SCIENCE. 



583 



Sea, which it must have nearly closed, the North Europe 

 glaciers extended over Scotland, all the north of England, 

 and probably all of Ireland. On the north its limits were 

 perhaps the polar ice itself, and in the west the deeper 

 waters of the Atlantic. The southern limit of this ice- 

 sheet was in the south-central part of England " ( p. 40). 

 This was probably " the southern edge of the polar ice 

 tops [icecap?] rather than a local system of glacial 

 sheets" (p. 40). In North America the accumulation 

 of ice was stiil more extensive, and of somewhat different 

 character; " here the ice lay as a continuous mass, stretch- 

 ing down from the polar regions to the central parts of 

 the continent, overlapping the shores for a great distance 

 to the south along the Atlantic and Pacific coasts, and 

 giving a continuous though irregular ice front across the 

 land from sea to sea" ( p. 41 ). The teiminus of this 

 sheet is yet marked by moraines as constituting the Banks 

 of Newfoundland, George's Banks, Cape Cod, Martha's 

 Vineyard, and Block and Long Islands, and extending 

 thence across central New Jersey, and south as far as 

 Washington. The attenuated margin left less distinct 

 traces of its existence in the hills of southern Virginia, 

 and thence into the higher Appalachians in North Caro- 

 lina, whence it returned hugging the western mountain 

 slope, and extending through West Virginia, crossing the 

 Ohio river near the mouth of the Kanawha. Thence 

 the southern edge of the ice skirted the north shore of 

 the Ohio to Cincinnati, near which place it sent a lobe 

 across the river a few miles into Kentucky. " West of 

 Cincinnati the front of the ice sheet inclined rapidly to the 

 north-west, and becomes hard to trace. It probably pas- 

 sed somewhat south of Chicago, through Iowa, and thence 

 through Minnesota, following near the line of the Mis- 

 souri to the Rocky Mountains " (p, 42 ). In the Cordil- 

 leras the ice was mainly confined to the higher mountains, 

 and probably partook of the character of local or alpine 

 glaciers within, the limits of the United States ; while 

 north of our domain " we know little of its distribution " 

 (p. 42). " There can be little doubt that the ice sheet 

 was continuous from its southern face to the poles during 

 the depths of the last ice time. *** This glaciated re- 

 gion of North America includes more than half the conti- 

 nent ; in fact over two thirds of its surface felt the weight 

 of the ice during the last geological period, and works 

 its work in the existing geography " ( p. 44 ). The thick- 

 ness of the ice is not definitely known except in the vi- 

 cinity of Mt. Washington, where it exceeded a mile. In 

 South An. erica it is probable that continental ice never 

 extended north of the Rio de la Plata over the plains, nor 

 beyond the Chilian coast on the Andes. 



V. The work of the glacial time. — Water, whether 

 liquid or solid, is a most efficient agent of erosion; but 

 the mode of action of the two forms is quite different. 

 Liquid water itself operates in a two-fold manner : 1st, 

 as a chemical agent, penetrating the earth and disorganiz- 

 ing its constituents, forming caverns, mineral veins, and 

 residuary products; and 2nd, as a mechanical agent, 

 loosening, removing, and comminuting the rocky parti- 

 cles, and finally bearing them to the sea to form new 

 lands ; but in the solid form only mechanical activity is 

 manifested. There is, first, the enormous weight of the 

 glacier (more than a ton per square inch beneath a gla- 

 cier a mile in thickness), enough in itself to comminute 

 rocks not strongly coherent and well supported laterally; 

 there is then the abrading action of this tremendous 

 weight dragged slowly forward — the ice being armed 

 with fragments of rock frozen into its mass; and there is 

 finally the corroding action of the sub-glacial streams 

 (sometimes, perhaps, under great hydrostatic pressure) 

 which constantly bear away the finer detritus and pre- 

 vent the clogging of the giinding faces of the glacial 

 mill. The rapidity of operation of these forces must be 

 almost beyond conception. Even in the diminutive 

 Alpine glaciers the sub-glacial streamlets are so fully 

 charged with impalpable mud as to carry away more 



material in a few days than is moved by a sub-serial 

 stream of like size in a year. Now, since the erosive 

 action of the ice is proportional to its thickness, and 

 since, moreover, this action is most effectively supple- 

 mented by sub-glacial streams in valleys, it is manifest 

 that the tendency of glaciation is to increase the depth 

 of existing depressions, and thus intensify topographical 

 irregularities. Accordingly, glaciated regions are char- 

 acterized by deep bays and fords along the coast line, V 

 shaped valleys intersecting mountainous areas, and 

 elongated basin-like ponds and lakes dotting more uni- 

 form surfaces — the longer axes coinciding with the direc- 

 tion of ice-motion; while at the same time abrupt peaks 

 and irregular knobs are replaced by gracefully rounded 

 swells with trains of fragments to the leeward. Since 

 the average rate of glacial erosion is so high (it was a 

 foot in a thousand years, or more than seven times as 

 rapid as sub-a;rial erosion in New England) it would ap- 

 pear that important geographical changes ought to follow 

 the visitation of an ice-sheet, not only by the carrying 

 out of a new series of hills and dales, but by heaping up 

 of piles of earth and stones of such magnitude as to ne- 

 cessitate the development of a new drainage-system ; and 

 accordingly just such geographical vicissitudes are 

 abundantly attested in north-eastern United States and 

 elsewhere. 



The most conspicous evidence of glacial action is the 

 mantle of drift occupying areas formerly overspread by 

 ice. This drift consists of the materials torn up and in- 

 discriminately intermingled by the glacier, and is gener- 

 ally a confused, unstratified mass of stones of all sizes 

 and shapes, generally much worn, cemented together by 

 sand and clay. It is sometimes heaped up in irregular 

 moraines ; some of which dcubtless mark the lines of 

 greatest extension of the ice, while others probably indi- 

 cate temporary pauses or re-advances in its secular re- 

 treat. Along the coast this deposit has been re-arranged 

 superficially by wave and tide, and has afforded material 

 for immense accumulations of terrace drift ; the unmodi- 

 fied basal portion being sometimes left in the form of 

 gracefully arched lenticular hills of elliptical outline, the 

 longer axis extending in the direction of motion of the ice. 

 In regions not submerged at the close of the ice-period the 

 upper portion of the drift has been modified by the action of 

 running water and of vegetal growth. The moving wa- 

 ter was rendered effective during the retreat of the ice, not 

 so much by the increased volume to be borne sea-ward 

 as by the imperfection of the nascent drainage system. 

 The valleys were clogged with glacial waste, forming 

 hosts of pools and lakelets which burst from time to time 

 and shifted the hetrogeneous mass here and there in a 

 series of pygmy debacles. The terraces of this period 

 along the Connecticut river and its tributaries contain 

 scores of cubic miles of drift thus re-arranged, and indi- 

 cate by their altitude that much more material than that 

 now remaining has been removed. Among the minor 

 drift phenomena are the isolated hills and greatly elonga- 

 ted ridges of sand, gravel, or stratified clay, denominated 

 aasar, kames,ox eskers. " No sufficient explanation has 

 yet been given of their origin " (p. 66). In the best in- 

 stance known the deposit is probably a central terminal 

 moraine, deposited in a valley of uniform slope, by a re- 

 treating local glacier. Other examples, however, appear 

 to be not morainal ; and it may be long before we under- 

 stand the method of their formation" (p. 68). 



VI. The origin and ?iature of glacial periods. — The 

 earliest of the several hypotheses which have been put 

 forth to explain the cause of the glacial period referred 

 the phenomenon to the secular refrigeration of the globe ; 

 but the hypothesis is untenable, since it does not contem- 

 plate the several successive transitions from warmth to 

 cold. A second hypothesis is that of Poisson, who sug- 

 gested that in the proper motion of the solar system it 

 might from time to time come into such proximity to, or 

 recede to such a distance from, neighboring stellar bodies 



