Principles oj 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, granite ; 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 diffei'ent 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; then crosses vales of clay 

 and sandstone, ascends a range of oolitic hmestone; traverses wide 

 plains of blue and red marl; 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 

 hmestone ranges, blue and red clays, coal, iron, and limestone tracts, 

 succeeded by the slate rocks which compose the well-known summit 

 of Skiddaw, vVill 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, regularity of arrangement must prevail ? 



To such a conclusion we are forcibly impelled by exploring the 

 relative 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 

 in layers or tabular masses, placed one upon another, like the leaves 

 of a book. These layers are called strata. 



The sea-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 

 rock, 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 

 opportunities of this nature, yet the principle of stratification among 



