THE PETROLOGY OF SEDIMENTARY ROCKS 727 
There are many minor points, however, which the field geologist 
remembers, but which were considered of too small importance to 
write down at the time, and which may yet derive an unexpected 
significance from the results of a simple petrologic study. When 
possible therefore, it is advantageous for the field man to examine 
his own material. 
SUMMARY AND CONCLUSIONS 
The amount of petrologic work done on sedimentary rocks, 
especially the more or less incoherent Cretaceous and Tertiary rocks 
of the West has been comparatively small, and most of the study 
made has been directed toward the rock as a rock type, rather than 
as a part of the rock mass or formation. Of late years attention 
has been directed to the broader structural features of clastic rocks 
and much information concerning the manner of deposition has been 
accumulated. It is believed that petrologic study of the mineral 
constitutents and the character of their grains will prove an impor- 
tant aid in determining the history of the rock, and that such study 
may also lead to facts of value in correlation and in the fixing of 
formation boundaries. An area of dark-gray shale near Terry, 
Montana, was correlated with the Lebo shale in the Bull Mountains 
to the west, largely by petrologic evidence. In this particular 
instance the determining constituent was andesitic ash, but in other 
cases the presence of other more or less distinctive constituents may 
be found significant and of value in correlation. There are two 
apparent difficulties confronting a study of this kind: first, those 
arising from the fact that many sedimentary rocks are made up of 
badly altered minerals occurring in grains too small for satisfactory 
examination; and second, the difficulty of choosing the proper rocks 
for detailed study. The fact that our definitions of sedimentary 
rock types are very vague and loose is in itself an argument for more 
petrologic study; and while it is not probable that this work will be 
found to have a very broad application to correlation, as in the one 
instance described, it still seems that it may prove an important 
auxiliary in that direction to the other lines of research at our 
command, and will in addition shed valuable light on the history 
of the rock. 
