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England. Incomplete as this field evidence is, it suggests a reason why the post-Glacial deformation of Newfoundland has been more controlled by the adjacent, masterful cap centering in Quebec than by its own load of ice; such a relation would be expected on the recoil theory of the deformation.

Recent Drowning of Southern Newfoundland and of Southern Nova Scotia.-Yarmouth, Nova Scotia, clearly lies outside of the area uplifted since the Glacial period. The zero isobase cuts across the shore somewhere between that point and Digby, where the uplift has been about 40 feet. A visit to Pictou Landing on Northumberland Strait showed that this locality lies south of the zero isobase." The 1920 observations thus confirm the essential accuracy of De Geer's map, published in 1892. With the exception of a small area in the northwest, Nova Scotia has not been uplifted since the latest driftsheet was deposited. On the contrary, post-Glacial drowning is manifest all along the coast from Yarmouth to Halifax, and at Sydney. The same process has apparently affected most, if not all, of Cape Breton Island, and also the Newfoundland shore south of the zero isobase.

Causes of the Drowning.-The positive movement of the sea-level is in part referable to its general rise as the Pleistocene land-ice melted. If the rise of the glaciated tract north of the zero isobase was largely an elastic reaction of the earth, additional drowning outside the zero isobase is to be credited to gravitational disturbance. Under the weight of the ice-cap the material of the earth's interior was condensed. Each radial element was compressed with consequent lowering of its center of gravity. The observed lag in uplift implies that this condition existed for some time after the ice melted away. The horizontal component of the mass attraction exerted by the radial element on the ocean water was less before the elastic upheaval than after that upheaval. The mass of the element was not changed by its expansion, but the distribution of the mass was changed. The fraction of

W. H. Twenhofel (this Journal, vol. 28, p. 147, 1909) found raised beaches, at altitudes above sea of about 25, 75, and 125 feet, in the shorebelt only 25 miles east of Pictou Landing. The present writer had no opportunity of visiting Twenhofel's locality (Arisaig). Since the latest drift-cover around Pictou has evidently not been washed by the sea, it is not easy to understand Twenhofel's results, except on the assumption that the Arisaig benches antedate the last glaciation.

the element measured by the amount of post-Glacial uplift was, during the application of the ice-load and the subsequent lag, represented by subsurface matter compressively condensed by the load. If the earth's compressibility varies according to the law deduced from seismograms, the center of gravity of the excess mass thus developed in depth may have been many hundreds of kilometers below the surface. The horizontal component of the attraction exerted by the element, at the earth's surface, would therefore be less than that exerted by the same mass when expanded because of unloading. A moderate rise of sea-level near and within the glaciated area should be expected.

Drowning in the belt outside the zero isobase may also result from the isostatic restoration of crustal equilibrium after unloading. Jamieson, Munthe, Barrell, and the writer have found some evidence that the weight of an ice-cap produces a centrifugal, viscous flow of subcrustal material and consequent low bulges along the margin of the glaciated tract. After the melting of the ice a return viscous flow toward the center of the glaciated tract should be expected. Barrell pointed out that, during the process of attaining final equilibrium, the crust underlying the marginal bulge should be lifted somewhat too high; and that, after the central region had nearly reached its final position, the bulge would slowly subside. Any coastal part of this belt would undergo progressive drowning for some time after the purely elastic deformation was completed.

Thus, in the marginal belt the shore contour at sea-level would be first affected by the return of water to the sea, a process accompanied by an immediate elastic uplift of the glaciated tract, with concomitant effect on mass attraction: then by the delayed uplift due to elastic afterworking and by accompanying viscous inflow, with further change in mass attraction; lastly, by slow subsidence in the bulged, marginal belt, entailing a positive movement of the sea in any coastal part of that belt. This third cause of drowning would persist long after the action of

Cf. J. H. Pratt, The Figure of the Earth, 4th ed., London, 1871, p. 214; G. H. Darwin, Scientific Papers, Cambridge, England, 1910, vol. 3, p. 29. ST. F. Jamieson, Geol. Mag., vol. 9, p. 461, 1882; H. Munthe, Geol. Fören. Stockholm Förhandl., vol. 32, 1910-reprinted as Guide-book No. 25, Cong. Géol. Internat., Stockholm, 1910; J. Barrell, this Journal, vol. 40, p. 13, 1915; R. A. Daly, Bull. Geol. Soc. America, vol. 31, 1920, p. 303.

the other causes had ceased to be important, and it would not be surprising if the third cause is still locally at work.

The available evidence appears to warrant belief that the deformation of the earth's crust under glacial loads has been chiefly elastic. Assuming the largest probable volume for the marginal bulge around the composite North American ice-cap, computation seems to show the purely elastic deformation to have been from five to ten times greater than the deformation caused by viscous outflow. The very recent drowning of the marginal belt would therefore be quite moderate-in the regions here considered probably not surpassing a few tens of meters, even though the marginal bulge may have had a maximum height of 200 meters.

The testing of this theoretical set of deductions by field observations involves close dating of the submergence so clearly manifest in the coast region southwest of Boston and again along the southern shores of Nova Scotia and Newfoundland. In a case of this kind close dating is notoriously difficult, and the writer has been able to add few objective facts relevant to the date of drowning along the coasts studied in 1920. It is certain that the rise of sea-level is there very recent; the waves have not yet had time to cut wide benches in the little-resistant glacial drift which mantles most of Nova Scotia and certain stretches of the sea-front in southern Newfoundland.

On the other hand, tide-gauge records for eastern Canada, published by Dr. W. Bell Dawson, Superintendent of Tidal Surveys (Ottawa, 1917), show no measurable sinking of the land at Halifax, Charlottetown, and St. Paul Island (Cabot Strait) during periods of from 6 to 18 years. The New England coast seems to have been sensibly stable for at least one hundred years. Shimer describes proofs of submergence of the coast region at Boston within a period of 3000 years. He writes: "The remnants of the fish-weir, excavated on Boylston Street, give evidence of man in the Back Bay region of Boston, probably 2000 to 3000 years ago. He built this weir during a climatic period as warm as off the Virginia coast at present, and upon a sinking coast. Since its erection the region has sunk sixteen or eighteen feet and suffered a refrigeration to its present climate." Certain facts suggest the necessity of postulating a recent, negative, * H. W. Shimer, Proc. Amer. Acad. Arts and Sciences, vol. 53, p. 462, 1918.

eustatic shift of ocean-level to the extent of about 6 meters (20 feet).10 The shift is tentatively placed in late Neolithic times. The complete failure of the corresponding bench to appear in southern Nova Scotia and southern Newfoundland suggests that the local drowning progressed after the eustatic shift of sea-level took place, that is, within the last three or four thousand years. Barrell's reasoning on the march of events during isostatic adjustment following the melting of an ice-cap would agree with this suggestion. The warping of the "25-foot" post-Glacial bench of the British Isles and the drowning of Neolithic deposits outside the zero isobase in England and Denmark may conceivably be explained in the same way, if the local British and Scandinavian ice-caps caused deformation like that connected with the Labrador icecap.11 If, in each of the three regions, the local sinking occurred during post-Neolithic time, none of the regions would be likely to give convincing evidence of an earlier eustatic change of sea-level. On the other hand, strandmarks, corresponding to the higher position of sea-level in the glaciated area well inside the zero isobase, should not have been greatly disturbed during the final collapse of the marginal bulge. The remarkable low bench along the shore of the St. Lawrence estuary is a case in point.

The various facts and suggestions noted in the last few paragraphs suffice to show that the character and exact dating of the deformation in the marginal belt represent a delicate problem, which for geodynamics has scarcely less importance than a similar understanding of the uplift in the central area of a vanished ice-cap. Because of their specially favorable relations to the zero isobase and to the level-marking ocean, Nova Scotia and Newfoundland seem to be among the best of all the large areas in which to seek compelling evidence as to what really happened in the marginal belt.

10 R. A. Daly, Geol. Mag., vol. 57, p. 246, 1920. At many well exposed headlands of Newfoundland the nearly or quite vertical sea-cliffs were seen to be continued under low-tide level from one-half fathom to two fathoms or a little deeper. This prolongation of the cliff below sea-level is the result of marine erosion and does not mean so much sinking of the land. The principle illustrated is important in connection with the problem of locating former sea-level from elevated rock-benches. At exposed places the bench levels are likely to be three to twelve or more feet below the high-tide level ruling at the time when the benches were cut.

11 Cf. W. B. Wright, Geol. Mag., vol. 57, p. 382, 383, 1920.

ART. XXVI.-New Camels in the Marsh Collection; by RICHARD S. LULL.

[Contributions from the Othniel Charles Marsh_Publication Fund, Peabody Museum, Yale University, New Haven, Conn.]

The Oligocene camels in the Marsh Collection consist of a number of skulls and other skeletal material. Those from the White River Oreodon beds are clearly referable to three of the known species of Poëbrotherium and add nothing to our knowledge of these forms. From the Protoceras beds, on the other hand, comes one apparently new species, while from the upper John Day are several specimens worthy of description.

Pseudolabis (Paralabis) matthewi, subgen. et sp. nov.

(Fig. 1.)

Holotype, Cat. No. 10167, Y. P. M. Upper Oligocene (Protoceras beds), Sturgis, South Dakota.

The type material consists of a poorly preserved skull with the third incisor and a complete series of cheek teeth. Distinctive characters.-Size somewhat larger than Poëbrotherium labiatum. Skull mutilated so that few characters are observable. Auditory bullæ less rounded than are those of Poëbrotherium, although fully as large. Tympanohyal recess more widely open and inner posterior lobe thus narrower, more as in the later camels. Rear of orbit not preserved, but zygomatic arch and adjacent bones are rather heavy, so that its closure is probable. Infra-orbital foramen above P+, palatal foramina opposite the posterior half of P3.

Dental formula: I, C', P+, M. Alveoli of I12 not preserved, I caniniform, recurved, probably equal to, if not exceeding, the canine in size. Canine lacking, alveolus elliptical, separated from I by a short diastema. P1 double-rooted, crown not preserved, larger than P2, and separated from the adjacent teeth by diastemata, of which the anterior one is nearly two times the greater. P2 to M3 form a compact series, the crowns of medium height. Molars with prominent parastyle, mesostyle, and external ribs, no internal basal pillars present. P with internal cingulum, and P with an accessory internal crest connecting the crescent with the rear of the tooth, suggestive

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