e 76st ) 
BOOK VI. 
STRATIGRAPHICAL GHOLOGY. 
“Tuts branch of the science arranges the rocks of the earth’s crust in 
the order of their appearance, and interprets the sequence of events 
of which they form the records. Its province is to cull from the ~ 
other departments of geology the facts which may be needed to show 
what has been the progress of our planet, and of each continent and 
- country, from the earliest times of which the rocks have preserved 
any memorial. Thus from Mineralogy and Petrography it obtains 
information regarding the origin and subsequent mutations of 
minerals and rocks. From Dynamical Geology it learns by what 
agencies the materials of the earth’s crust have been formed, altered, 
broken, or upheaved. From Geotectonic Geology it understands 
how these materials have been built up into the complicated crust 
of the earth. From Paleontological Geology it receives in well- 
determined fossil remains a clue by which to discriminate the 
different stratified formations, and to trace the grand onward march 
of organized existence upon the planet. Stratigraphical geology 
thus gathers up the sum of all that is made known by the other 
departments of the science, and makes it subservient to the 
interpretation of the geological history of the earth. 
The leading principles of stratigraphy may be summed up as 
follows : 
1. In every stratigraphical research the fundamental requisite is 
to establish the order of superposition of the strata. Until this is 
accomplished it is impossible to arrange the relative dates and make 
out the sequence of geological history. 
2. The stratified portion of the earth’s crust, or Geological 
Record, may be subdivided into natural groups or formations of 
strata, each marked throughout by some common genera or species 
of organic remains, or by a general resemblance in their paleon- 
tological type or character. 
3. Living species of plants and animals can be traced downward 
into the more recent geological formations; but grow fewer in 
number as they are followed into more ancient deposits. With their 
disappearance we encounter other species and genera which are no 
longer living. ‘These in turn may be traced backward into earlier 
