Januaey 29, 1897.] 



SCmNGE. 



177 



it is not difl&cult to imagine that such cases 

 ■occurred quite often, especially when, with 

 the change of the climate, both birds and 

 mammals spread more and more into the 

 temperate regions where the spring move- 

 ments of the grazing animals fell together 

 with the bird's breeding time. 



By a combination of favorable circum- 

 stances this new way of reproduction proved 

 successful, and the parasitic oifspring be- 

 came more and more numerous. In the 

 course of time the art of building nests was 

 lost, the desire to incubate entirely gone, 

 paternal and conjugal affection deadened, 

 and parasitism had become a fixed habit. 



O. WiDMANN. 



CURRENT NOTES ON PHYSIOGBAPEY. 

 THE BRANCH STREAMS OF THE SCHUYLKILL. 



Miss F. Bascom recently discussed ' the 

 relation of the streams in the neighbor- 

 hood of Philadelphia to the Bryn Mawr 

 gravel' (American Geologist, XIX., 1897, 

 50-57), with the object of determining the 

 disputed age of the gravels from the amount 

 of work done by the branches of the Schuyl- 

 kill since the gravels were laid down. Wis- 

 sahickon. Valley and Gulf creeks are ex- 

 plained as of superposed origin, because 

 they flow at certain points transversely 

 through narrow gorges in resistant strata. 

 This conclusion tacitly postulates the oc- 

 currence of only longitudinal (subsequent) 

 branch streams in the Schuylkill district 

 before the gravels were spread over the re- 

 gion ; it remains to be proved whether so 

 perfect an adjustment of branch streams to 

 structures is necessary. It is entirely con- 

 ceivable that, before the gravels were de- 

 posited, the Cretaceous peneplain had some 

 transverse streams, although most of its 

 drainage may have well become longitudi- 

 nal. Whether the Wissahickon could have 

 maintained a transverse course so near the 

 ■Schuylkill through both the Cretaceous and 



Tertiary cycles of denudation is certainly 

 doubtful, but it has not been proved impos- 

 sible. Gulf creek and its neighbors are so 

 distinctly rectangular in pattern that ad- 

 justment and re-adjustment suflflce to ex- 

 plain them without superposition. The 

 elements of doubt and certainty are here 

 so blended as to illustrate the dangers as 

 well as the values of river analysis as a 

 means of deciphering geological history. 



hann's allgemeine eedkunde. 

 The Allgemeine Erdkunde of Hann, 

 Hochsetter and Pokorny now reaches its 

 fifth edition. The first part, treating the 

 earth as a whole, the atmosphere and the 

 hydrosphere being still prepared by Dr. 

 Julius Hann (Vienna, Tempsky, 1896, 336 

 p., 24 colored plates and 92 figures), while 

 volumes on the earth's crust and its forms 

 by Briickner, and on the distribution of 

 plants and animals by Kirchhofif, are prom- 

 ised for 1897. Hann's revised volume im- 

 presses one as a thorough work by a compe- 

 tent author, useful as a text for an advanced 

 collegiate course, or as a reference book for 

 advanced students. It is questionable 

 whether various elementary facts, such as 

 the obliquity of the ecliptic, the variation 

 of the length of the day and its cause, and 

 the weather-map facts as to cyclonic circu- 

 lation, deserve a place in such a work ; for 

 any one who is competent to use the rest of 

 the book should have been for some years 

 familiar with these fundamentals. The 

 more serious subjects may be inferred from 

 a rapid review of the contents ; the size 

 and shape of the earth, and their conse- 

 quences in the variation of gravity and 

 the determination of positions ; terrestrial 

 magnetism and auroras ; the atmosphere, 

 its temperature, pressure, winds, moisture, 

 rain and weather — with less attention to 

 the origin of cyclones than would be wel- 

 come; the ocean, its depth, composition, 

 temperature — this treated in much detail — 



