10 OBJECTS AND METHODS OP GEOLOGY. 
close of the last century, the single province of Mexico yielded a 
larger amount of silver, than was drawn from all the other mines 
of the world. The surface of the earth is constantly subject to 
slight modifications and changes. 
The object of one branch of this science, called Positive 
Geology by some authors, and Geognosy by others, is, to ascer- 
tain the different kinds of rock of which the exterior crust of the 
earth is made up ; their distribution and mutual relations or situa- 
tions with respect to each other ; the circumstances under which 
we find the valuable minerals they contain, and the alterations of 
texture and position which they undergo from age to age. 
These ends are accomplished by a careful observation of the 
common undulating surface of the soil and of the rocks it contains ; 
of the precipitous sides of hills and mountains ; of mines and 
other artificial excavations ; and of those points where the ele- 
ments and subterranean forces are exerting their greatest activity. 
3. A question arises whether the more remarkable and impor- 
tant of the peculiarities just noticed are coeval with the existence 
of the earth, or the result of changes it has undergone since its 
formation. Did the Creator in the beginning cover the plains of 
Arabia with sand, mingle in the soil of Indostan the elements of 
lasting fertility, and place in the mountains of Pennsylvania and 
Mexico the mineral treasures for which they are now explored ; 
or have all these and others, been collected into the situations in 
which we find them by causes that have operated in succeeding 
times ? 
Ample evidence will be furnished as we proceed, that the beds 
of earth and rock forming the outer crust of the globe, and the 
valuable minerals they embosom, are of different ages, and have 
been deposited in succession. It will be sufficient for the present 
to refer to the remains of shell fish and other marine animals that 
lie imbedded in them, often on the tops of high mountains, and in 
regions remote from the sea. 
Limestones composed of shells from the ridges of hills in 
Palestine. Those wonderful stones of the temple at Jerusalem, 
to which the disciples called the attention of Jejsus Christ when 
they drew from him a prediction of the impending ruin of that 
magnificence, abound in them ; and it is in rocks of this nature 
that the ancient Jewish sepulchres are excavated.* The pyramids 
of Egypt are both founded upon and built of a kind of oolite, full 
of small nummulites and other shells, once supposed to be petri- 
fied lentils and other seeds left by the workmen employed on 
those stupendous fabrics. In most of the countries of Europe, 
shells occur in greater or less abundance over extensive tracts ; the 
mountains of China, according to the Jesuits, are covered with 
them. Ramond observed them in the Pyrenees upon the summit 
*Jamieson in the Edinburg Encyclopedia. The shells are probably not as 
abundant in the rocks about Jerusalem as he represents them. See Still-man's 
Journal, Vols. 9 and 10. 
