A GEOLOGICAL TIME-SCALE, PREPARED FOR THE COMPARATIVE STUDY OF THE LIFE-HISTORY OF ORGANISMS.

The periods are taken as the smallest divisions of time which can be universally recognized, and hence it is assumed that they are units of equal length. This assumption probably exaggerates the length of the more recent periods.

Importance of a Standard Time-scale. For the comparative study of the history of organisms this time-scale may be used irrespective of estimates of actual length of time represented by each period.

The division of the eras into twenty successive periods is

a scheme which is actually recognized in the classification of the geological formations throughout the world, where the criteria of classification are the fossils contained in them. Geologists dealing with distinct series of strata have named the individual members of the series differently for different regions of the earth. Therefore, as the systems are made up of formations presenting local features, of stratification, of petrographic composition, of structure, and of thickness, which are given local names, the fossil fauna-floras representing each one of the periods are found in formations which have different names in separate regions.

In using such a scale it becomes necessary to correlate the faunas of formations having different names. While the formation names may well be retained, in the discussion of the time-relations of organisms it is essential to use a uniform scale of time-divisions expressed in a single series of names: the scale and names above given supply us with such a standard time-scale.

Actual Length of Geological Time. That geological time is immensely long, as compared with any human standards, all modern geologists admit; but as to how much time, in centuries or years, has elapsed since the beginning of the series of sedimentary rocks, opinions greatly differ. A few facts may be mentioned to illustrate what is meant by great length of time in terms of geological work accomplished:

(1) Since the close of the Cretaceous Period the greater part of the mountain elevation along the southern part of Europe and extending to the extreme southeastern part of Asia was accomplished; and the Himalayas were raised, so that at least 16,000 feet thickness of their mass is composed of marine strata of Tertiary or earlier era.

(2) The large part of the Rocky Mountain region was under marine water in the Cretaceous time. Since the close of the Eocene, or beginning of the Middle Tertiary, as Captain Dutton estimates, the region of the Colorado cañons has been elevated approximately 10,000 or 11,000 feet, and 10,000 feet of erosion has taken place. G. M. Dawson estimates the total amount of elevation which has taken place since Cretaceous time, in British Columbia, to have been

32,000 to 35,000 feet. There are now cañons from 5000 to 6000 feet deep, excavated entirely since the Eocene period.

(3) It is believed that all the lava outflows in the Northwest, which cover 150,000 square miles along the Columbia River and the neighboring states, and through which the Columbia has cut a channel, in some cases, from 3000 to 4000 feet deep, were erupted and laid down since Miocene Tertiary time.

(4) Niagara River gorge, from the falls down to the whirlpool, and thence to the cliffs of the lake at Lewiston, it is estimated, was cut out since the retreat of the glacial ice from the surface of the northern part of the continent, and this is believed by many geologists to represent closely the length of time since man first appeared upon the earth. The gorge is 7 miles long, one fourth of a mile wide below, narrower above the whirlpool, and varies from 200 to 500 feet in depth.* The length of time required for its excavation is estimated to have been from 10,000 to 32,000 years. Taking Dana's general estimate of relative length of time, it is seen that the time since the Cretaceous is not over one sixteenth of the time from the beginning of the Cambrian, and that the length of Quaternary time is not over one third that of the Tertiary. Whatever be the actual length of time taken for these and similar geological processes, it is evident that the same forces working at the same rate would require but the extension of time to include the whole history of the earth.

Data upon which Time-estimates are Made. Although we cannot go into full particulars respecting the theories proposed to determine the time-limits and extent of the geological ages, a few of the prominent attempts may be cited. The principal data upon which the theories have been based are as follows:

(1) Physical and Astronomical.—Estimates from the earth's heat, its rate of cooling, and the radiation of heat into space. (Kelvin.)

Estimates from influence of tidal friction, and thence to the length of time since the moon was separated off from the earth. (Darwin, G. H.)

*See J W. Spencer, "The Duration of Niagara Falls:" Am. Jour. Sci., vol. XLVIII. p. 455. December, 1894.

From the rate of the sun's loss of its stores of heat. (Tait.)
From other physical data. (Croll and others.)

(2) Geological.—(a) Calculations based upon the estimated thickness of the geological deposits of the total series of stratified rocks and the estimated rate of accumulation of deposits along the shores of continents at the present time. (Houghton, Dana, Croll, Wallace, Lyell, Humphreys and Abbott, etc.)

(b) Calculations based upon rate of erosion since the retreat of the glacial cover at the close of the Tertiary era. (Dana, Lyell, Hall, Gilbert, Winchell, etc.); and general estimates and sundry hypotheses as to the time since the glacial age. (Geikie, McGee, Croll, Prestwich, Wright, LeConte, and others.)

Method of Computing Time from Thickness of Rocks.-The elaborate report of Humphreys and Abbott on the "Physics and Hydraulics of the Mississippi River" furnishes the kind of evidence required for making the kind of calculations mentioned under (2a) above-that based upon the rate of deposition, or formation of deposits, at the mouth of rivers. The amount of silt borne down and deposited by the Mississippi River annually is estimated by Humphreys and Abbott to be equal to a mass with 1 square mile base and...... 241 feet deep, the earthy matter pushed along...

27

66

66

or a total of sediment 1 mile square by....... 268 But upon Humphreys and Abbott's estimate, and distributing the sedimentary deposit along the coast for a distance of 500 miles, and giving the strip 100 miles width (or spread it out for 1000 miles, and make it 50 miles wide), assuming the area of distribution of the product of erosion of the whole river to be 50,000 square miles,—on such assumptions the deposit in 6000 years would reach a depth of approximately 32 feet, or 53 feet in 10,000 years; or, if we put it in round numbers, 50 feet in 10,000 years. The thickness of sediments for the Devonian era is, according to Dana, 14,300 feet of clastic sediments and 100 feet of limestone; estimating the 100 feet of limestone to be equivalent in time-ratio to 500 feet of ordinary fragmental sediment, we thus obtain in terms of fragmental sediments a total of 14,800 feet. Reducing

this 14,800 feet of thickness of sedimentary deposits into timeequivalent, on the basis of the above rate of formation of sediments, we have 2,960,000 years for the duration of the Devonian era. If now we assume the Devonian to be approximately 10% of the whole time-duration from the base of the Cambrian to the present, the total time-duration would be 29,600,000, which is a little over one half the estimate suggested by Dana, viz., 48,000,000 years since the beginning of the Paleozoic time-Paleozoic 36,000,000, Mesozoic 9,000,000, and Cenozoic 3,000,000.*

Forshay's estimate makes the amount of annual deposit 964 instead of 268 feet on a base 1 mile square in 1 year's time, which is about four times as rapid accumulation as the estimate of Humphreys and Abbott, and the effect upon timeduration expressed by rock-thickness would be to reduce the time one fourth, making the Devonian 740,000 instead of 2,960,000 years long. This would bring the age of the earth, as a solid globe, nearer to the estimate of Clarence King (24,000,000 years), to which Lord Kelvin gave approval as lately as March, 1895.†

Errors arising from Estimated Values in the Computations.According to this estimate we notice that there are several important data which are assumed, and not observed or known.

(1) The thickness of the deposits themselves. Formations, as may be noticed, vary greatly in thickness for even the few localities or regions of America in which they have been studied. We find that the maximum thickness of the North American Paleozoic series is given as 55,000 feet, the general thickness of these deposits in the Appalachian region is 40,000, and in the interior of the continent it varies from 6000 to 3500. Since this estimate was made, Walcott has claimed for the Cambrian 7000 feet of fragmental rocks and 200 of limestones; in the Rocky Mountain province 10,000 feet of fragmental and 6000 feet of limestones, which, reduced to time-ratios (for limestone), gives, instead of (7000 + 1000 =) 8000, (10,000 + 30,000 =) 40,000, or five times the

*See "Manual of Geology," 3d edition, p. 591.

+ See Nature, vol. LI. pp. 438-450.