282 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



but this disturbance seemingly had no marked effect upon the world's climate, though 

 there was a considerable retreat of the seas from the continents. 



Cretacic. — The emergence of the continents at the close of the Upper Jurassic gave 

 rise to extensive accumulations of fresh-water deposits, known in western Europe as the 

 Wealden, and in the Rocky Mountain area of North America as the Morrison. These 

 are now regarded as of Lower Cretacic (more accurately Comanchic) age. Along the 

 Atlantic border of the United States occur other continental deposits, known as the Poto- 

 mac formations, in the upper part of which the modern floras or Angiosperms make their 

 first appearance. Before the close of the Lower Cretacic this early hardwood forest had 

 spread to Alaska and Greenland, where elms, oaks, maples, and magnoHas occurred. 

 Knowlton concludes from this evidence that the chmate ' ' was certainly much milder than 

 at the present time" and "was at least what we would now call warm temperate" (1910a: 

 205-6). It was therefore a climate somewhat cooler than that of the Jurassic. On the 

 other hand, the Neocomian series of IGng Karl's Land has silicified wood, the trunks of 

 which, according to Nathorst, are at least 80 cm. in diameter and show 210 annular rings. 

 These rings are far better developed than in stems of the same age found in Europe, "which 

 indicates that the trees lived in a region where the difference between the seasons was 

 extremely pronounced" (1912: 339). 



At this time, in the temperate and tropical belts, the world had the greatest of all land 

 animals, the dinosaurs, reptiles attaining a length in North America of 75 feet or more 

 and in equatorial German East Africa of probably 125 feet. Their bones range to 50° 

 N. latitude, and the animals must have lived in a fairly warm and moist chmate. 



While the Lower Cretacic seas were prolific in life, the most characteristic shellfish 

 of southern Europe, the Mediterranean countries, and Mexico, were the limestone-making 

 rudistids, large ground-living foraminifers (Orbitolina), and reef corals. In northern 

 Europe and in the United States from southern Texas to Kansas, nothing of these warm- 

 water faunal elements is known. It is recognized that the north European seas had Arctic 

 connections by way of Scandinavia and Russia, and along the west coast of North America 

 are seen many other boreal migrants as far south as California and even Mexico. These 

 waters, however, were not cold. The same geographic distribution prevailed in the Upper 

 Cretacic of Europe. Tliis distribution was first noted in Texas by Ferdinand Roemer in 

 1852, and he further observed that "in each case the European deposit is approximately 10° 

 farther north than its American analogue," and concluded "that the differences between 

 the northern and southern facies were due to climate and that the climatic relations l^etween 

 the two sides of the Atlantic were about the same in Cretaceous time as they are now" 

 (Stanton, 1910: 67). Even though Roemer's conclusion as to climatic zones was founded 

 on erroneous stratigraphic correlations, still his theory has long been looked upon favorably, 

 but in 1908 Gothan showed that the fossil woods of the late Upper Cretacic of central 

 Germany have distinct annual rings, while those of Egypt do not have a trace of them. 

 The late Cretacic woods of Spitzbergen also have decided growth rings. Berry (1912) states 

 that the climate of Upper Cretacic time was far more uniform than now and that there was 

 an increase of warmth southward, Alabama having then a climate that was subtropical 

 or even tropical. On the other hand, the early Upper Cretacic or Cenomanian flora of 

 Atane in western Greenland, according to Nathorst, "is particularly rich in the leaves of 

 Dicotyledonous trees, among which are found those of planes, tulip trees, and bread 

 fruits, the last mentioned closely resembling those of the bread-fruit tree (Artocarpus 

 incisa) of the islands of the southern seas" (1912: 340). 



In Middle Cretacic times the oceans began again to spread over the continents and this 

 transgression of the seas was one of the greatest of the geologic past. It is interesting to 

 note that even though there was great opportunity for expansive evolution, but few new 

 marine stocks appeared here, and it was rather a time of death to many characteristic 



