46 INTRODUCTION. 



tion had assumed the Kainozoic facies. That the " Dakota Beds " are 

 truly of Cretaceous age is further shown by the fact that part of the series 

 is marine in origin, and that in it are met with the remains of such 

 characteristic Cretaceous types of Invertebrates as Ammonites and 

 Belemnites. 



A somewhat more complicated case is that of the so-called " Laramie 

 Beds," or " Lignitic Series," of North America. These constitute a series 

 of beds, of some thousands of feet in thickness, which repose upon strata 

 with marine fossils of unquestionable Cretaceous type, and are unconform- 

 ably surmounted by strata containing equally unquestionable marine 

 Tertiary (Eocene) fossils. The " Laramie Beds " are admittedly of purely 

 inland origin, and were probably laid down in a vast brackish-water lake. 

 The Invertebrate fossils which they contain consist almost wholly of 

 brackish-water, fresh-water, and terrestrial Molluscs, and the characters 

 of these are such that they do not afford a decisive test of the age of the 

 series. The remaining fossils are mostly those of land-plants or of ter- 

 restrial Vertebrates, and the evidence as to age yielded by these respec- 

 tively is discrepant, the vegetable remains being of distinctly Tertiary type, 

 while the Vertebrates belong to the characteristic Mesozoic group of the 

 Dinosaurian Reptiles. When we consider, however, that the evidence 

 afforded by the " Dakota Beds " and the strata overlying these renders 

 it certain that the Tertiary vegetation had been introduced into America 

 at a time when the marine Invertebrates of the Cretaceous period still 

 existed in full force, we cannot attach much value to the plant-remains 

 of the " Laramie Group " as indicating a reference of these strata to the 

 Tertiary period. Adding to this consideration the presence of such 

 characteristic Cretaceous Vertebrates as the Dinosaurs, and the further 

 fact that the Laramie beds are surmounted unconformably by beds of 

 unequivocal Eocene age, it seems difficult to resist the conclusion that 

 these disputed strata are either truly of Cretaceous age, or that they 

 form a transitional group between the Cretaceous and the Tertiary 

 systems. 



A good example in the European area of the contradictory evidence 

 sometimes yielded by different classes of fossils as tests of age is afforded 

 by the " Pikermi Beds " of Attica. These beds contain an abundance of 

 the remains of terrestrial Mammals, which have generally been regarded 

 as of a distinctively Miocene type ; and on the strength of this the 

 " Pikermi Beds " have been referred to the Upper Miocene period. The 

 mammaliferous clays of Pikermi repose, however, upon strata containing 

 marine Molluscs of unquestionable Pliocene type. The explanation of 

 this appears to be, that the terrestrial Mammals of the Miocene period 

 had persisted unchanged in the south-east of Europe, while the Miocene 

 Mollusca of the Mediterranean had been replaced by the later Pliocene 

 forms. Since it is by the marine Invertebrates that the age of strata can 

 be most uniformly and most reliably judged, we are thus bound to refer 

 the " Pikermi Beds " to the Pliocene period. 



Contemporaneity and Homotaxis. 



The discovery of the use of fossils as tests of the age of the sedi- 

 mentary rocks, and the recognition of the fact that by their means 

 a chronological succession of the stratified deposits of any particular 

 region could be established, constituted great advances in geological 



