1846.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



349 



intersecting the hanks, and lastly enlarged on the long narrow promontory 

 formed hy the great river and its banks hetween New Orleans anil the Balize. 

 The advance of this singular tongue of land has been generally supposed to 

 have been very rapid; but Mr. Lyell and Dr. Carpenter, who accompanied 

 him, arrived at an opposite conclusion. After comparing the present state of 

 this region with the map published by Charlevoix 120 years ago, they doubt 

 whether the land has, on the whole, gained more than a mile in the course 

 of a century. A large excavation, eighteen feet deep, made for the gas- 

 works at New Orleans, and still in progress in March 1S4C, shows that much 

 of the soil there consists of fine clay or mud, containing innumerable stools 

 of trees, buried at various levels, in an erect position, with their roots at- 

 tached, implying the former existence there of fresh-water swamps, covered 

 ■with trees, over which the sediment of the Mississippi was spread during 

 inundations, so as slowly to raise the level of the ground. As the site of the 

 excavation is now about nine feet above the sea, the lowest of these upright 

 trees ijiiply that the region where they grew has sunk down about nine feet 

 below the sea-level. The exposure also in the vertical banks of the Missis- 

 sippi at low water, for hundreds of miles above the head of the delta, of the 

 stumps of trees buried with their roots in their natural position, three tiers 

 being occasionallv seen one above the other, shews that the river in its wan- 

 derings has opeired a channel through ancient morasses, where trees once 

 grew,'^and where alluvial matter gradually accumulated. The old deserted 

 bed also of the river, with its banks raised fifteen feet above the adjoining 

 low grounds, bears testimony to the frequent shifting of the place of the 

 main stream ; and the like inference may be drawn from the occurrence here 

 and there of crescent-shaped lakes, each many miles in length, and half a 

 mile or more in breadth, which have once constituted great curves or bends 

 of the river, but are now often far distant from it. The Mississippi, by the 

 constant undermining of its banks, checks the rise of large commercial 

 towns on its borders, and causes a singular contrast hetween the wealth and 

 splendour of SOO or more fine steamers, some of which may truly be called 

 floating palaces, and th e flat monotonous wilderness of uncleared land 

 which extends for hundreds of miles on both sides of the great navigable 

 stream. . 



Mr. Lyell visited, in March 18-(6, the region shaken for three months m 

 1811-12 hy the earthquake of New Madrid. One portion of it, situated in 

 the states of Missouri and Arkansas, is now called the sunk country. It ex- 

 tends about seventy miles north and south, and thirty east and west, and is, 

 for the most part, 'submerged. Many dead trees are still standing erect in 

 the swamps, and a far greater number lie prostrate. Even on the dry ground 

 in the vicinity all the forest trees which are of prior date to 1811 are leafless; 

 they are supposed to have been killed by the loosening of their roots by the 

 repeated shocks of 1811-12. Numerous rents are also observable in the 

 ground where it opened in 1811, and many "sink holes," or cavities, from 

 ten to thirty yards wide, and twenty feet or more in depth, interrupt the 

 general level of the plain, which were formed by the spouting out of large 

 quantities of sand and mud during the earthquake. 



Formation of the Delta occupied 67,000 years. 



In attempting to compute the minimum of time required for the accumu- 

 lation of the alluvial matter in the delta and valley of the Mississippi, Mr. 

 Lvell referred to a series of experiments made by Dr. Riddell at New Or- 

 leans, showing that the mean annual proportion of sediment in the river was 

 to the water -j^'j^ in weight, or about ^oVo in volume. From the observa- 

 tions of the same gentleman, and those of Ur. Carpenter, and of Mr. Furshay, 

 an eminent engineer of Louisiana, the average width, depth, and velocity of 

 the Mississippi, and thence the mean annual discharge of water were de- 

 duced. In assuming 528 feet, or the 10th of a mile, as the probable thick- 

 ness of the deposit of mud and sand in the delta, Mr. Lyell founds his con- 

 jecture on the depth of the Gulf of Mexico between the southern point of 

 Florida and the Balize, which equals on an average 100 fathoms. The area 

 of the delta being about 13,600 square statute miles, and the quantity of 

 solid matter annually brought down hy the river, 3,702,758,400 cubic feet, 

 it must have taken 67,000 years for the formation of the whole ; and if the 

 alluvial matter of the plain above be 2G4 feet deep, or half that of the delta, 

 it has requiied 33,500 more years for its accumulation, even if its area be 

 estimated as only equal to that of the delta, whereas it is in fact larger. If 

 aome deduction be made from the time here stated in consequence of the 

 effect of the drift-wood which roust have aided in filling up more rapidly the 

 space above alluded to, a far more important allowance must be made, on 

 the other hand, for the loss of matter, owing to the finer particles of mud not 

 settling at the mouths of the river, but being swept out far to sea, and even 

 conveyed into the Atlantic by the Gulf stream. Yet the whole period dur- 

 ing which the Mississippi has transported its earthy burden to the ocean, 

 though perhaps far exceeding 100,000 years, must be insignificant in a geo- 

 logical point of view, since the bluffs, or cliff's, bounding the great vallcv, and 

 therefore older in date, and which are from 50 to 250 feet in perpendicular 

 height, consist in great parts of loam containing land, fluviatile, and lacus- 

 trine shells, or species still inhabiting the same country. These fossil shells, 

 occuring in'a deposit resembling the loess of the Rhine, are associated with 

 the bones of the mastodon, elephant, tapir, m\lodon, and other megathoid 

 animals ; also a species of horse, ox, and other mammalia, most of them ex- 

 tinct species. The loam rests at Vicksburg and other places on eocene or 

 lower tertiary strata, which in their turn repose on cretaceous rocks. A sec- 

 tion from Vicksburg to Darien, through the states of Mississippi, Alabama, 

 aud Georgia, exhibits this superposition, as well as that of the cretaceous 



strata on carhonift rous rocks at Tuscaloosa. Mr. Lyell ascertained that the 

 huge fossil cetacean named Zeuglodon by Owen is confined to the eocene 

 deposits. In the cretaceous strata the remains of the mosasaurus and other 

 reptiles occur without any catecea. The coal-fields of Alabama were next 

 alluded to, from which fossil plants have been procurred by Professor Brum- 

 by and Mr. Lyell, of the genera sphenoptcris, neuropteris, calamities, lepido- 

 dendron, sigillaria, stigmaria, and others, most of them identical in species, 

 as determined hy Mr. C. ISunbury, with fossils of Northumberland. This 

 fact is the more worthy of notice, because the coal of Tuscaloosa, situated in 

 latitude 33° 10' north, is farther south than any region in which this ancient 

 fossil flora had previously been studied, whetlier in Europe or North Ameri- 

 ca ; and it aft'ords, tlierefore, a new proof of the wide extension of a uniform 

 flora in the carboniferous epoch. Mr. Lyell, adverting to the opinion recently 

 adopted by several idle botanists, that the climate of the coal period was re- 

 markable for its moisture, equability, and freedom from cold, rather than 

 tlie intensity of its tropical heat, stated that this conclusion, as well as the 

 oscillations of temperature implied by the glacial period, is confirmatory of 

 the theory first advanced by him in 1830, to explain the ancient geological 

 changes of climate by geographical revolutions in the position of land and 

 sea. 



The lapse of ages implied by the distinctness of the fossils of the eocene, 

 cretaceous, carboniferous, and other strata is such, tiiat were we to endea- 

 vour to give an idea of it, we must estimate its duration not hy years, as ia 

 the case of the delta, but by such units as would be constituted by the inter- 

 val between the beginning of the delta and our own times. 



Doctrine of the earth's antiquity not to be refuted by prejudice. 



" It is now fifty years," said Mr. Lyell, " since Playfair, after studying the 

 rocks in the neighbourhood of Edinburgh in company with Dr. Hutton and 

 Sir James Hall, was so struck with tlie evidences they afforded of the im- 

 mensity of past time, that he observed, ' How much farther reason may go 

 than imagination can venture to follow !' These views were commou to the 

 most illustrious of his contemporaries, and since that time have been adopted 

 by all geologists, whether their minds have been formed hy the literature of 

 France or of Germany, or of Italy, or Scandinavia, or England ; all have ar- 

 rived at the same conclusion respecting the great antiquity of the globe, and 

 that, too, in opposition to their earlier prepossessions, and to the popular 

 belief of their age. It must be confessed that while this unanimity is satis- 

 factory as a remarkable test of truth, it is somewhat melancholy to reflect 

 that at the end of half a century, when so many millions have passed through 

 our schools and colleges since Playfair wrote that eloquent passage, there 

 should still be so great a discordance between the opinions of scientific mea 

 and tlie great mass of the community. Had there been annual gatherings 

 such as this, where they who are entitl d to speak with authority address 

 themselves to a numerous assembly drawn from the higher classes of society, 

 who, by their cultivation and influence, must direct the education and form 

 the opinions of the many of humbler station, it is impossible that so unde- 

 sirable and unsound a state of things should have uow prevailed, as that there 

 should he one eieed for the philosopher, and another for the multitude. Had 

 there been meetings like this even for a quarter of a century, we should 

 have already gained for geology the same victory that has been so triumph- 

 antly won by the astronomer. The earth's antiquity, together with the his- 

 tory of successive races of organic beings, would have been ere this as cheer- 

 fully and universally acknowledged as the earth's motion, or the number, 

 magnitude, and relative distances of tiie heavenly bodies. I am sure it 

 would be superfluous if 1 were to declare, in an assembly like this, my deep 

 conviction which you — all of you — share, that the farther we extend our re- 

 searches into the wonders of creation in time and space, the more do we ex- 

 alt, refine, and elevate our conceptions of the Divine Artificer of the uni- 

 verse. 



Mr. Lyell concluded this discourse by announcing his corroboration of the 

 discovery recently made by Dr. King at Greensburg, thirty miles from Pitts- 

 burg in Pennyslvania, of the occurrence of fossil footprints of a large rep- 

 tilion in the middle of the ancient coal measures. They project in relief 

 from the lower surface of slabs of sandstone, and are also found impressed 

 on the subjacent layers of fine unctuous clay. This is the first well-estab- 

 lished example of a vertehrated animal more highly organised than fishes 

 being met with in a stratum of such high antiquity. 



COMPARISON OF SCREW PROPELLERS AND PADDLE 



WHEELS. 

 (With Engravings, see Plate XVI. J 



The B«<ird of Engineers of the United States Navy having been officially 

 directed to inquire respecting the merits of dillerent methods of propelling 

 steam-vessels, presented, at the commencement of the present year two re- 

 ports (dated in January aud May respectively), detailing the result of their 

 enquiries. We are euabled to give the following analysis of these docu- 

 ments. 



The first report contains a comparison of the relative effects and ralues 



43 



