100 MOUNTAINS AND THEIR ORIGIN. 
the relative position of the stratified and unstra- 
tified rocks gives us the key to their c/omparative 
age. To explain this I must enter into some de- 
tails respecting the arrangement of stratified de- 
posits on mountain-slopes and in mountain-chains, 
taking merely theoretical cases, however, to illus- 
trate phenomena which we shall meet with re- 
peatedly in actual facts, when studying special 
geological formations. 
We have, for in- 
stance, in Figure 1, a 
central granite moun- 
tain, with a succession 
of stratified beds slop- 
ing against its sides, 
while at its base are deposited a number of hori- 
zontal beds which have evidently never been dis- 
turbed from the position in which they were orig- 
inally accumulated. The reader will at once 
perceive the method by which the geologist de- 
cides upon the age of such a mountain. He 
finds the strata upon its slopes in regular super- 
position, the uppermost belonging, we will sup- 
pose, to the Triassic period ; at its base he finds 
undisturbed horizontal deposits, also in regular 
superposition, belonging to the Jurassic and Cre- 
taceous periods. Therefore, he argues, this 
mountain must have been uplifted after the Tri- 
assic and all preceding deposits were formed, 
Fig. 1. 
