8 NEW YORK STATE MUSEUM 
effects. The terms geography and geology are thus here used in 
the sense that the latter includes the former, as the cause includes 
the effect. Paleogeography has reference to the geography of the 
past epochs in the history of the earth. 
Physiography, or physical geography, deals with the configuration 
(relief) of the earth’s surface and how it was produced. 
As a result of the work of many able students of earth science 
during the past hundred years, it is now well established that our 
planet has a clearly recorded history of many millions of years, and 
that during the lapse of those eons revolutionary changes in geog- 
raphy have occurred; that there has also been from an early stage 
of the earth’s history a vast succession of living beings which have 
gradually passed from simple into more complex forms and have, 
in some particulars, reached their highest expression in the organ- 
isms of the present time. The geographic changes and the organisms 
of the ages gone by have left us no abundant evidence of their char- 
acter and the study of the rock formations has shown that within 
them we have a fairly complete record of the earth’s history. 
In the time of Alexander von Humboldt, less than one hundred 
years ago, the keen student of natural phenomena could carry in 
his own mind most of what was definitely known of earth history. 
‘Today, because of the tremendous growth of the science, it would 
be a presumption for any man to claim that he knows all of what 
has been learned about the geological history of even the single 
State of New York. While it is true that much yet remains to be 
learned of this old earth, it is a real source of wonderment that 
man, through the exercise of his highest faculty, has come to know 
so much about it. 
All the rocks of the earth’s crust may be divided into three great 
classes: igneous, sedimentary, and metamorphic. 
Igneous rocks comprise all those which have ever been in a 
molten condition, and of these we have the volcanic rocks (for 
example, lavas) which have cooled at or near the surface ;- plutonic 
rocks (e. g., granites) which have cooled in great masses at con- 
siderable depths below the surface; and the dike rocks, which when 
molten have been forced into fissures of the earth’s crust and there 
cooled. 
Sedimentary rocks comprise all those which have been deposited 
under water (except for some wind-blown deposits) and are nearly 
always arranged in layers (stratified). These rocks may be of 
mechanical origin, such as clay or mud which hardens to shale; 
sand, which consolidates to sandstone; and gravel, which when 
cemented becomes conglomerate. Or they may be of organic origin, 
