8 
AMERICA THE OLD WORLD. 
The rocks formed under the influence of heat 
are called, in geological language, the Igneous, 
or, as some naturalists have named them, the 
Plutonic rocks, alluding to their fiery origin, 
while the others have been called Aqueous or 
Neptunic rocks, in reference to their origin under 
the agency of water. A simpler term, however, 
quite as distinctive, and more descriptive of their 
structure, is that of the stratified and massive or 
unstratified rocks. We shall see hereafter how 
the relative position of these two classes of rocks 
and their action upon each other enable us to 
determine the chronology of the earth, to com- 
pare the age of her mountains, and, if we have no 
standard by which to estimate the positive dura- 
tion of her continents, to say at least which was 
the first-born among them, and how their char- 
acteristic features have been successively worked 
out. I am aware that many of these inferences, 
drawn from what is called “ the geological rec- 
ord,” must seem to be the work of the imagina- 
tion. In a certain sense this is true, — for imam- 
• ? to 
nation, chastened by correct observation, is our 
best guide in the study of Nature. We are too 
apt to associate the exercise of this faculty with 
woi ks of fiction, while it is in fact the keenest 
detective of truth. 
Besides the stratified and massive rocks, there 
is still a third set, produced by the contact of 
