

 information, it appears, exists to show that the quantity of allu- 

 vium in the deltas of such rivers as the Mississippi, Ganges 

 and Po, is so enormous, that the accumulation must have oc- 

 cupied a period of time during which it would not be possible 

 to conceive the sea-level stationary. 



Little progress could be made in an inquiry of this kind with- 

 out clear views of the operations of rivers. The recent reports 

 of engineers upon this subject supply an important link in 

 the chain of evidence, and enable us to understand the laws 

 which govern the formation of alluvial plains along the lower 

 parts of all river-courses. 



The diagram (fig. 1) represents a section of 600 ;miles of 

 North America, through the alluvial plains and delta ofHhe 

 Missisippi,* together with a section of the Gulf of Mexico 

 from a point 100 miles east of the Balize to the continent of 

 South America. The sea-bottom is marked from the sound- 

 ings on the Admiralty Chart, and the depth of the Mississippi 

 and its fluviatile deposit are inserted from statistics collected by 

 Sir C. Lyell.t 



* For a most valuable detailed description of the physical geogra- 

 phy, &c. of the Mississippi and Ohio valley, see Mr. C. EUet's paper, 

 Smithsonian Contributions, vol. ii, 1851. 



f See note, page 26. 

 600 /eei); area 14,000 square miles ; height of the river above the sea level 275 feet at ' 

 depth of river, supposed 80 to 200 feet in this diagram ; ditto of plains, supposed to average 26i feet ; area, 16,000 square miles. 



^—3 





GiOFof 



Mexico 



* Junction with River Ohio. 

 «. a. Fluviatile strata of the plains of the Mississippi ; the slope of these plains is determined by measurement to be about 1 foot in 10,000 towards the sea. 

 c. jVIarine strata- 

 Direct distances: — Junction "with Ohio to Balize, 580 miles. Head of Delta to Balize, 180 miles. New Orleans to Balize, 70 miles. 

 [Vertical scale 1 inch to 1000 f5et. Horizontal scale 1 inch to 150 miles.] 

 Fig. 2. — Transverse section of the Mississippi, xvhereitis 1500 /fe^ wide and 100/ee^ deep, runningin the midst of an alhivial plain 50 miles wide. 

 [This diagram shows the section of slow-flowing rivers in ge7ieraL^ Vertical scale 100 feet to the inch. 



'W/' 



a, a. The levol of water in the river during flood, which is 25 feet b, h. Ai-tificial banks or levees, 4 feet high, 



above the level of the distant marshes, m, vt. d, d. The banks and plains, 



c. c. The level of water in the dry season. 7n, m. Marshes, supplied with water by filtration from the river at all seasons of the year. 



The ivhole body of -vv.ater in the river must be in motion, so that even in flood time only a small per centage of the water and alluvium in the 



stream can escape over the banks. 



