ARRANGEMENT OF STRATA. 115 



It is common to judge of the age of igneous rocks by their composition ; but 

 it is an unsafe criterion. Some use may be made of it hereafter in settling cases 

 that remain doubtful, but not until the age of the greater part of such rocks 

 has been ascertained on evidence of a better kind than this. In the case of 

 metamorphic rocks of different ages it may prove of value when their dis- 

 tinctive peculiarities are thoroughly known. 



125. (3.) Fossils. — This criterion for determining the chronological 

 order of strata takes direct hold upon time, and, therefore, is sure 

 and sufficient. The life of the globe has changed with the progress of time. 

 Each epoch has had its peadiar species. Moreover, the succession of 

 life has followed a grand law of progress, involving under a single 

 system a closer and closer approximation in the species, as time 

 moved on, to those which now exist. It follows, therefore, that 



Identity of species of fossils proves approximately identity of age. 



The change has not consisted in a change of species alone, but 

 also in certain grand modifications of type or structure. Thus, for 

 fishes there are both ancient and modern types ; and in most 

 of the classes there are great groups which belong to the past and 

 mark the progressing ages, — as Trilobites mark the Palseozoic, 

 Sigillarise the Coal period, Ammonites and flying reptiles the 

 Eeptilian age. Hence the canon may have the broader form, — 



Identity of type or family in organic forms proves identity of age. 



The canon is a universal one. Had we a table containing a list 

 of the complete series of rocks, and of the families, genera, and 

 species of fossils which each contains, it would be a key for the 

 rocks of the whole world, — South and North America as well as the 

 Orient; and by comparing the fossils of any rock under investi- 

 gation with this key, the age would be approximately ascertained. 

 This is the method now pursued in studying the geology of the 

 globe. The key is, in fact, already so complete that it is constantly 

 appealed to by the geological observer. The list which is made 

 for the Silurian and Devonian rocks in New York State is used 

 for identifying the strata of the Mississippi basin ; and that 

 which has been prepared in Europe is constantly employed to 

 make out the true synchronism between the rocks of the two 

 continents. 



By such comparison of fossils it was discovered that the Chalk 

 formation exists in the United States, although there is no chalk 

 on the continent ; that the Coal formation of North America and 

 that of Newcastle, England, belong to the same geological age ; 

 and so in numberless other cases of identity between the strata 

 of distant continents. 



The commencement in the preparation of such a key was attended 



