proceedings: biological society 413 



Filled valleys of this character are rather common features along the 

 Missouri in both eastern Montana and western North Dakota. The 

 material filling the valleys is uniformly fine-grained like much of the 

 alluvium of the present flood plain. It is generally unstratified but in 

 places contains thin layers of debris from the Fort Union formation, 

 which crops out higher up on the valley sides. Where cut by the streams 

 it stands out in nearly vertical light-colored cliffs from a few feet to 

 30 feet in height, having a tendency to break with vertical joints from 

 top to bottom. That it is not ordinary alluvium is proved by the fact 

 that its surface consists of long even slopes from the valley sides and 

 not of level plains. The material is as unstratified as glacial till, but 

 unlike till it does not contain angular fragments of granite, and in 

 several places it was found resting on glacial till. That the valley fill- 

 ings are of comparatively recent age is shown by the fact that from one 

 of them the head of a mountain sheep that became extinct in this neigh- 

 borhood only a few years ago was found at an elevation of 150 feet above 

 the river. While the writer was camped near the Missouri in North 

 Dakota several severe wind storms occurred. The river was low and 

 exposed a great many expanses of sun-dried mud, and the winds gathered 

 up great quantities of dust and carried it high in the air. After a day of 

 such wind the tent floors showed a very perceptible coating of dust. 

 Some of this dust undoubtedly had been carried for long distances and 

 was deposited wherever there was a lodgment free from wind. In the 

 winds there is an agent of deposition which is not accounted for, and the 

 writer believes that such winds are responsible for valley fillings of this 

 character; in other words, that these valleys are filled with loess. If 

 this conclusion is correct, it is probable that a large part of the rich soil 

 of North Dakota and Montana was brought in and is constantly 

 replenished b}^ the wind. 



George L. Harrington: Late Tertiary and Quaternary history of 

 the lower Yukon River region. (Illustrated.) In the lower Yukon 

 Valley there appears to be no dividing line between the late Tertiary 

 and the Quaternary. No fossils have been found in the unconsoli- 

 dated silts, sands, and gravels, and interpretations of the geologic 

 history are based on the unconsolidated deposits, high terraces along 

 the river, and horizontal lava flows. 



The events of the late Tertiary and Quaternary in this region include 

 subsidence of the land surface, extrusion of basaltic lavas, further 

 subsidence, reelevation, adjustments of drainage in the silt-filled valleys, 

 and erosion by the processes normal to subarctic and arctic regions. 



EsPER S. Larsen, Secretary. 



BIOLOGICAL SOCIETY OF WASHINGTON 



The 585th regular meeting of the Society was held in the Meeting 

 House of the Friend's School, 1809 I Street N.W., Saturday, May 4, 

 1918; called to order at 8 p.m. by President Rose; 26 persons present. 



