486 



MESOZOIC TIME. 



it is not directly opposed to their existence ; and, further, it is highly 

 improbable that Mammals, the superior type of Vertebrates, should 

 have existed before Birds. In the Jurassic, the occurrence of species 

 is beyond question ; and some of them, if not all, had that striking 

 mark of inferiority, a long vertebrated tail, along with some other 

 peculiarities that allied them to Reptiles, and especially to the three- 

 toed Dinosaurs. In the Cretaceous era, the species were evidently 

 numerous ; and the most were of modern type. But among them 

 were kinds with teeth and biconcave vertebrae, which were probably 

 allied to the Jurassic birds. 



Mammals. — The class of Mammals began in the Triassic, according 

 to present knowledge, with species of the inferior tribe of Marsupials ; 

 and the same continued to be the prevailing kind through the rest of the 

 Mesozoic. It is questioned whether there may not have been among 

 them some species of Insectivores (the group to which the Mole and 

 Shrew belong) ; but no higher species of ordinary Mammals than 

 these have yet afforded even doubtful evidence of their existence. 

 The Mammals were evidently far inferior in size and numbers, and in 

 grade of life, to the Reptiles of the era. 



IV. Disturbances during Mesozoic Time. 



In American history, the displacements of the beds of the Triassic 

 or Triassico-Jurassic areas on the Atlantic Border, and the multitudes 

 of trap-dikes, which intersect these areas, indicate that their deposition 

 was followed by an epoch of disturbance. The facts, and the con- 

 clusions from them, are stated on page 417. The time was either in 

 some part of the Jurassic period, or at its close. The beds next in age 

 along the Atlantic Border, the Cretaceous, did not participate in the 

 upturning ; and thus it is known that the ejections of trap took place 

 anterior to the era they represent. The facts (1) that the trap-dikes 

 are mostly confined to the sandstone areas ; (2) that they consist of 

 the same kind of dolerytic rock throughout, and (3) that the areas 

 and the fractures are parallel to the preexisting Appalachian chain, 

 have been pointed to as evidence that all belong to one continental 

 mountain-making movement. 



On the Pacific border, at the close of the Jurassic, the lofty Sierra 

 Nevada was made (p. 452) ; for the Cretaceous beds of California rest 

 unconformably on the upturned Jurassic. Whether there were other 

 cotemporaneous elevations west of the Rocky Mountain summit is not 

 determined. In, and east of, the Wahsatch, the beds are all conform- 

 able from the base of the Paleozoic to the top of the Laramie group. 



The Cretaceous strata of North America are throughout conform- 

 able. The positions of the successive beds indicate some oscillations 



