Ch.IL] TERTIARY STRATA OF SUCCESSIVE PERIODS. 
17 
French naturalists brought to bear upon their geological re- 
searches so much skill and proficiency in comparative anatomy 
and conchology, as to place in a strong light the importance of 
the study of organic remains, and the comparatively subordinate 
interest attached to the mere investigation of the structure and 
mineral ingredients of rocks. 
A variety of tertiary formations were soon afterwards found 
in other parts of Europe, as in the south-east of England, in 
Italy, Austria, and different parts of France, especially in the 
basins of the Loire and Gironde, all strongly contrasted to 
the secondary rocks. As in the latter class many different 
divisions had been observed to preserve the same mineral cha- 
racters and organic remains over wide areas, it was natural that 
an attempt should first be made to trace the different subdivi- 
sions of the Parisian tertiary strata throughout Europe, for 
some of these were not inferior in thickness to several of the 
secondary formations that had a wide range. 
But in this case the analogy, however probable, was not 
found to hold good, and the error, though almost unavoidable, 
retarded seriously the progress of geology. For as often as a 
new tertiary group was discovered, as that of Italy, for exam- 
ple, an attempt was invariably made, in the first instance, to 
discover in what characters it agreed with some one or more 
subordinate members of the Parisian type. Every fancied point 
of correspondence was magnified into undue importance, and 
such trifling circumstances, as the colour of a bed of sand 
or clay, were dwelt upon as proofs of identification, while the 
difference in the mineral character and organic contents of the 
group from the whole Parisian series was slurred over and 
thrown into the shade. 
By the influence of this illusion, the succession and chrono- 
logical relations of different tertiary groups were kept out of 
sight. The difficulty of clearly discerning these, arose from 
the frequent isolation of the position of the tertiary forma- 
tions before described, since, a proportion as the areas occupied 
by them are limited, it is rare to discover a place where one 
Vol. III. c 
