THE CRUST OF THE EARTH. 



downwards from the superficial soil, which is the theatre of agri- 

 culture, through the alluvial and diluvial deposits upon which 

 it rests, and thence successively through the tertiary, secondary, 

 and primary stratified rocks to the igneous ones which form 

 the foundation of the terrestrial crust, may be formed from the 

 following tabular statement (page 51), which we have compiled 

 from the works of different geologists, and principally from that 

 of Sir H. de la Beche, representing the order of the strata in 

 "Western Europe, and which in its general character will be found 

 to correspond sufficiently with the condition of the terrestrial 

 crust in most parts of the world, especially as regards its major 

 divisions. 



The strata, which in this table are denominated primary or 

 palaeozoic, are by some geologists included under the general 

 denomination of secondary rocks. These must not be confounded 

 with the igneous rocks, which are often called primitive rocks, 

 and which are not stratified at all. The hypozoic or lowest beds 

 of stratified roojpis, together with the lower groups of the primary 

 or palaeozoic, are those which in the preceding paragraphs have 

 been denominated transition or metamorphic rocks. 



47. On contemplating the table, and on considering the 

 peculiar super- structure which it exhibits, combined with the 

 fact that each layer being a sedimentary deposit, must have been 

 the result of an interval of time of considerable duration, two 

 questions will naturally suggest themselves. 



What are the dimensions of these several strata, and what the 

 total thickness of the whole structure from the granite foundation 

 on which it rests to the vegetable soil upon the surface, and 

 what has been the probable interval of time required to pro- 

 duce each stratum. 



Although certain and definite answers cannot be given to 

 these questions, some degree of approximation may be made to 

 them. The thickness of the several strata and groups of strata is 

 subject to considerable local variation, nevertheless the indications 

 of the limits of these variations in certain parts of the earth, 

 which have been subject to geological survey of more or less 

 accuracy may be useful. 



Thus, the following are the estimated thicknesses of several 

 strata, proceeding upwards from the transition-system to the 

 surface in Great Britain. 



Gneiss system a few miles. 



Mica schist system from a few yards to a few miles. 

 Cambrian system from one to five miles. 

 Llandeilo formation J 200 feet. 



Caradoc formation . . . 2500 ,, 



50 



