PRINCIPLES OF GEOLOGY. 5 



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 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, regularity of arrangement must prevail ? 



To such a conclusion we are forcibly impelled by exploring the re- 

 lative 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 ex- 

 amining 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 opportu- 

 nities of this nature, yet the principle of stratification among rocks is 

 confined to no country ; for whether in the new or the old world, in con- 

 tinents or in islands, it is so remarkable and so constant, that colliers sink 

 deep pits, and miners undertake expensive levels, in full confidence that 

 no exception to its generality will affect the success of their enterprises. 

 It is not a speculative truth, but a practical law of nature, and is, 

 probably, the fact of most extensive influence in the whole system of 

 geology. 



The Wernerian school of geology held it to be a universal law of 

 structure, and even Cuvier says, '" All rocks are stratified." But such 

 expressions are incorrect. How can the term strata be applied to basalt, 



