124 
ELEMENTS OF GEOLOGY. 
Superposition. —The first and principal test of the age of 
one aqueous deposit, as compared to another, is relative po¬ 
sition. It has been already stated, that, where strata are 
horizontal, the bed which lies uppermost is the newest of the 
whole, and that which lies at the bottom the most ancient. 
So, of a series of sedimentary formations, they are like vol¬ 
umes of history, in which each writer has recorded the an¬ 
nals of his own times, and then laid down the book, with the 
last written page uppermost, upon the volume in which the 
events of the era immediately preceding were commemo¬ 
rated. In this manner a lofty pile of chronicles is at length 
accumulated; and they are so arranged as to indicate, by 
their position alone, the order in which the events recorded 
in them have occurred. 
In regard to the crust of the earth, however, there are 
some regions where, as the student has already been inform¬ 
ed, the beds have been disturbed, and sometimes extensively 
thrown over and turned upside down. (See pp. 73, 87.) But 
an experienced geologist can rarely be deceived by these 
exceptional cases. When he finds that the strata are frac¬ 
tured, curved, inclined, or vertical, he knows that the origi¬ 
nal order of superposition must be doubtful, and he then en¬ 
deavors to find sections in some neighboring district where 
the strata are horizontal, or only slightly inclined. Here, 
the true order of sequence of the entire series of deposits 
being ascertained, a key is furnished for settling the chronol- 
ogy of those strata where the displacement is extreme. 
Mineral Character. —The same rocks may often be ob¬ 
served to retain for miles, or even hundreds of miles, the 
same mineral peculiarities, if we follow the planes of strati¬ 
fication, or trace the beds, if they be undisturbed, in a hori¬ 
zontal direction. But if we pursue them vertically, or in 
any direction transverse to the planes of stratification, this 
uniformity ceases almost immediately. In that case we can 
scarcely ever penetrate a stratified mass for a few hundred 
yards without beholding a succession of extremely dissimilar 
rocks, some of fine, others of coarse grain, some of mechan¬ 
ical, others of chemical origin; some calcareous, others argil¬ 
laceous, and others siliceous. These phenomena lead to the 
conclusion that rivers and currents have dispersed the same 
sediment over wide areas at one period, but at successive 
periods have been charged, in the same region, with very 
different kinds of matter. The first observers were so as¬ 
tonished at the vast spaces over which they were able to 
follow the same homogeneous rocks in a horizontal direc- 
tion, that they came hastily to-the opinion, that the whole 
