THE PETROLOGY OF SEDIMENTARY ROCKS ieee 
ripple-marks and mud cracks, is very common throughout the 
whole of the Lebo. The lenticular and irregular character of these 
strata extends to their coals as well, which are generally dirty, 
and, with the exception of bed U, of no great extent. 
Petrologic evidence—mineralogical composition and character of 
grains.—The Lebo shale on the northeastern flank of the Crazy 
Mountains is described as a derivative of an andesite. Microscopic 
examination showed that certain facies are nearly pure tuffs, but 
that generally a considerable amount of plagioclase is present and 
Frc. 3.—View in badlands on Custer Creek showing irregular character of the 
Lebo shale sediments, and the intercalation of lenses of white sandy clay which con- 
tains about 50 per cent andesitic ash, sec. 11, T. 11 N., R. 49 E., P.M., Montana. 
that the whole is set in a fine chloritic groundmass. Fragments of 
hornblende and augite are also frequently found, and in places there 
is a considerable admixture of quartz. Eight thin sections were 
made of the more coherent members of the dark shale of the district 
discussed in this paper and in addition a number of sections of 
incoherent material were prepared without grinding; and all these 
were examined by the writer. Two of the thin sections were fine- 
grained dark gray shale, so fine in fact that the microscope revealed 
no recognizable grains. They seem to consist of kaolin stained 
