Principles of Geology. . 5 
_fessed geologists, I thought it desirable to furnish them with a plain © 
distinct introduction to the science, in order to avoid obscurity and 
tedious repetition. 
Formerly, the materials near the surface of the earth were thought 
to be every where alike, just as agriculturists now speak of the veg- 
etable mould; and the internal parts were supposed to be a mere 
heap of minerals confusedly blended together ; a very little experi- 
mental investigation was sufficient to overthrow so groundless a no- 
tion. One district has beneath the surface, chalk; another, oolitic 
limestone ; a third, coal; a fourth, granité; and these are never 
mixed or confounded together; so that the most careless observer 
finds himself constrained to admit that not disorder, but method, ap- 
pears in the situation of different rocks. 
A person proceeds from London to North Wales. After passing 
low diluvial plains about London, he climbs, by a long slope, the 
chalk-hills of Oxfordshire and Berkshire 3 then crosses vales of clay 
and sandstone, ascends a range of oolitic limestone; traverses wide 
plains of blue and red mar]; arrives in districts where coal, iron, and 
limestone abound ; and finally sees Snowdon composed of slate. 
And if, in proceeding from London to the Cumberland lakes, he 
finds the same succession of low plains, chalk-hills, clay vales, oolitic 
limestone ranges, blue and red clays, coal, iron, and limestone tracts, 
succeeded by the slate rocks which compose the well-known summit 
of Skiddaw, will he not conclude that something beyond mere 
chance has brought together these rocks in such admirable harmony ? 
Will he not have reason to conjecture, that, in the interior of the 
earth, r egularity of arrangement must prevail ? 
© such a conclusion we are forcibly impelled by exploring the 
telative Position of rocks, as it is displayed in wells, quarries, and 
mines, the works of human industry, or laid bare in cliffs and ravines 
by the hand of nature. Here every one has seen the rocks formed 
n layers or tabular masses, placed one upon another, like the leaves 
of a book, These layers are called strata. 
., © 8ea-coast of Yorkshire affords excellent opportunities of exam- 
ining into this matter; for there cliffs of great altitude, in prominent 
and accessible situations, are composed of several distinct layers of 
TOCK, which are piled one upon another in a regular order, preserve 
a definite thickness, and appear under the same circumstances in many 
distant Places. But though one tract of country exceeds another in 
°Pportunities of this nature, yet the principle of stratification among 
