THE PETROLOGY OF SEDIMENTARY ROCKS (psi 
correlate the strata by the varying, though generally small, amount 
of andesitic tuff or ash which it contains, the Lebo member at the 
type locality consisting chiefly of andesitic material. Similarly if a 
formation, or some particular stratum of a formation, were known 
to contain tourmaline, or a large amount of mica, or spherulitic glass, 
or some other distinctive constituent, this information might prove 
a valuable auxiliary in correlation to stratigraphic and paleontologic 
evidence. Though the instances last cited are doubtless uncommon 
it is probable that ash is rather widely distributed through the 
Cretaceous and Tertiary rocks of the west, and it may prove to be 
an aid to correlation in many cases. Moreover, even where condi- 
tions are not favorable to using the petrology in this way it is 
possible that it may throw light on the exact position of a doubtful 
formation boundary. Thus, D. E. Winchester, United States 
Geological Survey, believes that evidence of this kind may lead to 
fixing the lower limit of the Mesaverde formation in the Zuni Indian 
Reservation, New Mexico, where by reason of a lack of diagnostic 
fossils at the critical horizon the exact base of the formation cannot 
be otherwise located. The case of the Lebo shale described below 
furnishes a simple example of the value which petrologic evidence 
may have. 
THE LEBO SHALE MEMBER OF THE FORT UNION FORMATION IN 
EASTERN MONTANA 
General statement.—During the summer of 1011 the writer 
examined for the United States Geological Survey an area in 
eastern Montana known as the Little Sheep Mountain coal field’ 
which extends about 60 miles westward from the town of Terry, on 
the Yellowstone River. In contiguous areas on the east and south 
the strata had been mapped as Fort Union and Lance. 
About 850 feet of the upper or yellow strata of the Fort Union 
formation are exposed in the Little Sheep Mountain field, the age of 
these rocks being definitely established by paleontological evidence. 
The beds are made up of sandstone, sandy shale, and clay shale, 
generally soft and in many places incoherent, and commonly yellow 
« G. S. Rogers, “Little Sheep Mountain Coal Field,” Bull. U.S. Geol. Survey No. 
531, 1913 (in press). 
