62 STRATA SEEN IN SECTIONS. 



gravelly plains in the drainage of the Thames we climb by a long 

 slope the chalk hills of Oxfordshire, cross vales of clay and sand- 

 stone, ascend a range of oolitic limestone, traverse wide plains of 

 blue and red marl, arrive in districts where coal, iron, and limestone 

 abound, and finally see Snowdon composed in great measure of igneous 

 rock, slates, and sandstones. And if, in proceeding from London to 

 the Cumberland Lakes, we find the same succession of gravelly plains, 

 chalk hills, clay vales, limestone ranges, blue and red clays, coal, iron, 

 and limestone tracts, succeeded by the slate rocks which compose the 

 well-known mountain of Skiddaw, we conclude that something beyond 

 mere chance has brought together these rocks with such perfect 

 sequence and order. May we not reasonably conjecture that also in 

 the interior of the earth regularity of structure must equally prevail ? 



Internal Arrangement of Rocks. This conjecture becomes cer- 

 tainty when we explore the relative position of rocks as displayed in 

 pits, quarries, railway cuttings, mines, and wells, or laid bare in cliffs 

 and ravines by the hand of nature. Here we see the rocks formed in 

 layers, strata, or tabular masses of various thickness, but always of 

 very great superficial or horizontal breadth or extent, and placed 

 parallel to or upon one another like the leaves of a book. These 

 layers are called strata. Along the edges and flanks of hills, in the 

 course of precipitous valleys, and by the margin of the sea, in the form 

 of cliffs, it is not difficult to recognise these facts or truths, it is 

 almost impossible to avoid perceiving them. 



Many parts of the English coast present what is termed a natural 

 section of the rocks, and accordingly whoever visits the shores of 

 Northumberland, Yorkshire, Kent, Hampshire, Dorsetshire, Cornwall, 

 South Wales, or Cumberland, may easily observe for himself the 

 stratification of most of the limestones, sandstones, clays, and slates of 

 the different geological formations exposed. For most of the cliffs 

 are composed of distinct layers of rock, which are placed upon or 

 succeed one another in regular order, preserve a definite thickness, and 

 appear under the same or similar circumstances in many distant 

 places. In the interior of the country the same conclusion is to be 

 drawn from examining precipitous hills and deep valleys ; and even 

 in the flattest country art supplies the means of investigation which 

 nature has denied. The wells, pits, quarries, and mines, which have 

 been constructed, all display the same general truth, and lead us to 

 conclude that the principles and laws of stratification among rocks 

 is confined to no particular country, but all over the world, in 

 continents or in islands, it is certain and constant, so much so, that 

 deep pits are sunk for coal, and miners undertake extensive levels, 

 in full confidence that no exception to the laws of stratification will 

 affect the result of their enterprises. It is not a speculative truth, 

 but a practical law of nature, having the most extensive influence in 

 the whole theory of geology. 



So many important facts respecting stratified rocks bear upon the 

 physical history of a county or district, that it is not easy to analyse 

 or realise them on paper in the exact order of their occurrence. But 



