GEOLOGIC VERSOS LITHOLOGIE 231 
extent and importance of the erosion which exposed the rock of 
that pebble, than is afforded by all other data combined. Two 
conglomerates in close proximity, and so situated at the locality 
where they are exposed that they seem to belong to the same 
general period of sedimentation, may possess in their pebbles 
evidence of events of great importance in the interval imper- 
fectly represented by the stratigraphic line between them. 
Lithologically a shell limestone is one largely composed of 
shells, and a coal of certain properties is bituminous coal, what- 
ever shells or plants, respectively, may have produced the rocks 
in question. But viewing the coal beds and limestones, or other 
fossiliferous strata, as geologic formations, not ‘‘lithologic 
units,” the significance which paleontology shows may be 
attached to the particular fossils found ina given case is of great 
importance. The record here referred to is not that of life his- 
tory, but of earth history. The fossils of a “lithologic indi- 
vidual” may be the most significant characters it contains 
bearing upon its age, the extent and importance of earth devel- 
opment in the epoch preceding its deposition, the characters of 
the waters in which it was formed, and the climatic conditions 
of the time. From the standpoint of the investigator striving 
to make out the full significance of a sedimentary deposit, a 
constituent pebble of a recognizable older rock of known struc- 
tural position is a feature possibly equal in importance to a 
shell, a bone, or a plant, of certain paleontologic character. 
Both are fossils, in a sense. 
The mode of origin and the characteristic features of igneous 
rocks render the evidence of geologic history to be obtained 
from them limited as compared with that which may be derived 
from sedimentary formations. But it appears to the writer that 
much more effort should be made to read these records and to 
represent the information obtainable on geological maps than is 
commonly attempted. 
The petrographic, or lithologic, character of an igneous rock 
serves to show its proper systematic position and leads to its 
