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has advanced three miles within the memory of middle-aged men.” These data give an increase of 990 feet in a year, requiring no more than 1160 years for the formation of the entire Delta.
These methods of computation are all too uncer tain to have any value in discussions like this. Many geologists frankly confess that they are wholly unreliable. Many ingenious calculations,” says Page, * ' have, no doubt, been made to approximate the dates of certain geological events; but these, it must be confessed, are more amusing than instructive. For example, so many inches of silt are yearly laid down in the Delta of the Mississippi – how many centuries will it have taken to accumulate a thickness of 30, 60, or 100 feet? Again, the ledges of Niagara are wasting at the rate of so many feet per century — how many years must the river have taken to cut its way back from Queenstown to the present falls? ... For these and similar computations, the student will at once perceive we want the necessary uniformity of factor ; and, until we can bring elements of calculation as exact as those of astronomy to bear on geological chronology, it will be better to regard our'eras,' and 'epochs,' and systems,' as so many terms, indefinite in their
* Advanced Text-book of Geology, by David Page, F. G. S. Edinburgh, 1861, p. 385.
duration, but sufficient for the magnitude of the operations embraced within their limits.”
4. Sir Charles Lyell mentions, but does not dwell upon, an alleged discovery of human remains in certain coral reefs on the coast of Florida. These reefs are in a process of growth by which it is estimated that the land advances upon the sea at the rate of one foot in a century. "In a calcareous conglomerate forming part of the above-mentioned series of reefs, and supposed by Agassiz, in accordance with his mode of estimating the rate of growth of those reefs, to be about 10,000 years old, some fossil human remains were found by Count Pourtalis. They consist of jaws and teeth, with some bones of the foot.” (Geol. Ev. p. 44.) This case is too indefinite to have any value. Nothing is stated as to the position of these remains, or the reasons for attributing to them an antiquity equal to that of the reef itself. For aught that appears, - they may be of a similar class with the famous Guadaloupe skeleton found in a ledge of shell limestone now in process of formation on the shore of that island, which is now ascertained to be the remains of a Carib Indian killed in battle about two hundred years ago.* 5. But the case most relied on to prove the re
* Dana's Manual of Geology, p. 580.
mote antiquity of man on earth appears to be that of the discovery of flint implements, constructed by man, in certain beds of river drift, accompanied by the remains of ancient animals in the valley of the Somme, in Picardy, France.
The above diagram will aid us in comprehending the phenomena of this valley. "It is situated geologically in a region of white chalk, with flints, the strata of which are nearly horizontal. The chalk hills which bound the valley are almost everywhere between 200 and 300 feet in height. On ascending to that elevation, we find ourselves on an extensive table-land, in which there are slight elevations and depressions. The white chalk itself is scarcely ever exposed at the surface on this plateau, although seen on the slopes of the hills as at a and b. The general surface of the upland region is covered continuously for miles in every direction by loam or brick earth (5), about five feet thick, devoid of fossils. To the · wide extent of this loam the soil of Picardy chiefly Cowes its great fertility: Here and there we also observe on the chalk outlying patches of tertiary sand and clay (6), with eocene fossils, the remnants
of a formation once more extensive, and which probably once spread in one continuous mass over the chalk, before the present system of valleys had • begun to be shaped out.” (Geol. Ev. p. 106.) In the bottom of the valley, which has an average width of one mile, there is a bed of gravel (1) from three to fourteen feet thick, and upon this, separated by a thin layer of impervious clay, a growth of peat (2) from ten to thirty feet in depth, through which the river now flows (c). Upon the sides of the valley (3 and 4) are beds of gravel, resembling ancient river banks, the lower one but little above the peat, the upper from eighty to a hundred feet higher. In these gravel beds are found the bones of numerous animals of races now extinct, such as the elephant, the rhinoceros, the horse, ox, deer, tiger, hyena, and others, and, mingled with these, various tools of flint, supposed to have been used for hatchets, spear-heads, knives, etc.
The geological history of this valley is assumed to have been as follows: Originally the chalk formation was continuous, filling the entire space. In some way a stream of water began to flow across this formation, by which the chalk was gradually worn away to the level of the upper gravel beds (4), and of a width equal to the present breadth of the valley at that level. Here the process was for a
time arrested, and the gravel bed, formed of the insoluble materials not carried away, settled itself in the then bottom of the valley, reaching, of course, from side to side. During this period lived and died the animals above named, and their remains were mingled and imbedded in the alluvium of the stream. At the same time, some of the primitive race of men lived there, who, not knowing the use of iron, fashioned for themselves rude instruments out of the flints once contained in the chalk formation, which they used for defense, and hunting, and for digging canoes, building huts, and the like; which implements, also, as they became worn or lost, were buried in the earth, along with the remains of the animals that perished there. After a long period, owing, probably, to an elevation of the land, the process of washing away was resumed, and the valley was further excavated to the level of the lower gravel beds (3), leaving behind the traces of the earlier alluvium, as we now find them. Then a like period of repose, followed by similar results, gave rise to the lower beds. Still another elevation caused a further scooping out of the valley to its present depth, leaving it filled with the bottom bed of gravel, which still remains. Upon this have since accumulated the vegetable remains which have covered it with a bed of peat in some places more than thirty feet in thickness.