1098 principl.es of stratigrappiy 



floras preceding, it was a comparatively simple matter to divide 

 the earth's history into periods or eras characterized by these suc- 

 cessive changes in the ancient inhabitants of the earth. That there 

 was much apparent justification for this belief in the characters of 

 the faunas and floras found in the strata of the earth cannot be 

 questioned. Thus, trilobites are even to-day unknown from strata 

 later than Palaeozoic, nor until recently have strata containing am- 

 monites been recognized as older than the Mesozoic. That sudden 

 disappearances of whole organic assemblages, and the equally sud- 

 den appearance of others of a different type occur repeatedly are 

 matters of common observation ; but it was not always recognized 

 that such sudden changes are seldom universal in extent, though 

 generally traceable over wide areas. While abrupt changes in or- 

 ganic content of the strata have come to be generally regarded as 

 marking the lines between the greater divisions in the earth's history, 

 they are correlated with, and, in fact, dependent on, widespread 

 physical breaks in the continuity of the strata which compose the 

 earth's crust. Such physical breaks w^ere, indeed, taken as the planes 

 of division by the pioneers in stratigraphy, who considered strati- 

 graphic succession rather than geologic chronology. Thus, about 

 the middle of the i8th Century Lehman, a German miner (11) 

 proposed a threefold division of the rocks of the earth's crust into 

 (i) "Primitive" {Primitiv) or "Urgebirge," including all the igne- 

 ous and metamorphic rocks in which there was no sign of life and 

 which showed no evidence of having been derived from the ruins of 

 preexisting rocks, and, therefore, of chemical origin, antedating the 

 creation of life; (2) Secondary, comprising the fossiliferous strata, 

 and largely composed of mechanical deposits, produced after the 

 planet had become the habitation of animals and plants; and (3) 

 Alluvial deposits, due to local floods, and the deluge of Noah. 

 Fiichsel, a contemporary of Lehmann, recognized that certain groups 

 of strata belonged together and constituted a geologic formation. 

 He held that each formation represented an epoch in the history of 

 the earth, and thus he brought into consideration the time element 

 in the earth's history. Half a century later Werner introduced his 

 "transition formations" between the primitive and secondary rocks, 

 comprising a series of strata, first found in northern Germany, which 

 were intermediate in mineral character between the crystallines and 

 sedimentaries and partook in some degree of the characters of both. 

 This Uchergangsgehirge, or transition formation, consisted princi- 

 pally of clay slates, argillaceous sandstones or graywackes and cal- 

 careous beds, which, in the region studied by Werner, were highly 

 inclined and unconformably overlain by the horizontal Secondary 



