io8 



SCIENCE. 



[Vol. XIX. No. 472 



in soft tertiary marls" it may be inferred that the streams are 

 little advanced in their present cycle of development. Professor 

 Hicks postulates that at the beginning of ihe current cycle of 

 river history, the several branches of the Loup River all pursued 

 independent courses to the Platte. The origin of those early 

 courses is not stated; whether they were consequent on the slant- 

 ing surface of the tertiary marls, or whether they represented the 

 finally adopted positions of old rivers of a previous cycle of 

 growth. 



Old rivers, revived by the uplift of the- plains into a new cycle 

 of growth are common enough in the western country, and per- 

 haps the Platte and Loup may be of that kind ; but, if so, it does 

 not seem possible to explain the present course of the main Loup 

 River as resulting from a recent capture of its several north-west 

 branches. River captures occur during the early maturity of a 

 river system. If the Platte and the Loup are revived from the 

 old stage of a previous cycle, the captures should have taken 

 place in the earlier part of that cycle; and when the river courses 

 had thus become well adjusted, they would be maintained even 

 after uplift and entrance into another cycle, unless distinctly new 

 conditions were thereby introduced. The possibility of this will 

 be considered in a later paragraph. 



If the rivers are not now in a second cycle of development, but 

 are in their first cycle, having first taken their course when their 

 region rose from the waters in which its strata were deposited, 

 and having since then done nothing more than cut their shallow 

 trenches in the general unbroken surface of the country, then we 

 must ask whether their initial courses must have been in the 

 arrangement postulated by Professor Hicks, or whether they may 

 not from the beginning have had courses essentially on their 

 present lines of flow. This latter alternative appears to be indi- 

 cated at the end of Professor Hicks's article, if I read it aright. 

 Assuming that the last great tertiary lake not only submerged the 

 area of the Loup River, but spread its lacustrine sediments over the 

 surface so as to obliterate any channels of earlier date, then on 

 the disappearance of the lake, the rivers would be newly devel- 

 oped on the faint slopes of its deposits. The Platte, bringing 

 down silt in large amount, may have been at that time a construe 

 tive rivrr, busied in building up a broad delta like flood-plain, 

 further and further out on the lacustrine deposits as they were 

 revealed. If so, it would turn its lateral tributaries down-stream, 

 and the existing arrangement would be produced without the aid 

 of headwater erosion and capture. Hence, until the process of 

 flood-plain deflection is excluded, it does not seem necessary to 

 include the process of headwater erosion and capture. 



But even if it be supposed that the courses of the rivers at the 

 beginning of the present cycle were arranged as postulated by 

 Professor Hicks, and that all of them from the Beaver to the 

 South Loup entered directly into the Platte, it seems impossible to 

 explain their present arrangement by the headwater erosion and 

 piracy of the Loup. The condiaons for so systematic a process do 

 not occur in the region under consideration, as will appear from 

 the following analysis. 



In the first place, it is important to remember that it is not the 

 river but the little trickhng headwater streams on the slopes of 

 the divides that do the capturing' in cases of the kind here dis- 

 cussed. The capture of one river by another, or lateral abstrac- 

 tion, as described by Gilbert in his most original examination of 

 this problem in his report on the Henry Mountains is a compara- 

 tively rare occurrence, and is not applicable here. 



In the second place, capture by little headwater^- is most com- 

 mon in regions of tilted rocks of varied hardness, and on the 

 headwaters of "subsequent" streams; that is, streams whose 

 headwater growth is dependent on the opportunity given by the 

 we-thering of some especially weak stratum, along whose strike 

 j .ue stream extends. No such special opportunity has been offered 

 [to the Loup River in this region of horizontal beds. 

 ^ In the third place, as one headwater stream grows, all other 

 lidjacent headwaters of the -same kind grow at about the same 

 ,pa6^. Hence, if the Loup River has so greatly extended itself by 

 heac'water erosion, all the other headwater streams should have 

 growfn also, and the country thereabouts would be much moredis- 

 I secteqi by channels than it now is. 



Finally, the location of Prairie Creek seems to contradict the 

 supposition that the branches of Loup River ever joined the 

 Platte directly; for, if they had, then Prairie Creek must be, like 

 the supposed extension of the Loup, an example of headwater 

 erosion; and this is not to be thought of in a stream so systemati- 

 cally located between two parallel and larger rivers in a district 

 of horizontal beds. 



Taken all together, it does not seem necessary to give any es- 

 pecial emphasis to headwater erosion and capture in this river 

 system. The natural result of excessive deposition along the 

 Platte, as described by Professor Hicks, is alone sufficient to ac- 

 count for the present arrangement of the streams. The growth 

 of the Platte flood-plain may have dammed back some of its tribu- 

 taries, as certain branches of the Red River in Louisiana are 

 dammed back and converted into shallow lakes; and the present 

 main Loup River would then be developed by lateral overflow 

 along the margin of the flood-plain; but this is quite another pro- 

 cess from headwater erosion and capture. 



These suggestions are only tentative; for not having seen the 

 region and having no full account of its geological history or of 

 its topography, I can only submit them for criticism. 



W. M. Davis. 



Harvard College, Feb. 10. 



Origin of the Frigid Period in the Northern Hemisphere. 



In my letter, published in your issue of Oct. 16, I stated that 

 the independent circulation of the southern ocean waters was the 

 main cause of ice-sheets forming on the lands situated in the high 

 latitudes of the southern hemisphere; and that such currents were 

 caused by the strong westerly winds, which blew the surface 

 waters of the southern ocean constantly around the globe, and 

 thus prevented the tropical surface currents from largely entering 

 its waters. Consequently, through this cause and the constant 

 gathering of ice in the antarctic regions the temperature of the 

 southern latitudes was slowly lowering; and that the growing 

 coldness would continue until the southern ice-sheets filled the 

 Cape Horn channel and prevented the further independent move- 

 ment of the southern ocean waters. This being accomplished, the 

 westerly winds would blow the surface waters of the sea away 

 from the eastern side of the ice-formed isthmus and the southern 

 lands of South America, and so cause a low sea-level, that would 

 attract the surface waters of the tropical seas far into the southern 

 latitudes, and thus in time furnish heat sufficient to melt the ice 

 from the southern lands I also stated that an ice period could 

 not be perfected in the northern hemisphere without the assistance 

 of cold derived from a frigid period in the southern hemisphere. 

 The independent circulation of the arctic waters is not complete, 

 owing to land obstructions; but it is able to largely prevent the 

 tropical Gulf Stream waters from entering the higher northern 

 latitudes. The prevailing westerly winds blow the surface waters 

 of the A-tlantic away from the eastern shore of North America 

 from Georgia to Labrador; consequently the low sea-level thus 

 caused attracts the high-level tropical waters of the Gulf of Mex- 

 ico through the Florida channel well into the northern latitudes; 

 and during the same time the westerly winds which blow the sur- 

 face waters of the Atlantic auay Ironi tue Amei-ican coasl alg slsO 

 causing a high sea level on the seas abreast north-western Europe, 

 which creates a return current through the Arctic Ocean, passing 

 through the several straits leading into Baffins Bay, and also down 

 the eastern coast of Greenland. Thus the ocean waters of the 

 high northern latitudes maintain a partly independent circulation, 

 which serves to crowd the Gulf Stream away from the higher lati- 

 tudes, and thus lower the temperature of the arctic regions. 

 Through this exclusion of tropical waters, glaciers have formed 

 on Greenland and other arctic shores; and these glaciers are proba- 

 bly slowly increasing, as every iceberg launched from the frigid 

 lands and floated to the Gulf Stream lowers somewhat the tem- 

 perature of the north Atlantic, and so causes conditions more 

 favorable for larger accumulations of ice. Still it is probable that 

 a northern ice period could not be perfected by this process alone 

 should the tropical and southern oceans maintain their present 

 temperature. But with the assistance of a frigid period in the 



