MEDIEVAL OR MESOZOIC TIMES. 175 



deposition, as we have seen, of clay and calcareous marls and fine 

 limestones, where previously sand and other shallow water de- 

 posits were taking place. At least in the Jurassic, water communi- 

 cation existed directly with the ocean, as is indicated bv the abun- 

 dant marine life that is preserved in these deposits. 



Now, the length of these periods must have been exceedingly 

 great, during which 3,^00 feetof sediment was deposited, especially 

 as a large part of them were of a character that never, so far as is 

 now known, accumulate rapidly. The Jurassic beds at least, which 

 are made up almost exclusively of soft clays, clayey calcareous 

 marls and intercalated beds of thin lithographic limestone, must 

 have been deposited with extreme slowness. Some authorities esti- 

 mate the increase of sediment at a f©ot to the century, and others 

 at only a few inches. Even at the larger figures, a foot to the cen- 

 tury, the time involved would be 180,000 years for the Jurassic alone. 

 Elsewhere, especially in Europe, the deposits of the Jurassic are 

 thicker even than this, and therefore the probabilities are that this 

 estimate is far too low. The preceding Triassic period was only 

 one-fourth shorter than the Jurassic. This would give for the two 

 periods combined 315,000 years. (See Dana's Manual, page 491.) 

 During all these long centuries, therefore, and far into the Cre- 

 taceous, as we shall presently see, the greater part, and perhaps the 

 whole of Nebraska existed as an extended land surface. The events 

 that occurred here during these periods can never be certainly 

 known. The imagination alone can, with the few data from the 

 vegetable and animal life of the time, fill out imperfectly this lost 

 page in our geological history. 



Vegetable Life. — Nebraska during these periods, owing to its 

 position, and because bounded on the west and southwest bv seas of 

 great extent, had a warm, temperate and moist climate. The pecu- 

 liar vegetable forms of the Mediaeval world must then have flour- 

 ished here. Among these, in the Triassic period, were huge tree 

 ferns, cycads and conifers, these last being principally araucarians, 

 a family which is now mainly confined to South America and Aus- 

 tralia. In the succeeding Jurassic, the vegetation was similar, and 

 the conditions on the whole still more favorable for a gigantic 

 growth. In this period were re-introduced the conditions favorable 

 to the production and preservation of a vegetation for the formation 

 of coal. To this period belong some of the coal fields of Scotland 

 and England, of India and China. Either to this or the preceding 



