570 HISTORICAL GEOLOGY 



or ran in search of food or water and left their tracks. It has been 

 shown, however, that a number of hours at least must elapse under 

 known conditions, before mud can dry sufficiently to form sun cracks 

 and to retain the footprints of animals. If these conclusions are 

 correct, we must believe that the deposits of the Connecticut valley 

 and New Jersey were laid down in river valleys analogous to the Great 

 Valley of California and other structural basins. In the shallow 

 lakes which occurred, such for example as Tulare Lake in California, 

 the depth of the water was greatly reduced by evaporation during 

 longer or shorter periods, and the bottom of the shallower portions 

 of the seas was exposed for several days or weeks at a time. 



There is abundant proof, however, that in certain regions the rain- 

 fall during the Triassic was plentiful, due probably, as to-day, to the 

 presence of mountain ranges which caused abundant precipitation 

 on one side and deserts on the other. In Virginia, for example, beds 

 of coal aggregating 30 to 40 feet in thickness indicate long-continued 

 swamp conditions. Horsetails four to five inches in diameter, ferns of 

 large size, some of them tree ferns, prove that the climate was favorable 

 for luxuriant growth. The petrified trees of Arizona, some of which 

 were eight feet in diameter and more than 120 feet high, do not indicate 

 aridity, nor, for that matter, do they prove a moister climate than that 

 of Arizona to-day, in which the great pines south of Flagstaff flourish. 

 The complete or nearly complete absence of rings in the tree trunks 

 indicates that there were no, or but slight, seasonal changes, due to 

 alternations of heat and cold or wet and dry periods. 



Jurassic. — The presence of luxuriant ferns, many of them tree ferns, 

 horsetails of large size, and conifers, the descendants of which live 

 in warm regions, all point to a moist, warm, subtropical climate during 

 the greater part of the Jurassic ; although arid regions unquestion- 

 ably existed. The animals also indicate a warmer climate in the 

 northern regions than at present. Saurians and ammonites lived 

 within the Arctic circle, and corals 3000 miles farther north than now. 

 The presence in the late Jurassic of rings in the tree trunks of northern 

 species shows that slight seasonal changes occurred. 



Cretaceous. — The climate of the Lower and Upper Cretaceous 

 seems to have been milder than at present, even that of Greenland 

 being temperate or warm temperate. The distribution of marine 

 fossils indicates the existence of climatic zones according to latitude, 

 but the vegetation does not show this so clearly; for example, oaks, 

 maples, and magnolias grew in Greenland and nearly as far north as 



