THE PETROLOGY OF SEDIMENTARY ROCKS 723 
from the incoherent material; several were too fine-grained to 
permit of satisfactory identification, but the remainder contained 
varying quantities of glass, commonly badly altered. The sandy 
white material is thus derived directly from andesitic effusive 
material; all of the other rocks examined which were coarse enough 
to allow of study indicate a derivation, in large part at least, from 
a basic igneous rock; the shale is in general too fine to warrant an 
opinion as to its origin, but its evidence is not adverse. Further- 
more, specimens of the yellow Fort Union above and of the Lance 
below were examined and all indicate a probable derivation from a 
much more siliceous rock; at least it may be said that they differ 
decidedly from the dark shale. 
Owing to the fact that most of the important constituents of 
these rocks—glass and ferro-magnesian minerals—are prone to 
comparatively rapid alteration the evidence as to the shape of the 
grains in this instance cannot be satisfactorily obtained. The 
feldspar grains are in general: nearly or quite fresh and are angular. 
The quartz is commonly in decidedly angular grains and only a few 
were noticed which were subangular or rounded. Similarly in the 
case of the glass it seems that the fresher the grain the more angular 
- it is, some being found which were fairly fresh and distinctly angular. 
While these rocks are not favorable for a detailed study of the shape 
of the grains it may be said that they show the complete assortment 
as to size which characterizes Scherzer’s aqueous type, but with the 
angularity which is regarded as diagnostic of the volcanic, glacial 
and residual types. Taking the minerat composition into account 
therefore they may be classified as aqueo-volcanic rocks. 
Inter pretation —Because of this concurrence of the stratigraphic 
and petrologic evidence therefore it seems proper to corre ate these 
strata with the Lebo member. According to this hypothesis a large 
part of the material has been transported from the Crazy Moun- 
tains, although a considerable increment of foreign detritus has been 
received along the route. The striking cross-bedding and the 
decided lenticularity and lack of persistence of the beds argue for a 
fluvial origin, while the angularity of the grains themselves and their 
assortment as to size offer a similar suggestion and preclude the 
possibility of any long eolian transportation. It is possible that the 
