some action subsequent to their original deposition, so as to 
partially obliterate their original structure. These are often called 
also Crystalline rocks. The rocks of New York Island are ex- 
amples. Granite, gneiss, roofing slate and statuary marble be- 
long here. These crystalline rocks being the lowest, and con- 
sequently the oldest, formed a floor upon which the more recent 
ones were deposited. They also contain many forms of minerals 
and metals, especially iron (see Case a). 
The Noii-nietamorphic divisions of the stratified rocks are 
those which afford the great interest to students of geology, as 
being those which present, in their fossil contents, a record of the 
past history of the globe since the commencement of life on its 
surface. The character of these rocks shows that they at one time 
formed the bed of the ocean, and are made up of the sediments 
from the water, together with the remains of different forms of 
animal or plant life that existed at the time the deposits were 
made, and in that way preserve a record (as fossils) of the beings 
which existed in the seas at that time. As they lie one upon the 
other in regular order, the lowest being of course the oldest, each 
successive bed, from below upward, would record the events of a 
succeeding period of time ; and as these periods and epochs are 
divided according to their fossil contents, they represent the dif- 
ferent stages of the earth existence, by the history as recorded 
therein. 
By subsequent changes, which took place in the level of the 
earth's surface, these rocks, together with their contents, were 
raised to their present position, and again partially worn away. 
They are now seen protruding from beneath the surface covering 
of soil, sand, gravel or clay, and are exposed to view on the sides 
of hills, or along ravines, where they are cut through by streams ; 
and as the various beds differ in composition or in their mineral 
or fossil contents, they can be traced and mapped as roads or 
streams are. In this way geological maps are constructed which 
show by their colors over what part of a country any certain rock 
forms the surface, beneath this superficial covering. 
There are several divisions of geological science, as Dynamical, 
Chemical, Mining, Economic, Geographical, Historical, &c. The 
collections in this room are illustrative principally of Historical 
