84 Outlines of Geology' 



upon chalk, which is succeeded by calcareous freestone and 

 a species of argillaceous limestone ; then comes a zone of 

 red sandy marl, and mines of coal and iron succeed, surrounded 

 by limestone, and followed by slate and granite. 

 • If, instead of proceeding westward, his route lie to the north, 

 the same succession of strata, the same rocks and mineral masses, 

 exactly in the same general order, will present themselves, and 

 various opportunities will occur to lead him to another important 

 fact respecting these stratified beds, which is, that they are evi- 

 dently disposed at an angle, and neither vertical nor parallel to 

 the horizon ; so that their edges may be observed successively to 

 emerge from under each other. The term outcrop or hasset of the 

 strata is applied to the successive zones thus formed, and their 

 inclination or dip is found subject to much variation. 



It is impossible to contemplate this arrangement without dis- 

 cerning the important secondary purposes to which it so effi- 

 ciently contributes ; for those strata which at one place are at im- 

 penetrable depths, are at another so brought to the surface, as to 

 enable us to examine and obtain them and their contents : nume- 

 rous useful products and mineral treasures are thus collected at, 

 or comparatively near, the surface, with which we otherwise must 

 have remained wholly unacquainted, or which could only have 

 been procured by almost insurmountable labour and expense. 

 Without such obliquity of stratification, there would have been no 

 succession of soils. *' In the whole machinery of springs and 

 rivers, and the apparatus that is kept in action for their duration 

 through the instrumentality of a system of curiously constructed 

 hills and valleys, receiving their supply occasionally from the 

 rains of heaven, but dispensing them 'perpetually in thousands of 

 never-failing fountains," we see other important consequences of 

 this arrangement of rocks. Waters, collected upon the hills and 

 on high ground, filter and flow through the softer and perme- 

 able layers, producing springs in the valleys, and feeding streams 

 and rivers, instead of accumulating in marshes and swamps as 

 they would do, were the strata horizontal, or the surface plane. 



Among the strata or formations of the vicinity of London, 



