60 THEORETICAL GEOLOGY.— INTRODUCTORY. 
fore the sand was brought over it, so that the thickness of the seam 
or bed of coal, varies from a few inches, to forty or fifty feet- 
But the principal coal fields of the United States are amongst 
the ridges of the Alleghany Mountains, and on the west side of 
them, especially in Pennsylvania, which appears to abound 
beyond every other part of our country in this mineral. The 
great anthracite formations of Pennsylvania are on the eastern 
side of the Susquehanna river, on the head waters of the Schuylkill 
and Lehigh, and also on both sides and beneath the bed of the 
Lackawanna, and of the Susquehanna, where it traverses the 
Wyoming valley. For the number and thickness of the beds, 
there is no parallel on the eastern continent. At Mauch-Chunk, 
on the Lehigh, the coal mine is an open quarry, exhibiting on its 
sides, precipices of solid coal, from twelve to thirty-five feet in 
height. Bituminous coal abounds in the western part of Pennsyl- 
vania, especially about Pittsburg/and is found in a number of other 
places in the Valley of the Mississippi* — in Ohio, Kentucky, 
Tennessee, Alabama, but the fact only of its existence has been 
ascertained, very little being known about the number, thickness 
or extent, of the beds. There is probably no country in the 
world that will compare with Pennsylvania in the number, 
extent, and riches of its coal-fields, a circumstance that is destined 
at a future day, to have an important influence upon the relative 
power and standing of that state amongst the members of this 
confederacy. 
The shales and sandstones accompanying coal, present impres- 
sions of vegetables, and other appearances, which haveinduced 
a general belief amongst geologists, that the beds of this mineral, 
in all parts of the World, have proceeded from the decomposition 
of vegetrbles, bearing little resemblance in form, or the manner 
of their growth, to such as now occupy the soil: of the same 
countries. 
THEORETICAL GEOLOGY.— INTRODUCTORY. 
35. From documents that have descended to us from other 
ages, we draw up an account of the most important and remark- 
able of the transactions of men, and call it a history of the world. 
This record of the events of other times is carefully studied, 
and the names of the great men of Greece and Italy, are as 
familiar to us as those of our intimate friends and acquaintance. 
We require minute accuracy in the account given of the causes 
by which the rise, advancement, and decay of empires has been 
produced and promoted, even when no lesson of wisdom or vir- 
tue can be drawn from the change, and the only object in view, 
is the gratification of an enlightened curiosity. 
May we not be permitted to devote a few pages to the history 
of the earth itself, of the revolutions it has undergone, and by 
which it has been brought into the condition in which we now 
