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tion of the deposit, though not so abundant as Cymbella gastroides Kuetz., or Cysto pleura turgida (Ehr.) Kuntze. Also

common in Nebraska deposits. Cymbella gastroides Kuetz. Very common, and next to Cysto

pleura turgida (Ehr.) Kuntze, the most important species in the deposit. Common in the Nebraska deposits at Mullen,

Thedford, and Greeley county. Cymbella levis Naeg. Very rare. Occurs only rarely in the de

posit at Mullen. Cystopleura occellata (Ehr.) Kuntze. Rare. Rather common in

the top layer of the Mullen deposit. Cystopleura turgida (Ehr.) Kuntze. The most abundant species in

the deposit. Varies greatly. A very common species in the

Nebraska deposits. Cystopleura zebra (Ehr.) Kuntze. Rather common. About as

common in Nebraska deposits. Encyonema caespitosum Kuetz. Rare. Found in Nebraska only

in the Mullen deposit. Gomphonema intricatum Kuetz. Rare. Common in the Greeley

county deposit. Gomphonema montanum Schum. The form called var. subclava

tum Grun. is rather common. Found in Nebraska only in

the Wheeler county deposit. Navicula cuspidata Kuetz. Rare. Not

very common in Nebraska deposits. Navicula oblonga Kuetz. Rare. Rather common in deposits at

Thedford and in Wheeler county. Stauroneis phoenicenteron Kuetz. Only one specimen was found.

Rather common in Nebraska deposits. Synedra sp. Only a fragment was found, and this was too small

to identify. The material from the Denver deposit was taken from a railroad cut. The leading species in this deposit are the same as those in the St. Joseph deposit, but there are differences in the less frequent species.

The following species were found in it: Cocconeis placentula Ehr. Common, but forming a very small

portion of the deposit. About equally common in Nebraska

deposits. Cymbella cuspidata Kuetz. Rare. Rather common in Nebraska

deposits. Cymbella gastroides Kuetz. Common. Cystopleura gibba (Ehr.) Kuntze. Rather common, as is also the

form called var. ventricosa (Ehr.) Grun. Cystopleura turgida (Ehr.) Kuntze. Very common. Cystopleura zebra (Ehr.) Kuntze. Rather more common than in

Nebraska deposits. Encyonema caespitosum Kuetz. More common than in Nebraska

deposits. Fragilaria construens (Ehr.) Grun. The form called var, venter

Grun. is more common that the type forming a considerable

portion of the deposit. Fragilaria elliptica Schum. Common, but less abundant than in

some Nebraska deposits. Gomphonema acuminatum Ehr. Rare. Gomphonema constrictum Ehr. Less common than in Nebraska

deposits. Gomphonema herculeanum Ehr. Rare. Also rare in Nebraska

deposits. Melosira distans (Ehr.) Kuetz. Common, but not so abundant

as in Nebraska deposits. Navicula radiosa Kuetz. Rare. Not very common in Nebraska

deposits. Synedra capitata Ehr. Not very common. About equally com

mon in Nebraska deposits. Synedra ulna (Nitz.) Ehr. Not very common.

Besides the diatoms, both of these deposits contain a large number of sponge spicules of at least two distinct forms. Although all of the region in which these deposits occur was at one time covered by salt water, none of them were made at that time, for all of the diatoms found belong to fresh-water species. So it is evident that these deposits were made after the land of this region had risen out of the ocean, but when there were still freshwater lakes covering part of the region. These deposits must have been made in lakes rather than in rivers, for river conditions are too changeable to allow the forming of a large deposit. Diatoms live in rivers as well as in lakes and ponds, but the formation of a large deposit requires quiet water and practically constant conditions. So these diatom deposits tell us that during Tertiary times there were lakes in Missouri, Nebraska, and Colorado. They also tell us that the conditions were practically alike in all of these places, for the species in all of the deposits show a great similarity, a large number of them being identical. The most abundant genus is Cystopleura, and this grows attached to some filamentous algae. So we also have evidence that other algae than diatoms lived in these Tertiary lakes.

The number of diatoms in these deposits is enormous. Ehrenberg calcuated that there were 41,000,000,000 individuals in a cubic inch of diatomaceous earth. Taking the largest specimen of Stauroneis phoenicenteron that I ever found, and which is larger than any of the fossils in these deposits, we would have only about 230,000,000 individuals per cubic inch. As this number is based on the largest diatoms, it is farther from the truth than Ehrenberg's. But Ehrenberg's estimate allows a cube of only about 7 micromillimeters for each specimen, and this is probably too small for our deposits. But even taking the number obtained in using the largest diatoms, a cubic inch contains enough to give three to every person in the United States.

The time required for making these deposits is impossible to determine. If the diatoms multiplied at their most rapid rate, it would take an incredibly short time; but practically, such deposits are made rather slowly. If we started with a single dia

tom, and this diatom should divide every hour for a week, there would be 168 divisions, but for convenience we may take two hours more than a week, making 170 divisions. At the end of this time the number of diatoms would be one doubled 170 times, or about 512,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000. Now taking Ehrenberg's estimate, which is based on very small specimens, this number of diatoms would make 12,000,000,000,000,000,000,000,000,000,000,000,000,000 cubic inches of diatomaceous earth, the product of a single diatom in a week's time. Now if on every square inch we had one diatom to start with, so that these cubic inches could be placed one above another, they would make a deposit 1,000,000,000,000,000,000,000,000,000,000,000,000,000 feet deep; or 200,000,000,000,000,000,000,000,000,000,000,000 miles deep; or, to bring it nearer to our comprehension, 2,000,000,000,000,000,000,000,000,000 times the distance from the earth to the sun. At this rate, the progeny of half a dozen diatoms would in a few days fill all the space occupied by the solar system, with diatomaceous earth, enough to satisfy fully the most ardent diatom collectors. It is hardly necessary, however, to mention that diatoms do not ordinarily reproduce at this rate. This will serve as a warning to scientists to make mathematics their servant and not their master. It is quite evident that the supposition that diatoms do divide at this rate is entirely hypothetical. The “struggle for existence” kept diatoms within bounds as well in ancient as in modern times, and it is likely that the formation of these deposits occupied several, or even many years.

AN OBSERVATION ON ANNUAL RINGS.

FRED W. CARD.

The question often arises as to whether the rings of growth observed in trees are strictly annual rings. The opinion appears to be generally prevalent that they represent rather periods of growth. Even if that be true they will still be in most cases annual, as that is the normal period of growth in temperate climates. It may then be asked whether depredations of insects which defoliate the tree, or periods of drought which check its growth, will cause the formation of another ring for that year.

In order to throw some possible light on this subject a simple experiment was made in the summer of 1894. On May 19 a piece of bark about one and one-half inch square was removed from the north side of an ash tree about four inches in diameter and from a maple about three inches in diameter. Both trees were in full growth at the time and the bark lifted readily.

July 10 the leaves were stripped from both these trees, with the exception of a very few which were purposely left. By the end of the month both trees were leaving out again.

On the 10th of November both trees were cut down. A cross section cut through the points from which the bark was removed showed no evidence of the treatment which the trees had received. The ring of growth for that year was apparently as uniform as for other years.

This experiment, it should be noted, does not contradict the general opinion that there may be more than one ring formed in one year, but it does seem to indicate that a greater interference with the normal conditions of growth is needed to produce that effect than has often been supposed. It is quite possible, to be sure, that at some other part of the season the effect might have

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