SEQUENCE OF STRATA. II 



nature long ago attracted the attention of William Smith ; 

 and as early as 1822, the Rev. W. D. Conybeare remarked — 



"If we suppose an intelligent traveller taking his departure from 

 our metropolis, to make from that point several successive journeys 

 to various parts of the island, for instance to South Wales, or to 

 North Wales, or to Cumberland, or to Northumberland, he cannot 

 fail to notice (if he pays any attention to the physical geography of 

 the country through which he passes) that before he arrives at the 

 district in which coal is found, he will first pass a tract of clay and 

 sand [Eocene, etc.] ; then another of chalk ; that he will next 

 observe numerous quarries of the calcareous freestone [Oolites] 

 employed in architecture ; that he will afterwards pass a broad 

 zone of red marly sand ; and beyond this will find himself in the 

 midst of coal mines and iron furnaces. This order he will find 

 to be invariably the same, whichever of the routes above indicated 

 he pursues ; and if he proceeds further, he will perceive that near 

 the limits of the coal-fields he will generally observe hills of the 

 same kind of compact limestone [Carboniferous Limestone], 

 affording grey and dark marbles, and abounding in mines of lead 

 and zinc ; and at a yet greater distance, mountainous tracts in 

 which roofing slate abounds [Silurian and Cambrian], and the 

 mines are yet more valuable ; and lastly, he will often find, sur- 

 rounded by these slaty tracts [Devonian, etc.], central groups of 

 granitic rocks." ^ 



It is this regular order which enables us to form our tables 

 of strata, showing the oldest known rocks to be the Archaean 

 and Cambrian, and the newest the Alluvial deposits of our 

 rivers and the Beaches along our coasts. 



This order is, however, nowhere absolutely complete. We 

 find, as a rule, that the older the rocks, the more wild, rugged, 

 and mountainous is the nature of the ground they occupy ; 

 for these rocks have often been elevated to form land for long 

 periods, while around them were forming newer deposits, 

 which indeed were made up to a large extent from their 

 destruction. And throughout all time, while deposits have 

 been forming very much as they do now in limited areas, 

 these areas have many of them changed again and again from 

 dry land to water, and vice versd. The deposits have been 

 upheaved, and partly worn away or denuded ; other strata 

 have afterwards been spread over their worn surfaces ; and 

 thus, although there is regularity, the series is here and there 

 marked by the local absence of some of its members. Where 

 denudation has taken place before the newer strata were laid 

 down, the deposits are said to be iincoiiforniable^ and this 



1 Outlines of the Geology of England and Wales (1822), p. ii. 



