GEOLOGY AND PALEONTOLOGY. 
29 
work is the particular province of the section of petrology. Rocks 
may be ground down to sections so thin that they become transparent, 
and then, with a particular form of microscope, the minerals compos- 
ing them may be determined. 
On the return of the geologist from the field such microscopic sec- 
tions as are necessary are made and are studied in consultation with 
the chief of the section of petrology. Analyses of rocks or ore are 
made in the chemical laboratory, and the fossils collected are submitted 
to the paleontologists for determination and interpretation. The field 
notes are plotted, sections are drawn, and the report is written. 
Accompanying each report is a geologic map on which is shown, by 
appropriate colors and symbols, the distribution of the various forma- 
tions. Geologic time — the time during which the rocks were made — 
is divided into several large parts called periods. Smaller time divi- 
sions are called epochs, and still smaller ones stages. The age of a rock 
is expressed by naming the time interval in which it was formed. 
The sedimentary rocks are grouped together into systems. The 
principal divisions of a S3^stem are called series. Any aggregate of 
formations less than a series is called a group. Where a part of 
a formation is specially designated it is known as a member. In the 
work of the Geological Survey the following s}^stems and series are 
recognized : 
Rock systems and series. 
System. 
Cenozoic 
Quaternary 
-Tertiary 
Series. 
r Recent. 
IPleistocene. 
Pliocene. 
Miocene. 
Oligocene. 
Eocene. 
'Cretaceous. . 
Mesozoic . .• 
Jurassic 
Triassic 
Carboniferous 
C Permian. 
\ Pennsylvaniaii. 
[Mississippian. 
Devonian 
Paleozoic ..- 
Silurian 
Ordovician 
Cambrian 
fSaratogan. 
< Acadian. 
Algonkian . 
[Georgian. 
Archean 
