C3. X] OF TERTIAEY FOEMATIONS. Ill 



(tlie Valley of the Loire), to overlie the Parisian formation, and in an- 

 other (in Piedmont) to underlie the Subapennine beds. The first exam- 

 ple of these was pointed out in 1829 by M. Desnoyers, who ascertained 

 that the sand and marl of marine origin called Faluns, near Tours, in 

 the basin of the Loire, full of sea-shells and corals, rested upon a lacus- 

 trine formation, which constitutes the uppermost subdivision of the 

 Parisian group, extending continuously throughout a great table-land 

 intervening; between the basin of the Seine and that of the Loire. The 

 other example occurs in Italy, where strata, containing many fossils sim- 

 ilar to those of Bourdeaux, were observed by Bonelli and others in the 

 emarons of Turin, subjacent to strata belonging to the Subapennine 

 group of Brocchi. 



Without pretending to give a complete sketch of the progress of dis- 

 covery, I may refer to the facts above enumerated, as illustrating the 

 course usually pursued by geologists when they attempt to found new 

 chronological divisions. The method bears some analogy to that pur- 

 sued by the naturalist in the construction of genera, when he selects a 

 typical species, and then classes as congeners all other species of animals 

 and plants which agree with this standard within certain limits. The 

 genera A and C having been founded on these principles, a new species 

 is afterwards met with, departing widely both from A and C, but in 

 many respects of an intermediate character. For this new type it be- 

 comes necessary to institute the new genus B, in which are included all 

 species afterwards brought to light, Avhich agree more nearly with B than 

 with the types of A or C. In like manner a new formation is met with 

 in geology, and the characters of its fossil fauna and flora investigated. 

 From that moment it is considered as a record of a certain period of the 

 earth's history, and a standard to which other deposits may be com- 

 pared. If any are found containing the same or nearly the same organic 

 remains, and occupying the same relative position, they are regarded in 

 the light of contemporary annals. All such monuments are said to re- 

 late to one period, during which certain events occurred, such as the 

 formation of particular rocks by aqueous or volcanic agency, or the con- 

 tinued existence and fossilization of certain tribes of animals and plants. 

 When several of these periods have had their true places assigned to 

 them in a chronological series, others are discovered which it becomes 

 necessary to intercalate between those first known ; and the difiiculty of 

 assigning clear lines of separation must unavoidably increase in propor- 

 tion as chasms in the past history of the globe are filled up. 



Every zoologist and botanist is aware that it is a comparatively easy 

 task to establish genera in departments which have been enriched with 

 only a small number of species, and where there is as yet no tendency 

 in one set of characters to pass almost insensibly, by a multitude of con- 

 necting links, into another. They also know that the difficulty of classi- 

 fication augments, and that the artificial nature of their divisions becomes 

 more apparent, in proportion to the increased number of objects brought 

 to light. But in separating families and genera, they have no other al- 



