





CHAPTER VI. 



PETROLOGY. 



Stratification. 



SUPPOSING that the student has made himself acquainted, by exami- 

 nation, with the more common and important rocks, as limestone, 

 sandstone, and clay, various kinds of slates, basaltic, porphyritic, and 

 granite rocks, we proceed to inquire in what manner they are arranged 

 in the earth. 



The best way of prosecuting this inquiry is to examine sections in 

 the field, open railway cuttings, quarries, and natural sections in the 

 cliffs on the seashore; comparing one area with another, so as to 

 class the phenomena and deduce general results. 



The stratification of the aqueous rocks is the basis and foundation 

 of all geological investigation the great problems and deductions of the 

 science are based upon a clear understanding of lamination, bedding, 

 and stratification. 



Arrangement of Rocks on the Surface. It might be very excus- 

 able before countries were cleared and cultivated, and before their 

 various mineral productions were employed and understood, to imagine 

 that the materials of the earth were heaped together in confusion ; but 

 at present such a notion will not stand the test of a moment's reflec- 

 tion. One district has chalk beneath the surface, another limestone, 

 a third coal, and a fourth granite, and these are never mixed or con- 

 founded together ; so that the most careless observer must conclude 

 that the different rocks are arranged after some definite and ascertain- 

 able method. These different rocks are not mere insulated patches 

 irregularly scattered through the country, but generally connected on 

 or beneath the surface in long ranges, which, as in the eastern half of 

 England, have their prevailing direction or strike from north-east to 

 south-west. Thus the chalk of the Yorkshire wolds is prolonged 

 through Lincolnshire, Norfolk, Suffolk, Bedfordshire, and Wiltshire, 

 into Dorsetshire, Sussex, and Kent ; the oolitic limestones range 

 through Lincolnshire, Northamptonshire, Gloucestershire, and Somer- 

 setshire ; and many other limestones, sandstones, and clays hold a 

 parallel direction. Hence it is that in proceeding from London 

 toward the south-west, west, or north-west of England, we cross so 

 great a variety of rocks and formations, and so many ranges of hills. 



On proceeding from London to North Wales, after passing low, 



