250 N. S. SIIALKR LOESS DEPOSITS OF MONTANA 



sioii of irrigation in tlie naturally arid valleys. If the fires were pre- 

 vented this effect would soon be important. As it is, however, the 

 action of the wind is now, year by year, on the increase, and will doubt- 

 less be augmented in a rapid manner until this process of devastation 

 is arrested; for with each augmentation in the amount of material in 

 the blast it becomes able more effectively to destroy the lowly plants 

 and thus to obtain a larger share of the soil matter. This fine matter, 

 as before noted, is carried to the margins of the streams, whence it is 

 quickly taken to the sea. Acting in this manner the wind is rapidly 

 taking awa}^ from the higher benches of the valleys the at best rather 

 scanty layer of fine detritus which constitutes the true soil, leaving the 

 pebbly waste, so that wide areas which in time might be won to agri- 

 culture by irrigation from storage reservoirs are ever becoming less suited 

 to that use. 



Remedies for injurious Effects' of Loess-forming 



This evil can be in part met by an adequate protection of the forests 

 from the fire and by systematic plantation, and also by the introduc- 

 tion from other arid countries of species of plants which are better 

 fitted to thrive in desert conditions. The relatively recent coming of 

 an arid climate to this part of the Cordilleras may account for the ])au- 

 city of species which are adapted to a very dry soil. In other fields 

 where like climatal conditions have endured for a much longer time, as, 

 for instance, in the desert regions of central Asia, we may hope to find 

 plants Avhich can be naturalized here and which will aid in protecting 

 the soil from the wind. 



Summary of Conclusions 



The foregoing considerations may be summed up as follows : 



1. The loess deposits of central Montana indicate that the arid condi- 

 tions existing in the central section of the Cordilleras of North America 

 came about suddenly and have been of no very long duration. In this 

 regard the area probably differs from the districts to the southward. 



2. The present arid state followed immediately on a time when the 

 streams were much larger than the}' are at ]iresent ; when the}' were able 

 to build very extensive detrital cones which are now, wherever in their 

 natural state, not receiving additions thi-ough the torrent work, but are 

 being sheeted in beneath loess deposits. 



