102 



CHRONOLOGICAL ARRANGEMENT 



[Oh. IX 



thinning out, No. 2 and No. 5 are absent at one extremity of the sec- 

 tion, and No. 4 at the other. 



In the annexed diagram, fig. 105, a real section of the geological 

 formations in the neighborhood of Bristol and the Mendip Hills, is pre- 

 sented to the reader as laid down on a true scale by Professor Eamsay, 

 where the newer groups 1, 2, 3, 4 rest unconformably on the formations 



Fig. 105. 

 Dundry Hill. 



Section South of Bristol. A. C. Kana&sy, 



Length of section 4 miles. a, 5. Level of the sea. 



1 . Inferior oolite. 5. Coal measure. 



2. Lias. 6. Carboniferous limestone. 



3. New red sandstone. 7. Old red sandstone. 



4. Magnesian conglomerate. 



5 and 6. Here at the southern end of the line of section we meet with 

 the beds No. 3 (the New Red Sandstone) resting immediately on No. 6, 

 while farther north, as at Dundry Hill, we behold six groups superim- 

 posed one upon the other, comprising all the strata from the inferior 

 oolite to the coal and carboniferous limestone. The limited extension of 

 the groups 1 and 2 is owing to denudation, as these formations end ab- 

 ruptly, and have left outlying patches to attest the fact of their having 

 originally covered a much Avider area. 



In many instances, however, the entire absence of one or more forma- 

 tions of intervening periods between two groups, such as 3 and 5 in the 

 same section, arises, not from the destruction of what once existed, but 

 because no strata of an intermediate age were ever deposited on the in- 

 ferior rock. They were not formed at that place, either because the 

 region was diy land during the interval, or because it was part of a sea 

 or lake to which no sediment was carried. 



In order, therefore, to establish a chronological succession of fossilifer- 

 ous groups, a geologist must begin with a single section, in which sev- 

 eral sets of strata lie one upon the other. He must then trace these 

 formations, by attention to their mineral character and fossils, continu- 

 ously, as far as possible, from the starting point. As often as he meets 

 with new groups, he must ascertain by superposition their age relatively 

 to those first examined, and thus learn how to intercalate them in a tab- 

 ular arrangement of the whole. 



By this means the German, French, and English geologists have de- 

 termined the succession of strata throughout a great part of Europe, and 

 have adopted pretty generally the following groups, almost all of which 

 have their representatives in the British Islands. 



