Deckmkki: 14, 1883.] 



SCIENCE. 



763 



of south, ,ind then as raucli to the west of south, giv- 

 ing the boundary an eastward angle in its middle part. 

 The general strike of the rocks in eastern Iowa is 

 south-east; and the dip, which is gentle, is south-west. 

 The broadest oiU crops are those of the Niagara and 

 Hamilton formations. The Niagara, having resisted 

 the prcquaternary planation, holds an escarpment, 

 the crest of which runs from the extreme eastern 

 point of the state to a point on (lie Minnesota line 

 fifty miles west of the Mississippi. From this lino 

 there is a somewhat rapid descent to the Mississippi, 

 and a gentle slope south-westward to the broad, shal- 

 low depression marking the position of the Flamilton. 

 From this valley the ascent is gentle to the water- 

 parting between the Mississippi and Missouri. The 

 general slope of the region west of the Niagara es- 

 carpment, considered as a whole, is with the dip to 

 the south-west. 



Beside the south-east trending depression marking 

 the Ilaniilton outcrop, there is a gently sloped and 

 indefinitely outlined but continuous ami actual pro- 

 quaternary valley, extending southward across the 

 eastward projection of the state, and traversing diago- 

 nally the upper Silurian, Devonian, and carboniferous 

 rocks. 



The north-eastern angle of the state, from the crest 

 of the Niagara escarpment to the Mississippi, belongs 

 to the driftless region. The remainder of the state 

 is covered with drift, and is affected by the undula- 

 tions characteristic of drift topography. 



The general directions of the rivers are from north- 

 west to south-east ; but their upper courses swerve a 

 little toward the meridian, and their lower are de- 

 flected slightly toward the east, so as to give them a 

 gentle curvature with concavity to the north-east. 

 There is, moreover, a convergence northward, as 

 though they radiated from .some point in Minnesota. 

 The variations from this normal system are so few 

 that the drainage is almost unique in its regularity. 

 It is likewise independent of the general topography; 

 for not only do the principal streams flow at right 

 angles to the prevailing slope, and cut through the ele- 

 vated escarpment when it lies in their way, but, with 

 a single exception, they preserve their courses across 

 the ancient north and south valley. 



In their relaticuis to minor topographic features, 

 they conform to two antagonistic laws, — first, they 

 follow in general the ill-defined shallow valleys which 

 characterize the drift-plains; and, second, they flow 

 for one-third of their total courses in narrow gorges, 

 following the axes of a system of elongated ridges 

 which constitute the leading features in the local 

 topography. Moreover, th.ey have in many instances 

 gone out of their direct courses, and deserted valleys 

 seemingly prepared for them, to attain the anoma- 

 lous positions assumed under the second law of as- 

 sociation: and in every such case the gorges have 

 demonstrably been carved by the streams themselves. 

 The avoided valleys are evidently pre-existent : they 

 have not been appreciably eroded since the quater- 

 nary, and there has been no recent localized oro- 

 graphic movement. 



So the drainage is essentially independent of the 

 general topogrjiphy, though affected by local topog- 

 raphy ; and its relations to local topography are largely 

 anomalous. 



The loess of the region is continuous stratigraphi- 

 cally, but follows different laws of distribution in dif- 

 ferent districts. It constitutes the surface throughout 

 the driftless region, and at the margin it overlaps the 

 drift. In the northern part of the drift-covered area 

 it forms narrow bands with a general north-west trend, 

 each of which caps a ridge. Farther south it covers 

 the entire plain, eminences and depressions alike. 

 In the driftless area it rests on and merges into a 

 thin stratum of water-worn erratic material. In the 

 belts traversing the contiguous drift-plain it passes 

 downward into sand, which may, or may not, merge 

 into drift. Elsewhere it reposes on the drift, into 

 which it graduates insensibly. The ridges in which 

 the rivers have carved their anomalous cafions are 

 always loess-topped ; and, wherever streams avoid 

 low-lying valleys for high-lying plateaus, the plateaus 

 are of loess exteriorly. 



So in its distribution the loess of eastern Iowa is 

 intimately connected with the driftless region, with 

 the drainage, and with the topographic configura- 

 tion. 



In the communication referred to, Mr. McGee offers 

 no explanation, but merely sets forth the facts. His 

 working hypothesis has, however, been published in 

 an earlier paper (Amer. journ. sc, Sept., 18S2), and 

 may properly be restated in this connection. 



It is now many years since Powell first proposed to 

 cl.ass all inconsequent drainage as either antecedent or 

 superimposed; and no later writer has .added to the 

 number of categories. In incbn/tcquent drainage the 

 courses of the streaius are independent of Ihedipaud 

 other structure-elements of the rocks across which they 

 run. If the drainage is older than the rock-structure, 

 — if, for exam]>le. the dip has been given to the rock 

 after the establishment of the stream-coiuses, — tl>e 

 drainage is said to be antecedent. If the drainage 

 was established by the configuration of an overlying 

 and unconformable formation, which has disappeared 

 by denudation, the drainage is said to be superim- 

 posed. In eastern Iowa, the .sviperficial formation 

 being northern drift, which lies with little modifica- 

 tion as originally dejiosited, the hypothesis of ante- 

 ci'dent drain.ige appears quite out of the question, 

 while that of superimposed drainage in the ordinary 

 sense is equally inapplicable. Mr. McGee's working 

 hypothesis is, that the drainage was superimposed in 

 an extraordinary manner; namely, by the ice-sheet. 

 This, he finds reason to believe, was so thin in that 

 region as to have its superficial configuration material- 

 ly modified by the small inequalities of its bed. Where 

 the ice was retarded by ridges underneath, more time 

 was allowed for superficial w.aste by melting: so th.at 

 hollows were produced, and the rivers of the ice-sur- 

 face came to be established over the ridges of the 

 glacier bed. With the disappearance of the ice, they 

 were stranded upon the hill-tops. 



G. K. Gilbert. 



