186 GEOLOGY. 



There are only two ways in which we can account for the sudden 

 appearance of this Dakota Group flora. One is, that it appeared 

 without any connection with antecedent types. It involves the 

 theory that by some fiat of Nature's God it was spontaneously and 

 suddenly produced. Few naturalists now accept this view. They 

 regard the vegetable world as a connected chain. They are there- 

 fore in this case driven to use the "scientific imagination" and sue- 

 gest the following explanation — the second explanation already 

 referred to. 



We have already seen that throughout the unnumbered centuries 

 of the latter Permian, Triassic, Jurassic, and Lower Cretaceous 

 Nebraska was an extended land surface, and covered by a colossal 

 vegetation of which no memorials have been preserved. The pecu- 

 liar animal life of the time flourished here as elsewhere. Now, it 

 is conceivable that during these long periods, whose length is 

 simply incalculable, vegetable l}fe underwent many changes, be- 

 cause the conditions of climate and environment changed many 

 times. The transformation therefore from primitive types was 

 gradual, all the intermediate links of which have been lost, and 

 the last factor, the flora of the Dakota Group alone preserved. 



Climate of the Dakota Groiip Epoch. — Many of the genera of 

 plants of the Dakota Group period are still flourishing- in Nebraska, 

 Kansas, and even in Minnesota. Professor Heer has also pub- 

 limed a memoir on a group of Cretaceous plants from Greenland, 

 whose facies resembles that of the Dakota Group. If the Green- 

 land fossil Cretaceous flora is cotemporaneous with that of the 

 Dakota Group, which seems probable, then a similar climate pre- 

 vailed from southern Kansas to near the Arctic circle. However 

 that may be, little difference can be detected between the fossil 

 vegetable forms in Kansas and Minnesota, and therefore a tem- 

 perate climate must have prevailed over this entire region, during 

 Dakota group times, not greatly different from the one that now 

 exists in Nebraska. The similarity of the vegetable forms that 

 then existed, to those that now obtain here, proves that the climate 

 of that epoch was much like ours to day. It was, judging from 

 the presence of some species, only slightly warmer than our pres- 

 ent climate. It was colder, however, than the preceding Triassic 

 and Jurassic, and also colder than the climate that subsequently 

 prevailed in the Niobrara Cretaceous, and during Eocene and Mio- 

 cene times. 



