CHAPTER II. 



Arrangement of the materials composing the earth's crust The existing con- 

 tinents chiefly composed of subaqueous deposits Distinction between sedi- 

 mentary and volcanic rocks Between primary, secondary, and tertiary 

 Origin of the primary Transition formations Difference between secondary 

 and teitiary strata Discovery of tertiary groups of successive periods Paris 

 basin London and Hampshire basins Tertiary strata of Bordeaux, Pied- 

 mont, Touraine, &c. Subapennine beds English crag More recent deposits 

 of Sicily, &c. 



GENERAL ARRANGEMENT OF THE MATERIALS COMPOSING 

 THE EARTH'S CRUST. 



WHEN we examine into the structure of the earth's crust (by 

 which we mean the small portion of the exterior of our planet 

 accessible to human observation), whether we pursue our investi- 

 gations by aid of mining operations, or by observing the sections 

 laid open in the sea cliffs, or in the deep ravines of mountainous 

 countries, we discover everywhere a series of mineral masses, 

 which are not thrown together in a confused heap, but arranged 

 with considerable order ; and even where their original position 

 has undergone great subsequent disturbance, there still remain 

 proofs of the order that once reigned. 



We have already observed, that if we drain a lake, we fre- 

 quently find'at the bottom a series of recent deposits disposed 

 with considerable regularity one above the other ; the upper- 

 most, perhaps, may be a stratum of peat, next below a more 

 compact variety of the same, still lower a bed of laminated 

 shell marl, alternating with peat, and then other beds of 

 marl, divided by layers of clay. Now if a second pit be sunk 

 through the same continuous lacustrine deposit, at some distance 

 from the first, we often meet with nearly the same series of 

 beds, yet with slight variations ; some, for example, of the 

 layers of sand, clay, or marl may be wanting, one or more of 



