103 



to the south tropical zone, arising from its being then above the sea 

 level. It will, therefore, be observed, that although the sedi- 

 mentary series of rocks are commonly described as if they were 

 regularly built on each other in the following order, Cambrian, 

 Silurian, Coal formation, Lias, Oolite and Chalk, with a series 

 of beds called Tertiary, they are never found so complete in 

 nature. It is true that the order of the beds is never found 

 inverted, yet a great number are always absent, and their re- 

 spective development varies considerably in different localities. 

 England contains almost all the series from the south frigid to 

 the north temperate, but not piled on one another, as described 

 in the usual geological sections, but overlapping at the edges at 

 different extremities of each other, thus showing that although 

 some parts were constantly under the sea receiving new deposits, 

 they were not always the same, but alternately changing accord- 

 ing to circumstances. By reading some geological works it may 

 be supposed that the earth was once actually covered by all the 

 variety of beds, like the concentric coats of an onion ; but such 

 an idea is very erroneous, because it is abundantly evident that 

 the depositions were extremely local. 



In order to give a more general idea, and comprehend the re- 

 lations of superposition of the sedimentary rocks, as actually 

 observed in the different zones from south to north, and to show 

 that although the order is never inverted, the beds are often 

 wanting, and seldom seen in their complete numerical order, 

 we shall enumerate the series in the following manner : — 



Deposits. Contents. 



1. South frigid Trilobite, Orthocera- 



zone. tite, &c. 



2. South tern- Terebratula, Producta, 



perate. Encrinite, Spirifera, 



&c. 



3. South tropic. Ammonites, Belemnite, 



Gryphite, &c. 



4. North tropic. Echinus, Fusus, Pec- 



ten, &c. 

 1. Commencing our examination south in Patagonia, we find, 

 by a transverse section from Rio Santa Cruz to the base of the 

 Cordilleras, and another on the Rio Negro, that the whole sedi- 



Geological names. 

 Cambrian and Si- 

 lurian. 

 The great coal for- 

 mation. 



Oolite. 



Chalk and clay. 



