100 
HISTORICAL SKETCH. 
to the sequence in age of the strata in which they were 
found. Such an arrangement, however, is only applicable to the 
fossils of a particular region, owing to the great difficulties 
in accurately determining the correspondence in age of forma- 
tions occurring in distant parts of the earth's surface ; hence 
a large and varied palaeontological collection, such as that of 
the British Museum, is best arranged in the main upon a 
systematic or zoological and botanical basis. A limited series 
sliowing the more characteristic rock-formations with their 
included fossil remains, of the British Isles, placed in chrono- 
logical sequence, is arranged in one of the galleries of the 
Geological Department. 
