Epochs of Glaciation.

G K Gilbert

With the observations of Gilbert, Chamberlin, and King in mind, the terminal moraine was traced by various workers across the United States and into Canada and the extent of glacial cover revealed. Following 1875 the pages of the Journal contain many contributions dealing with the origin and structure of moraines, eskers, kames, and drumlins. Before 1890 twenty-eight papers on the glacial phenomena of the Erie and Ohio basin alone had appeared. By 1900 substantial agreement had been reached regarding the significant features of the drift, the outline history of the Great Lakes had been written, and the way had been paved for stratigraphic studies of the Pleistocene, which bulk large in the pages of the Journal for the last two decades.

For a decade following the general acceptance of the glacial origin of “diluvium,” the deposits were embraced as “drift” and treated as the products of one long period of glacial activity, and throughout the controversy of iceberg and glacier the unity of the glacial period was unquestioned. Beds of peat and fossiliferous lacustrine deposits in Switzerland, England, and in America and the recognition of an “upper” and a “lower” diluvium by Scandinavian geologists suggested two epochs, and as the examples of such deposits increased in number and it became evident that the plant fossils represented forms demanding a genial climate and that the phenomena were seen in many countries, the belief grew that minor fluctuations or gradual recession of an ice sheet were inadequate to account for the phenomena observed.

It is natural that this problem should have found its solution in America, where the Pleistocene is admirably displayed, and where the State and Federal surveys were actively engaged in areal mapping. In 1883 Chamberlin[64]presented his views under the bold title, “Preliminary Paper on the Terminal Moraine of the Second Glacial Epoch,” and the existence of deposits of two or more ice sheets and the features of interglacial periodswere substantially established by the interesting debate in the Journal led by Chamberlin, Wright, Upham and Dana.[65]Contributions since 1895 have been concerned with the degree rather than the fact of complexity, and continued study has resulted in the general recognition of five glacial stages in North America and four in Europe.

A curious side-product of the study of glaciation in North America is the controversy over the origin of loess. The interest aroused is indicated by scores of papers in American periodicals and State reports of the last quarter of the 19th century—papers which bear the names of prominent geologists.

The “loess” in the valley of the Rhine had long been known, but the subject assumed prominence by the publication in 1866 of Pumpelly’s Travels in China.[66]Wide-spread deposits 200 to 1,000 feet thick were described as very fine-grained yellowish earth of distinctive structure without stratification but penetrated by innumerable tubes and containing land or fresh-water shells. Pumpelly considered these deposits lacustrine, a view which found general acceptance though combated by Kingsmill (1871),[67]who argued for marine deposition. Baron Von Richthofen’s classic on China, which appeared in 1877, amplifies the observations of Pumpelly and marshals the evidence to support the hypothesis that the loess is wind-laid both on dry land and within ancient salt lakes. The conclusions of Von Richthofen were adopted by Pumpelly whose knowledge of the Chinese deposits, supplemented by studies in Missouri, of which State he was director of the Geological Survey in 1872–73, placed him in position to form a correct judgment. He says:[68]

“Recognizing from personal observation the full identity of character of the loess of northern China, Europe and the Missouri Valley, I am obliged to reject my own explanation of the origin of the Chinese deposits, and to believe with Richthofen that the true loess, wherever it occurs, is a sub-aerial deposit, formed in a dry central region, and that it owes its structure to the formative influence of a steppe vegetation.

The one weak point of Richthofen’s theory is in the evidentinadequacy of the current disintegration as a source of material. When we consider the immense area covered by loess to depths varying from 50 to 2,000 feet, and the fact that this is only the very finest portion of the product of rock-destruction, and again that the accumulation represents only a very short period of time, geologically speaking, surely we must seek a more fertile source of supply than is furnished by the current decomposition of rock surface.

It seems to me that there are two important sources: I. The silt brought by rivers, many of them fed by the products of glacial attrition flowing from the mountains into the central region. Where the streams sink away, or where the lakes which receive them have dried up, the finer products of the erosion of a large territory are left to be removed in dust storms.

II. The second ... source is the residuary products of a secular disintegration.”

The evidence presented by Pumpelly for the eolian origin of loess—structure, texture, composition, fossil content and topographic position—is complete, and to him belongs the credit for the correct interpretation of the Mississippi valley deposits. Unfortunately his contribution came at a time when the geologists of the central States were intent on tracing the paths and explaining the work of Pleistocene glaciers, and the belief was strong that loess was some phase of glacial work. Its position at the border of the Iowan drift so obviously suggests a genetic relation that the fossil evidence of steppe climate suggested by Binney in 1848[69]was minimized. Students of Pleistocene geology in Minnesota, Iowa, Nebraska, Missouri, although less vigorous in expression, were substantially in agreement with Hilgard (1879).[70]“The sum total of anomalous conditions required to sustain the eolian hypothesis partakes strongly of the marvellous.” The last edition of Dana’s Manual, 1894, and of LeConte’s Geology, 1896, the two most widely used text-books of their time, oppose the eolian theory, and Chamberlin, in 1897,[71]states: “the aqueous hypothesis seems best supported so far as concerns the deposits of the Mississippi Valley and western Europe” (p. 795). Shimek, in papers published since 1896 has shown that aquatic and glacial conditions can not account for the loess fossils, and the return to the views of Pumpelly that the loess was deposited on landby the agency of wind in a region of steppe vegetation is now all but universal.

Within the present generation sculpture by glaciers has received much attention and has involved a reconsideration of the ability of ice to erode which in turn involves a crystallization of views of the mechanics of moving ice. The evidence for glacier erosion has remained largely physiographic and rests on a study of land forms. In fact, the inadequacy of structural features or of river corrasion to account for flat-floored, steep-walled gorges, hanging valleys, and many lake basins, rather than a knowledge of the mechanics of ice has led to the present fairly general belief that glaciers are powerful agents of rock sculpture. The details of the process are not yet understood.

Erosion by glaciers enters the arena of active discussion in 1862–63. The possibility had been suggested by Esmark (1827) and by Dana (1849) in the description of fiords and by Hind (1855) with reference to the origin of the Great Lakes. It appears full-fledged in Ramsay’s classic, which was published simultaneously in England and in America.[72]The argument runs as follows: There is a close association of ancient glaciers and lakes especially in mountains; glaciers are amply able to erode; evidences of faulting, special subsidence, river erosion, and marine erosion are absent from the lake basins of Switzerland and Great Britain. To quote Ramsay:

“It required a solid body grinding steadily and powerfully in direct and heavy contact with and across the rocks to scoop out deep hollows, the situations of which might either be determined by unequal hardness of the rocks, by extra weight of ice in special places, or by accidental circumstances, the clue to which is lost from our inability perfectly to reconstruct the original forms of the glaciers.”

“I believe with the Italian geologists, that all that the glaciers as a whole effected was only slightly to deepen these valleys and materially to modify their general outlines, and, further (a theory I am alone responsible for), to deepen them in parts more considerably when, from various causes, the grinding power ofthe ice was unusually powerful, especially where, as in the lowlands of Switzerland, the Miocene strata are comparatively soft.”

Whittlesey (1864)[73]considered that the rock-bound lakes and narrow bays near Lake Superior were partly excavated by ice. LeConte (1875)[74]records some significant observations in a pioneer paper on glacier erosion which has not received adequate recognition. He says:

“... I am convinced that a glacier, by its enormous pressure and resistless onward movement, isconstantly breaking off large blocksfrom its bed and bounding walls. Its erosion is not only a grinding and scoring, but also acrushing and breaking. It makes by its erosion not only rock-meal, but also largerock-chips.... Its erosion is a constant process of alternaterough hewingandplaning.

If Yosemite were unique, we might suppose that it was formed by violent cataclysms; butYosemite is not uniqueinformand therefore probably not inorigin. There are many Yosemites. It is more philosophical to account for them by theregularoperation of known causes. I must believe that all these deep perpendicular slots have been sawn out by the action of glaciers; thepeculiar verticality of the walls having been determined by the perpendicular cleavage structure.”... A lake in Bloody Canyon “is apure rock basin scooped out by the glacierat this place.... These ridges [separating Hope, Faith, and Charity valleys] are in fact the lips of consecutive lake basins scooped out by ice.

... Water tends to form deep V-shaped canons, while ice produces broad valleys with lakes and meadows.... I know not how general these distinctions may be, but certainly the Coast range of this State is characterized by rounded summits and ridges, and deep V-shaped canons, while the high Sierras are characterized on the contrary by sharp, spire-like, comb-like summits, and broad valleys; and this difference I am convinced is due in part at least to the action of water on the one hand, and of ice on the other.”

King (1878)[75]assigned to glacial erosion a commanding position in mountain sculpture. In regard to the Uintas, he says:

“Glacial erosion has cut almost vertically down through the beds carving immense amphitheatres with basin bottoms containing numerous Alpine lakes.... Post-glacial erosion has donean absolutely trivial work. There is not a particle of direct evidence, so far as I can see, to warrant the belief that these U-shaped canons were given their peculiar form by other means than the actual ploughing erosion of glaciers....”

These contributions from the Cordilleras corroborating the conclusions of Ramsay (1862), Tyndall (1862), Jukes (1862), Hector (1863), Logan (1863), Close (1870), and James Geikie (1875), made little impression. The views of Lyell (1833), Ball (1863), J. W. Dawson (1864), Falconer (1864), Studer (1864), Murchison (1864, 1870), Ruskin (1865), Rutimeyer (1869), Whymper (1871), Bonney (1873), Pfaff (1874), Gurlt (1874), Judd (1876), prevailed, and the conclusions of Davis in 1882[76]fairly expressed the prevailing belief in Europe and in America:

“The amount of glacial erosion in the central districts has been very considerable, but not greatly in excess of pre-glacial soils and old talus and alluvial deposits. Most of the solid rock that was carried away came from ledges rather than from valleys; and glaciers had in general a smoothing rather than a roughening effect. In the outer areas on which the ice advanced it only rubbed down the projecting points; here it acted more frequently as a depositing than as an eroding agent.”

During the past quarter-century the cleavage in the ranks of geologists, brought about by Ramsay’s classic paper, has remained. Fairchild and others in America, Heim, Bonney, and Garwood in Europe argue for insignificant erosion by glaciers; and Gannet, Davis, Gilbert, Tarr in America followed by Austrian workers present evidence for erosion on a gigantic scale. A perusal of the voluminous literature in the Journal and elsewhere shows that the difference of opinion is in part one of terms, the amount of erosion rather than the fact of erosion; it also arises from failure to differentiate the work of mountain glaciers and continental ice sheets, of Pleistocene glaciers and their present diminished representatives. The irrelevant contribution of physicists has also made for confusion.

It is interesting to note that the criteria for erosion of valleys by glaciers has long been established and by workers in different countries. Ramsay (1862) inEngland outlined the problem and presented generalized evidence. Hector (1863) in New Zealand pointed out the significance of discordant drainage, the “hanging valleys” of Gilbert. The U-form, the broad lake-dotted floor, and the presence of cirques and the process of plucking were probably first described by LeConte (1873) in America. The truncation of valley spurs by glaciers pointed out by Studer in the Kerguelen Islands (1878) was used by Chamberlin (1883) as evidence of glacial scouring.

During the past century many principles of land sculpture have emerged from the fog of intellectual speculation and unorganized observation and taken their place among generally accepted truths. Many of them are no longer subjects of controversy. Erosion has found its place as a major geologic agent and has given a new conception of natural scenery. Lofty mountains are no longer “ancient as the sun,” they are youthful features in process of dissection; valleys and canyons are the work of streams and glaciers; fiords are erosion forms; waterfalls and lakes are features in process of elimination; many plains and plateaus owe their form and position to long-continued denudation. Modern landscapes are no longer viewed as original features or the product of a single agent acting at a particular time, but as ephemeral forms which owe their present appearance to their age and the particular forces at work upon them as well as to their original structure.

It is interesting to note the halting steps leading to the present viewpoint, to find that decades elapsed between the formulation of a theory or the recording of significant facts and their final acceptance or rejection, and to realize that the organization of principles and observations into a science of physiography has been the work of the present generation. Progress has been conditioned by a number of factors besides the intellectual ability of individual workers.

The influence of locality is plainly seen. Convincing evidence of river erosion was obtained in central France, the Pacific Islands, and the Colorado Plateau—regionsin which other causes were easily eliminated. Sculpture by glaciers passed beyond the theoretical stage when the simple forms of the Sierras and New Zealand Alps were described. The origin of loess was first discerned in a region where glacial phenomena did not obscure the vision. The complexity of the Glacial period asserted by geologists of the Middle West was denied by eastern students. The work of waves on the English coast impressed British geologists to such an extent that plains of denudation and inland valleys were ascribed to ocean work.

In the establishment of principles, the friendly interchange of ideas has yielded large returns. Many of the fundamental conceptions of earth sculpture have come from groups of men so situated as to facilitate criticism. It is impossible, even if desirable, to award individual credit to Venetz, Charpentier, and Agassiz in the formulation of the glacial theory; and the close association of Agassiz and Dana in New England and of Chamberlin and Irving in Wisconsin was undoubtedly helpful in establishing the theory of continental glaciation. From the intimate companionship in field and laboratory of Hutton, Playfair and Hope, arose the profound influence of the Edinburgh school, and the sympathetic cooperation of Powell, Gilbert, and Dutton has given to the world its classics in the genetic study of land forms.

The influence of ideas has been closely associated with clarity, conciseness, and attractiveness of presentation. Hutton is known through Playfair, Agassiz’s contributions to glacial geology are known to every student, while Venetz, Charpentier, and Hugi are only names. Cuvier’s discourses on dynamical geology were reprinted and translated into English and German, but Lamarck’s “Hydrogéologie” is known only to book collectors. The verbose works of Guettard, although carrying the same message as Playfair’s “Illustrations” and Desmarest’s “Memoirs,” are practically unknown, as is also Horace H. Hayden’s treatise (1821) on the drift of eastern North America. It has been well said that the world-wide influence of American physiographic teaching is due in no small part to the masterly presentations of Gilbert and Davis.

It is surprising to note the delays, the backward steps, and the duplication of effort resulting from lack of familiarity with the work of the pioneers. Sabine says in 1864:[77]

“It often happens, not unnaturally, that those who are most occupied with the questions of the day in an advancing science retain but an imperfect recollection of the obligations due to those who laid the first foundations of our subsequent knowledge.”

The product of intellectual effort appears to be conditioned by time of planting and character of soil as well as by quantity of seed. For example: Erosion by rivers was as clearly shown by Desmarest as by Dana and Newberry 50 years later. Criteria for the recognition of ancient fluviatile deposits were established by James Deane in 1847 in a study of the Connecticut Valley Triassic. Agassiz’s proof that ice is an essential factor in the formation of till is substantially a duplication of Dobson’s observations (1826).

The volumes of the Journal with their very large number of articles and reviews dealing with geology show that the interpretation of land forms as products of subaërial erosion began in France and French Switzerland during the later part of the eighteenth century as a phase of the intellectual emancipation following the Revolution. Scotland and England assumed the leadership for the first half of the nineteenth century, and the first 100 volumes of the Journal show the profound influence of English and French teaching. In America, independent thinking, early exercised by the few, became general with the establishment of the Federal survey, the increase in university departments, geological societies and periodicals, and has given to Americans the responsibilities of teachers.

(In the following list “this Journal” refers to the American Journal of Science.)

4. Wilson, J. W., Bursting of lakes through mountains, this Journal,3, 253, 1821.

5. Whitney, J. D., Progress of the Geological Survey of California, this Journal,38, 263–264, 1864.

6. Playfair, John, Illustrations of the Huttonian theory of the earth, Edinburgh, 1802.

7.Kain, J. H., Remarks on the mineralogy and geology of northwestern Virginia and eastern Tennessee, this Journal,1, 60–67, 1819.

8. Hitchcock, Edward, Geology, etc., of regions contiguous to the Connecticut, this Journal,7, 1–30, 1824.

9. Buckland, Wm., Reliquiæ diluvianæ, this Journal,8, 150, 317, 1824.

10. Phillips, John, Geology of Yorkshire, this Journal,21, 17–20, 1832.

11. Scrope, G. P., Excavation of valleys, Geol. Soc., London, No.14, 1830.

12. Hayes, G. E., Remarks on geology and topography of western New York, this Journal,35, 88–91, 1839.

13. Seventh Meeting of the British Association for the Advancement of Science, this Journal,33, 288, 1838.

14. Darwin, Charles, Geological observations on the volcanic islands and parts of South America, etc., second part of the Voyage of the “Beagle,” during 1832–1836. London, 1844.

15. Hildreth, S. P., Observations, etc., valley of the Ohio, this Journal,29, 1–148, 1836.

16. Geddes, James, Observations on the geological features of the south side of Ontario valley, this Journal,11, 213–218, 1826.

18. Warren, G. K., Preliminary report of explorations in Nebraska and Dakota, this Journal,27, 380, 1859.

19. Lesley, J. P., Observations on the Appalachian region of southern Virginia, this Journal,34, review, 413–415, 1862.

17. Conrad, T. A., Notes on American geology, this Journal,35, 237–251, 1839.

20. Dana, J. D., On denudation in the Pacific, this Journal,9, 48–62, 1850.——, On the degradation of the rocks of New South Wales and formation of valleys, this Journal,9, 289–294, 1850.

20. Dana, J. D., On denudation in the Pacific, this Journal,9, 48–62, 1850.

——, On the degradation of the rocks of New South Wales and formation of valleys, this Journal,9, 289–294, 1850.

21. Hubbard, O. P., On the condition of trap dikes in New Hampshire an evidence and measure of erosion, this Journal,9, 158–171, 1850.

22. Hayden, F. V., Some remarks in regard to the period of elevation of the Rocky Mountains, this Journal,33, 305–313, 1862.

23. Newberry, J. S., Colorado River of the West, this Journal,33, review, 387–403, 1862.

24. Jukes, J. B., Address to the Geological Section of the British Association at Cambridge, Quart. Jour. Geol. Soc., 18, 1862, this Journal,34, 439, 1862.

25. Powell, J. W., Exploration of the Colorado River of the West, 1875. For Powell’s preliminary article see this Journal,5, 456–465, 1873.

26. McGee, W. J., Three formations of the Middle Atlantic slope, this Journal,35, 120, 328, 367, 448, 1888.

27. Davis, W. M., Topographic development of the Triassic formation of the Connecticut Valley, this Journal,37, 423–434, 1889.

28. Percival, J. G., Geology of Connecticut, 1842.

29. Kerr, W. C., Origin of some new points in the topography of North Carolina, this Journal,21, 216–219, 1881.

30. McGee, W. J., The classification of geographic forms by genesis, Nat. Geogr. Mag.,1, 27–36, 1888.

31. Davis, W. M., The rivers and valleys of Pennsylvania, Nat. Geogr. Mag.,1, 183–253, 1889.——, The rivers of northern New Jersey with notes on the classification of rivers in general, ibid.,2, 81–110, 1890.

31. Davis, W. M., The rivers and valleys of Pennsylvania, Nat. Geogr. Mag.,1, 183–253, 1889.

——, The rivers of northern New Jersey with notes on the classification of rivers in general, ibid.,2, 81–110, 1890.

32. Silliman, Benjamin, Notice of Horace H. Hayden’s geological essays, this Journal,3, 49, 1821.

33. Cornelius, Elias, Account of a singular position of a granite rock, this Journal,2, 200–201, 1820.

34.Finch, John, On the Celtic antiquities of America, this Journal,7, 149–161, 1824.

35. Finch, John, Geological essay on the Tertiary formations in America, this Journal,7, 31–43, 1824.

36. Conybeare and Phillips, Outlines of the geology of England and Wales, this Journal,7, 210, 211, 1824.

37. Hayden, Horace H., Geological essays, 1–412, 1821, this Journal,3, 47–57, 1821.

38. Jackson, C. T., Reports on the geology of the State of Maine, and on the public lands belonging to Maine and Massachusetts, this Journal,36, 153, 1839.

39. Gibson, J. B., Remarks on the geology of the lakes and the valley of the Mississippi, this Journal,29, 201–213, 1836.

40. Phillips, John, Geology of Yorkshire, this Journal,21, 14–15, 1832.

41. Granger, Ebenezer, Notice of a curious fluted rock at Sandusky Bay, Ohio, this Journal,6, 180, 1823.

42. Dobson, Peter, Remarks on bowlders, this Journal,10, 217–218, 1826.

43. Murchison, R. I., Address at anniversary meeting of the Geological Society of London, this Journal,43, 200–201, 1842.

44. Peter Dobson (1784–1878) came to this country from Preston, England, in 1809 and established a cotton factory at Vernon, Conn.

45. Buckland, W., On the evidence of glaciers in Scotland and the north of England, Proc. London Geol. Soc.,3, 1841.

46. Hitchcock, Edward, First anniversary address before the Association of American Geologists, this Journal,41, 232–275, 1841.

47. Third annual meeting of the Association of American Geologists and Naturalists, this Journal,43, 154, 1842; Abstract of proceedings of the fourth session of the Association of American Geologists and Naturalists, ibid.,45, 321, 1843.

48. Rogers, H. D., Address delivered before Association of American Geologists and Naturalists, this Journal,47, 275, 1844.

49. Agassiz, Louis, The erratic phenomena about Lake Superior, this Journal,10, 83–101, 1850.

50. Desor, E., On the drift of Lake Superior, this Journal, 13, 93–109, 1852; Post-Pliocene of the southern States, etc.,14, 49–59, 1852.

51. Dana, J. D., Manual of geology, 546, Philadelphia, 1863.

52. Dana, J. D., on the Quaternary, or post-Tertiary of the New Haven region, this Journal,1, 1–5, 1871.

53. Matthew, G. F., Surface geology of New Brunswick, this Journal,2, 371–372, 1871.

54. Maclaren, Charles, The glacial theory of Prof. Agassiz, this Journal,42, 365, 1842.

55. Daly, R. A., Problems of the Pacific Islands, this Journal,41, 153–186, 1916.

56. Catlin, George, Account of a journey to the Côteau des Prairies, this Journal,38, 138–146, 1840.

57. Hilgard, E. W., Remarks on the drift of the western and southern States and its relation to the glacier and iceberg theories, this Journal,42, 343–347, 1866.

58. Hall, C. E., Glacial phenomena along the Kittatinny or Blue Mountain, Pennsylvania, this Journal,11, review, 233, 1876.

59. Stevens, R. P., On glaciers of the glacial era in Virginia, this Journal,6, 371–373, 1873.

60. Rogers, W. B., On the gravel and cobble-stone deposits of Virginia and the Middle States, Proc. Boston Soc. Nat. Hist., 18, 1875; this Journal,11, 60–61, 1876.

61. Kerr, W. C, Origin of some new points in the topography of North Carolina, this Journal,21, 216–219, 1881.

62. Gilbert, G. K., On certain glacial and post-glacial phenomena of the Maumee valley, this Journal,1, 339–345, 1871.

63. Chamberlin, T. C., On the geology of eastern Wisconsin, Geol. of Wisconsin,2, 1877; this Journal, 15, 61, 406, 1878.

64.Chamberlin, T. C, Preliminary paper on the terminal moraine of the second glacial epoch, U. S. Geol. Survey, Third Ann. Rept., 291–402, 1883.

65. Wright, G. F., Unity of the glacial epoch, this Journal,44, 351–373, 1892.Upham, Warren, The diversity of the glacial drift along its boundary, ibid.,47, 358–365, 1894.Wright, G. F., Theory of an interglacial submergence in England, ibid.,43, 1–8, 1892.Chamberlin, T. C., Diversity of the glacial period, ibid.,45, 171–200, 1983Dana, J. D., On New England and the upper Mississippi basin in the glacial period, ibid.,46, 327–330, 1893.Wright, G. F., Continuity of the glacial period, ibid.,47, 161–187, 1894.Chamberlin, T. C. and Leverett, F., Further studies of the drainage features of the upper Ohio basin, ibid.,47, 247–282, 1894.

65. Wright, G. F., Unity of the glacial epoch, this Journal,44, 351–373, 1892.

Upham, Warren, The diversity of the glacial drift along its boundary, ibid.,47, 358–365, 1894.

Wright, G. F., Theory of an interglacial submergence in England, ibid.,43, 1–8, 1892.

Chamberlin, T. C., Diversity of the glacial period, ibid.,45, 171–200, 1983

Dana, J. D., On New England and the upper Mississippi basin in the glacial period, ibid.,46, 327–330, 1893.

Wright, G. F., Continuity of the glacial period, ibid.,47, 161–187, 1894.

Chamberlin, T. C. and Leverett, F., Further studies of the drainage features of the upper Ohio basin, ibid.,47, 247–282, 1894.

66. Pumpelly, Raphael, Geological researches in China, Japan, and Mongolia, Smithsonian Contributions, No. 202, 1866.

67. Kingsmill, T. W., The probable origin of “loess” in North China and eastern Asia, Quart. Jour. Geol. Soc.,27, No. 108, 1871.

68. Pumpelly, Raphael, The relation of secular rock-disintegration to loess, glacial drift and rock basins, this Journal,17, 135, 1879.

69. Binney, A., Some geologic features at Natchez on the Mississippi River, Proc. Boston Soc. Nat. Hist.,2, 126–130, 1848.

70. Hilgard, E. W., The loess of Mississippi Valley, and the eolian hypothesis, this Journal,18, 106–112, 1879.

71. Chamberlin, T. C, Supplementary hypothesis respecting the origin of the loess of the Mississippi Valley, Jour. Geol.,5, 795–802, 1897.

72. Ramsay, A. C., On the glacial origin of certain lakes in Switzerland, the Black Forest, Great Britain, Sweden, North America, and elsewhere, Quart. Jour. Geol. Soc., 1862; this Journal,35, 324–345, 1863. Preliminary statements of this theory appeared in 1859 and 1860.

73. Whittlesey, Charles, Smithsonian Contributions, No. 197, 1864.

74. LeConte, Joseph, On some of the ancient glaciers of the Sierras, this Journal,5, 325–342, 1873, 10, 126–139, 1875.

75. King, Clarence, U. S. Geol. Expl. 40th Par., 1, 459–529, 1878.

76. Davis, W. M., Glacial erosion, Proc. Boston Soc. Nat. Hist.,22,58, 1882.

77. Sabine, Sir Edward, Address of the president of the Royal Society, this Journal,37, 108, 1864.

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