41 



CHAPTER IV. 



ON STRATIFICATION, AND THE RELATIVE POSITION OF ROCKSo 



Strata and Geological Formations explained. — Various Appearances presented 

 by plane Strata. — Appearances presented by curved Strata, and Errors respect- 

 ing them. — Distinction between Strata Seams and Natural Fissures or Cleava- 

 ges. — On the conformable and unconformable Positions of stratified and unstra- 

 tified Rocks. — The Continuity of stratified Rocks broken by Valleys. — Longitu- 

 dinal Valleys. — Transverse Valleys. — Lateral Valleys. — Denudations. — On the 

 Elevation of Mountains and Mountain Chains. — On the Direction of Mountain 

 Chains in the new and old Continents. — On vertical Beds in Mountains. — On 

 the apparent Devastation in Alpine Districts. — On the Passages in the Alps call- 

 ed Cols; and Observations respecting their Formation. — Diflferent Ages of Moun- 

 tain Ranges. 



When we have ascertained what are the most common or pre-* 

 vailing rocks in a part of any country, and observed that any one 

 stratum or rock which attracts our attention is, in that part of the 

 country, invariably covered by a peculiar rock or stratum of a dif- 

 ferent kind, or invariably covers any particular stratum ; we hence 

 learn, that there is a certain order of superposition, and we natural- 

 ly feel desirous to know whether the same order is observable in ev- 

 ery country where similar rocks occur. Thus, in the vale of Thames 

 round London, there is, at the depth of a few feet under the sur- 

 face, a dark-coloured clay, called London Clay, much intermixed, 

 in the lower part, with beds of sand. If we bore through this clay, 

 we shall find its average thickness to be nearly 300 feet. When 

 we have pierced through this, we invariably come to chalk ;* and 

 were we to continue to bore in the chalk, after piercing through 

 many hundred feet of that rock, we should come to a stratum of 

 sand or sandstone, filled with green particles, and hence called 

 Green Sand. 



The observer, who had confined his researches to this part of the 

 country only, would form a very erroneous conclusion, were he to 

 infer that the outer crust of the globe was, invariably, composed of 

 London clay, chalk, and green sand. But, wherever similar beds 

 occur together, they lie over each other in the same order of super- 

 position. Thus, the London clay is never found under the chalk or 

 the green sand. 



But, it is not always necessary to bore through the upper beds to 

 ascertain this order ; for, the different strata scarcely ever occur in a 

 flat or horizontal position : they, generally, rise in a certain direction, 

 and come to the surface, as represented in Plate L fig. 1. Now, 



*^The lower clay is by some geologists denominated plastic clay. See Chap, 



6 



