REVIEWS 381 
covered in large part with subaérial detritus. In a humid region with 
free drainage no great thickness of detritus can usually be built up on 
the floor of a basin without increasing the gradient so as to suspend 
the process of aggradation, unless movements of deformation or 
changes of sea-relationship intervene to renew and perpetuate the 
conditions of aggradation. This of course may happen, but it is 
rather to be classed as an accidental intervention than as a systematic 
process. 
The presumptions therefore seem to lie on the side of lacustrine 
deposition, with incidental fluvial aggradation, in humid regions, while 
in arid regions they lie on the side of fluvial aggradation, with inciden- 
tal lacustrine deposition. ‘To the reviewer, therefore, the question has 
a specific climatic relationship and this relationship seems much the 
most important phase of the subject. Given the same humidity, and 
the ratio of lacustrine to fluvial deposition is dependent on surface 
adjustments of a local nature. Given the same surface adjustments, 
and the ratio of lacustrine to fluvial deposition is dependent on states of 
humidity or aridity. But the humidity or the aridity of an area large 
enough to have geological importance, implies atmospheric states that 
are a function of the whole atmosphere, and of its modes of circulation, 
and hence has far-reaching significance. 
If these considerations have any validity, the question which 
Professor Davis pointedly raises regarding the Rocky Mountain Terti- 
aries, as a specific example of the class under question, deserves the 
most critical attention. The value of an academic discussion, which is 
often unwisely underrated by the working field geologist, lies chiefly 
in deploying the problem and laying the groundwork for discrimi- 
native observations. Professor Davis seems to be altogether correct 
in pointing out a lack of critical observation and interpretation in 
most previous studies of the Tertiaries in question, and his discussion 
can hardly fail to call forth incisive studies upon these formations. 
Obviously their true character can only be determined by such critical 
field studies. A first step is the establishment of criteria of discrimi- 
nation between lacustrine and fluvial deposits; by no means an easy 
task where the products of relatively shallow lakes are to be distin- 
guished from those of rivers, which is really the critical case. It is 
not clear that the criteria given in the paper will always hold good, 
but there are several additional ones that may be brought into service, 
such as the distribution of the remains of land animals in the midst of 
