360 PROCEEDINGS OP THE AMERICAN ACADEMY. 



(Monogr. I., U. S. G. S., 193). Beds of cross-bedded gravel and sand, 

 associated with sandy loam, occur between a lower and a higher marl in 

 the Lahontan basin : of these Russell says : " the remarkable similarity 

 of the middle member of the Lahontan section, as exposed in certain 

 localities, to the . . . deposit formed by meandering streams, leads us to 

 refer its origin with considerable confidence to similar causes " (Monogr. 

 XL, U. S. G. S., 129). Indeed the whole theory of the variations of 

 Quaternary climate in the Great Basin depends on a subaerial origin 

 of certain gravel and sand deposits which are in many ways similar to 

 deposits that have been repeatedly described as lacustrine in accounts of 

 Tertiary formations. 



8. Continental Deposits. — It was during a western excursion with 

 Professor Penck of Vienna in the summer of 1897 that a possible or 

 probable non-lacustrine origin of many of our western fresh-water 

 Tertiaries was first clearly presented to me. Since then, I have had 

 opportunity of seeing something of the great fluviatile plain of thePo, and 

 of recalling what I had long before seen of similar plains in California 

 and in northern India, as well as of reviewing several essays that bear 

 on the general problem here considered ; and the problem has thus come 

 to have an importance that warrants the present review and summary. 



Penck's views on this subject may be found in his " Morphologie der 

 Erdoberfljiche " (ii, 24-36), where he discusses the occurrence of deposits 

 formed on subaerial plains in the older geological systems. Recognizing 

 that non-mai'ine formations may result under the action of various sub- 

 aerial agents as well as within lakes, he suggests the name, continental, 

 to include all such formations, leaving the discrimination of particular 

 deposits to further study. Penck's term deserves acceptance among 

 geologists, as an aid in the general consideration that it seems desirable 

 to give to the problem of our western Tertiaries ; they might be called 

 "continental" in order to avoid implication of either lacustrine or 

 fluviatile origin. Yet as far as the published descriptions of these 

 deposits afford evidence of their detailed structure, it appears to me 

 probable that streams and rivers have had more than lakes or winds to 

 do with their formation, and hence that "fluviatile" might often to 

 advantage replace "lacustrine" in describing them. 



9. Fluviatile Deposits. — It is perhaps because so much has been 

 written regarding the erosive power of rivers that their constructive 

 powers have been too little considered; but their capacity to aggrade a 

 sinking area deserves as careful examination as their capacity to degrade 

 a rising area. When acting as aggrading agents, they spread out broad 



