790 GEOLOGY: R. J. RUSSELL PROC. N. A. S.

which underlie the restoration of nerve fibers by the anodal current (cf. ref. 6,chap. XIII).A certain periodicity of the variations of threshold appears in curve V (Fig. 5),

the period corresponding approximately to internodal lengths. The question,however, must be left open whether such a periodicity was coincidental or was due tothe fact that with the nerve fiber in a certain state the anesthetic acts more stronglyupon certain parts of the internodes than upon other parts.Summary.-Evidence has been presented to show that all points of the inter-

nodes of peripheral myelinated fibers produce action potentials in response to ap-plied cathodal currents, the stimulation threshold being practically uniformthroughout each internode. Anesthetics act upon all points of the internodes.Model experiments have shown that the nodes of Ranvier can have only a negligibleeffect on the electrotonic flow of action currents, or applied currents, throughoutthe myelinated internodes.

* This work has been supported in part by a grant (NB 02650) from the U.S. Public HealthService.

1 Lorente de No, R., and V. Honrubia, these PROCEEDINGS, 52, 305 (1964).2 Adrian, E. J., and D. W. Bronk, J. Physiol., 66, 81 (1928).3 Working with isolated nerve fibers, evidence has been obtained, to be presented elsewhere, that

the nodes of Ranvier either do not produce an action potential, or produce an action potentialwhich is too small to be detectable with present-day techniques.

4 Kato, G., in Excitation Phenomena, Cold Spring Harbor Symposia on Quantitative Biology,vol. 4 (1936), p. 200.

6 Tasaki, I., Nervous Transmission (Springfield, Illinois: Charles C Thomas, 1953).6 Lorente de No, R., A Study of Nerve Physiology (Studies from the Rockefeller Institute, 1947),

vols. 131, 132.

DURATION OF THE QUATERNARY AND ITS SUBDIVISIONS

BY RICHARD J. RUSSELLCOASTAL STUDIES INSTITUTE, LOUISIANA STATE UNIVERSITY, BATON ROUGE

Communicated July 9, 1964

Estimates of the length of the Quaternary vary tremendously, as based on widelydivergent lines of evidence such as astronomical calculations, rates of weathering,rates of sediment accumulation, and isotope abundance. The model presentedhere is based on changes of level of land and sea.The essential reason for recognizing the Quaternary is climatic.' The record of

its history is stratigraphic.2 By the time the earliest accumulation of continentalice attained sufficient volume to lower sea level appreciably, rivers began to inciseall land surfaces along routes leading to the oceans. Sediment deposition wasaccelerated around continental margins and shifted to progressively lowering levelsduring the waxing of the first glaciation. There is certainly no merit in postulatingthe start of the Ice Age during pre-Quaternary time on biological evidence. Theessential criterion is the time when Quaternary sea level began to lower.At-the initiation of the Quaternary, sea level approximated the stand that would

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be attained were most Antarctic and other large ice accumulations to melt. Presum-ably this level stood at least 200 ft above today's stand.3 Postglacial time willnot arrive until that level is regained. This condition has not been closely approxi-mated since the start of the Quaternary. During five major oscillations, however,sea level has dropped to much lower stands, each of which undoubtedly marks a timeof maximum continental ice accumulation. The low stands are established byseveral lines of evidence, among which are the depths to which rivers leading tooceans cut their valleys. The record of each sea level recovery during times whenice volumes diminished is preserved by sedimentary deposits that first accumulatedbeyond continental margins, crept upward across the shelves, and eventually ex-tended inland, along valleys. The surfaces of four of these depositional sequencesnow stand as terraces bordering valleys and coasts. The fifth is exhibited as today'svalley flood plains and coastal flats.4 Each of these surfaces caps a Quaternaryformation.

Across a zone of coastal flexing, each terrace formation increases its seaward slopeand, in progression, passes beneath the level of the next-younger member of theseries, to arrive in its appropriate stratigraphic position under coastal plains and thecontinental shelf.5 This zone varies in width front about 10 miles in the Rh"one toover 200 in the Lower Mississippi Valley.From oldest to youngest, Fisk named Louisiana terraces and formations Williana,

Bentley, Montgomery, and Prairie, and demonstrated their genetic equivalencewith the Recent alluvium.6 He did not propose these nanmes for universal adoption,but applied them locally, without suggesting stratigraphic correlations that latermight be found in error. However, he suggested synchronism with widely recog-nized glacial and interglacial stages of the American Pleistocene. As there is notcomplete agreement about these, either in North America or their equivalents acrossthe Atlantic, it appears desirable at present to regard times of maximum ice coverand lowest sea level as Glaciations I, Il, III, IV, and V. In the following discussionFisk's terminology will be followed, and times of minimum ice cover will be identifiedas W, B, M, P, and F. Each marks the start of major lowering of sea level and timewhen a Quaternary surface was completed.

Inland from the zone of coastal flexing, the vertical intervals between terracesurfaces remain about constant for hundreds of miles. This was established manyyears ago in Europe7 and is the case along the east front of the Ozarks and on Crow-leys Ridge in Arkansas. At Forrest City the intervals are W-B, 150 ft; B-M, 100;M-P, 60; and P-F, 40. These are plotted on the right in Figure 1. An olderinterpretation would regard these intervals as evidence of a progressive eustaticlowering of sea level.8 This hypothesis is suggested by the dotted line in the figure,which starts at the elevation of the Williana and ends at F, the flood plain related totoday's sea level.For the reason that all Quaternary deposits appear in normal stratigraphic

order and are well known along the northern Gulf Coast, the traditional eustatichypothesis is rejected in favor of recognizing uplift of continental interiors as ex-plaining the elevated positions of terraces.9 As remnants of the Williana formationin the vicinity of Forrest City lie more than 500 ft above sea level and well above anypossible sea level when the Quaternary was initiated, it is necessary to recognizethat continental uplift has occurred. A similar explanation is demanded to explain

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792 GEOLOGY: R. J. RUSSELL PROC. N. A. S.

LEVEL (FT)400Peitcee-. -- 2 --R00

N.600 ? b |* 500 |- 440 > | *14 0 100

B F -00

-_300r-400I-ok--Glaciations 11..I III IV V 500

-.O-~Plei s tocene -- - -- R-600 ?-.j4---500 --I--------*~44 -1 80

IN [HOUSANDS OF vEARS

FIG. l.-Oscillations of Quaternary sea level. Terrace elevations above adjacent flood plainat Forrest City, Arkansas, and suggested minimum sea levels of five glacial stages are indicatedon the vertical scale to the right. Measured highest stands during interglacial stages are designatedB, M, P, and F. Estimates of earlier stands are designated S and W. If the estimate of 80,000years since the last major interglacial is realistic, the Quaternary started at least 1.8 million yearsago.

terrace elevations along the Rhone, Rhine, Tarsus, various rivers in north Africa,Western Australia, and presumably most large rivers leading to the oceans.Whether the elevation of S (start of the Quaternary) is 200 ft, as indicated in thediagram, has no bearing on the following discussion.

If uniformity in rate of uplift in Arkansas is assumed, the W terrace wouldhave attained an elevation of 150 ft by B time, and finally its elevation of 350 fttoday, as shown along a dashed line. In a similar manner the surfaces of B, M, andP attained their observed elevations above today's Mississippi flood plain in thevicinity, as indicated by other dashed lines.

Evidence of higher stands of Pleistocene seas should be apparent on manycoasts and, indeed, this appears to be the case, but there is tremendous variabilityin reported elevations.'0 One explanation of this is the use of faulty criteria forshoreline determinations, and another is the crustal instability of almost all coasts. 1'With a background of having studied coasts facing all oceans, on the shores of allcontinents excepting Antarctica, Russell and McIntire conclude that minimalcoastal deformation occurs in parts of Western Australia and on the crystalline-rockislands of the Seychelles. During 1961 and 1963 they studied these coasts in detailand now have several reports in preparation. Interior continental uplift is evi-denced in Western Australia by the seaward slopes of Pleistocene terraces, but thehorizontality of Quaternary shorelines remains within narrow limits for hundreds ofmiles along separated parts of the coast, and coastal flexing is minimal. What wasmost impressive in our studies is the fact that elevations established in WesternAustralia agreed exactly with those in Seychelles, on the opposite side of theIndian Ocean.Along all tropical coasts the best guide for establishing low-tide level is the con-

tact between reef-flat accumulations and overlying incrusting organic growths.This contact is the most reliable criterion for determining either today's or earliersea levels. To establish a Pleistocene sea level, all that is necessary is to measurethe vertical distance between this contact and its older equivalent. Beaches, seacliffs, notches, abrasion platforms, and similar guides are too variable in positiontoday to be regarded as satisfactory for determining earlier changes of level."On the basis of observations at the Quobba-Gnaraloo estate boundary north of

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Carnarvon, in the vicinity of Perth, and at many places near Geraldton, as well ason the islands of Mah6, Silhoutte, and La Digue, in the Seychelles, there is clearevidence of three high Pleistocene sea stands, which in chronological order fromoldest to youngest occur at approximately 0, 32, and 16 ft above today's sea level.The 0 level in Western Australia is a reef flat overlain by shelly beach deposits

that continue inland under sand dunes now cemented to eolianite ("Coastal Lime-stone") in the vicinity of Perth and other places north to Wallal Downs. The 32-and 16-ft levels are represented by reef-flat deposits occurring along the seawardface of the eolianite and therefore are younger. The higher bench occurs as scatteredremnants that have experienced more solution, travertine development, and ero-sional change than the comparatively fresh deposits along the much better preserved16-ft bench, which is certainly the youngest member of the series. These conditionsare duplicated in the Seychelles. Several radiocarbon assays show that the 16-ftlevel is older than assay potentialities (greater than 37,000 years), and hence isPleistocene in age if the Recent (R) is defined as the time during which sea levelexperienced its last major rise. 12

There is general agreement that the elapsed time since the latest Pleistocene(Late Wisconsin) high sea-stand is short. Trenches now filled with Recent allu-vium but which were cut by rivers leading to the last low-level sea are narrow andsteep-walled. 13 The gradient along the Lower Mississippi trench between Cairo,Illinois, and the inner border of the coastal marshes is remarkably uniform at about0.83 ft/mile.14 When this gradient is projected to the shore of the Gulf of Mexicoat the time of low stand, a sea level of -450 ft is indicated. The reason for pro-jecting this gradient is to avoid the effects of faulting and compaction of sedimentsacross the coastal marshes. These have lowered the Recent-Pleistocene contact to adepth of - 550 ft on the nearby continental shelf, a level shown in many cores fromoil wells, which reflects both sea-level rise and local subsidence. " A level of -450ft across a broad platform west of Kauai, Hawaii, has been cited as indicating thelast major low-stand of sea level. 16

While radiocarbon ages of organic materials filling river trenches may suggestthat valley fills accumulated during the last 18,000 years, those obtained from therecent formation on the shelf include samples beyond assay range.' It appearsreasonable to assume that sea level attained its lowest position about 50,000 yearsago and that about 30,000 years were involved in the drop from its P.level. Therecognition of a shorter interval for lowering appears justified for several reasons.Ice volumes, in all probability, increased more rapidly than they dissipated. Theintervals between halts in the retreat of the last ice sheet across Poland and countriesnorth of the Baltic, and central North America, appear to account for more than halfof the time since the preceding interglacial. Where low-sea-level valley trenches areknown in most detail, as along the northern Gulf Coast, they exhibit few tributariesand all have steep gradients, indicating that little time was available for developingnew drainage systems, even in poorly consolidated bedrock. Little evidence ofvalley-widening occurs along major trench walls. If the assumption of 80,000 yearsfor the last glacial stage proves incorrect, the effect will be that of changing timeestimates, but not ratios between glacial-stage intervals. If we adopt the estimateof 80,000 years as the time between P and F (Fig. 1), assume that uniformity existedin the rate of uplift in Arkansas, and regard Western Australian and Seychelles sea-

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794 GEOLOGY: R. J. RUSSELL PROC. N. A. S.

level determinations as representing B, M, and P time, we have a basis for estimat-ing the duration of four of five subdivisions of the Quaternary.The assumption of uniform rate of uplift in northern Arkansas, in an inland loca-

tion where only slight crustal deformation has occurred since Precambrian time,seems more reasonable than alternative hypotheses. The uplift resembles that ofother continents and may be a result of change of phase in the crust or mantle.Whatever the reason, it is unlikely that significant changes in rate would occur dur-ing a few million years.The 40-ft interval between F and P indicates an uplift of 24 ft at Forrest City,

be ause P was established when sea level stood 16 ft higher than it does today.The 60-ft interval between P and M, for similar reason, indicates an-uplift of 44ft. The M-time sea level stood 16 ft above that of P time. As in B time sea levelwas at today's position, the 100-ft interval between M and B indicates an uplift of132 ft. If we assume that the stand at W time stood somewhat below today'slevel, an uplift of at least 150 ft occurred between W and B. The sum of theseintervals is the 350 ft separating the flood plain from the Williana surface inArkansas.Our failure to find evidence of any W stand on the stable coasts of Western

Australia and Seychelles, even though terrain conditions are favorable for its recog-nition, leads us to the conclusion that the highest level attained was somewhat lowerthan sea level today. The thickness of the Williana formation on the Ozark frontsouth of Batesville, Arkansas, is so similar to the thickness of the Recent alluviumin the vicinity that there is no reason to believe that the magnitude of the pre-Woscillation differed appreciably from that of the pre-Recent.The greatest uncertainty in assigning values in years to subdivisions of the

Quaternary is the estimate that the last period of minimum Pleistocene ice coveroccurred 80,000 years ago. All ratios would remain fixed but the times representedby each would be shortened or lengthened by a more certain value of the time be-tween P and F. The indicated rate of uplift in Arkansas, of 0.03 ft/century, wouldalso be altered slightly. If the P-F interval is 80,000 years, Al-P amounts to 147,-000; B-M, 440,000; and W-B, 500,000.

Less certain is the initial pre-Quaternary elevation (S) and the time betweenthen and W. There is no reason to believe that S-W was shorter than W-B. If weassume that 600,000 years were involved, the total duration of the Quaternary wouldamount to 1.8 million years. This should be regarded as a minimum estimate andone based entirely on the slow uplift of a region that has remained tectonically in-active during Quaternary sea-level oscillations. There was little factual supportfor the long-accepted estimate of one million years and it might be well to take a hardlook at estimates based on geochemical evidence that suggest even shorter duration.Although adequate funding could solve the problem, not much is known about

volumes of continental ice and levels to which the seas dropped during any but thelast glaciation. The volume was probably greatest during Glaciation III, and sealevel is placed somewhat lower in the figure than the others, which are at -450 ft. 18

When better data are available, the lower limits of oscillation may be corrected. Theproposed curves are essentially sinusoidal, but are distorted in the direction of allow-ing more time for sea-level rise in each instance. While suggested by the most re-cent case, this pattern may not be factual, but neither the shapes of the curves nor

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VOL. 52, 1964 GEOLOGY: R. J. RUSSELL 795

the depths to which sea levels actually dropped affects the time estimates presentedhere. It is well known that many halts occurred during the last general rise, and evi-dence from major moraines demonstrates that some reversals in trend occhrred,19but these are details in the generalization attempted here.Many observational facts appear to support the validity of the suggeste oscilla-

tion pattern. Earlier interglacial stages are regarded by most investigators aslonger than those that followed. The fluvial surfaces of Prairie and Montgomeryterraces in the states north of the Gulf of Mexico are comparatively narrow bandsthat are restricted to areas near present flood plains, whereas the older terraces aretypically much more widespread and their deposits occupy broader and less clearlydefined valleys. The drainage systems filled by their formations had sufficient timeto widen valleys and reduce the slopes of their confining walls. Contrasting in-clinations of terrace slopes in the zone of coastal flexing provide ratios that agree wellwith the suggested intervals between times of terrace development. The Prairiesurface slopes seaward in central Louisiana and southeastern Texas at gradientsonly slightly steeper than those of nearby flood plains, but the contrast betweenWilliana and Bentley is much greater than between other adjacent pairs. Althoughrates of leaching, oxidation, and accumulation of secondary minerals are not com-pletely satisfactory indicators of terrace antiquity, it is true that these and relativedegrees of erosional dissection agree in a general way with a spacing of Quaternaryevents at intervals such as are presented in Figure 1.

H. N. Fisk and W. G. McIntire participated in field investigations. Investigations in Turkeyand the Indian Ocean were supported financially by the Geography Branch of the Office of NavalResearch, contract no. Nonr 1575 (03). Radiocarbon assays were furnished by the GeochemicalLaboratory and Exploration Department of Humble Oil and Refining Co., Houston, Texas.

1 Flint, R. F., Glacial Geology and the Pleistocene Epoch (New York: John Wiley and Sons,1947).

2Russell, R. J., Intern. Geol. Congr., 9th, London, 1948, part 9, 94.3Ahlmann, H. W:S., Glacier Variations and Climatic Fluctuations (New York: Am. Geogr.

Scc., 1953).4Fisk, H. N., Louisiana Dept. Conserv. Geol. Surv., 10 (1938); ibid., 18 (1939); Dubois, G.,

Bull. Soc. Geol. France, (4 ser.), 24, 857 (1925).5Frink, J. W., Louisiana Dept. Conserv. Geol. Surv., 19, 367 (1941); Fisk, H. N., J. Geomorphol.,

2, 181 (1939); Fisk, H. N., and E. McFarlan, Jr., "The crust of the earth," Geol. Soc. Am. Spec.Papers, 62, 279 (1954).

6 Fisk, H. N., Louisiana Dept. Conserv. Geol. Surv., 18 (1939).7Lamothe, Le G6neral de, Bull. Soc. GMol. France, (4 ser.), 1, 297 (1901); 15, 3 (1915); 18,

1 (1918); 21, 97 (1921); Chaput, E., Ann. de Gdogr., 28, 81 (1919); Deperet, C., Compt. Rend.Acad. Sci., 166, 480, 636, 884 (1918); 174, 1502, 1594 (1922).

8 F. E. Zeuner [The Pleistocene Period (London: Hutchinson, 1959)] presents the thesis ser-iously, with many references.

9 Russell, R. J., Intern. Geogr. Congr. (Amsterdam), 2d (1938), p. 406; Bull. Geol. Soc. Am., 51,1199 (1940); Fisk, H. N., Louisiana Dept. Conserv. Geol. Surv., 10 (1938); ibid., 18 (1939); J.Geomorphol., 2, 181 (1939).

10 Russell, R. J., ed., Z. Geomorphol. Suppl., 3 (1961).l1 Russell, R. J., Sonderheft, 8, 25 (1964).12 Russell, R. J., Z. Geomorphol. (1948).13 Fisk, H. N., Geological Investigation of the Alluvial Valley of the Lower Mississippi River (Miss.

River Comm., Vicksburg, 1944); Saucier, R. T., Recent Geomorphic History of the PontchartrainBasin, Louisiana (Baton Rouge: La. State Univ. Press, 1963), Coastal- Studies Series 9.

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14 Fisk, H. N., Geological Investigation of the AlUuvial Valley of the Lower Mississippi River (Miss.River Comm., Vicksburg, 1944).

16 Russell, R. J., Bull. Geol. Soc. Am., 69, 1 (1958).16Inman, 1). L., W. R. Gayman, and D. C. Cox, Pacific Sci., 12, 106 (1963).17 McFarlan, E., Jr., Bull. Geol. Soc. Am., 72, 129 (1961); Fisk, H. N., Geometry of Sandstone

Bodies (Tulsa: Am. Assoc. Petrol. Geol., 1961), vol. 29.18 Donn, W. I., W. R. Farrand, and M. Ewing, J. Geol., 70, 206 (1962); Fisk, H. N., J. Geol.,

59, 333, 1951.19Jelgerama, S., Mededel. Geol. Sticht., 7 (1961); Goreau, T. F., Report to Biology Branch, Of/ice.

of Naval Research (1961); Emery, K. O., Z. Geomorphol. Suppi., 3, 17 (1961); Kaye, C. A., U. S.Geol. Surv., Profess. Paper, 317B, 49 (1959).

THE ENZYMATIC SYNTHESIS OF A CIRCULAR DNA-RNA HYBRID*

BY ALIX BASSEL, M. HAYASHI, AND S. SPIEGELMAN

DEPARTMENT OF MICROBIOLOGY, UNIVERSITY OF ILLINOIS, URBANA

Communicated by Nelson J. Leonard, July 27, 1964

It was first noted by Doerflerl and his colleagues that RNA synthesized on asingle-stranded DNA template yielded a product markedly resistant to RNAase,suggesting2 the appearance of a DNA-RNA hybrid in the reaction mixture. Thissupposition was confirmed by Warner et al.3 who characterized the density andthermal transitions of the product. Recently, two groups4' 6 have used the single-stranded6 DNA of the bacteriophage 4X174 in a similar study. They confirmedthe conversion of the single-stranded templates to DNA-RNA hybrids and showedthat free RNA does not appear until the composition of the hybrid approaches alimiting value of 1:1.The virus 4X174 has proved to be a potent tool for the experimental analysis of

genetic transcription. With its aid it has been shown7 that RNA messages found inthe infected cell are complementary only to the complement of the mature strand.The development' of a chromatographic procedure provided pure preparations ofthe double-stranded replicating form (RF-DNA) of 4X174 which were shown9 toconsist of over 95 per cent intact circular structures. With the aid of these it waspossible to reproduce0 in the test tube the strand selection mechanism observed inthe cell.The ready availability of pure double-stranded circular RF-DNA and the single-

stranded component of the virus particle permitted an informative comparisonof the RNA polymerase reaction with these two sorts of templates, both intact andfragmented. We report here some of our results with the single-stranded DNAsince they both confirm and augment the available data. The experiments to bedescribed show that during the course of the reaction, RNA appears in threedifferent density regions of a Cs2SO4 gradient. One can be identified with a ribonu-clease-resistant hybrid structure. The other two are both sensitive to ribonu-clease, one corresponding to free RNA and the other found in the density region ofsingle-stranded DNA. It will further be shown by electron microscopy that thecollapsed coil of the single-stranded DNA template is gradually converted in thecourse of the reaction to a hybrid structure possessing circular morphology.

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