THE JURASSIC PERIOD 225 



A progressive marine transgression continued through the 

 middle of the period and well toward its close, extending farther 

 and farther eastward, till much of the continent was submerged, 

 as shown by Fig. 134. This was one of the greatest marine trans- 

 gressions in the known geological history of Europe. As would be 

 expected, the strata of later Jurassic age contain much more lime- 

 stone than those of the earlier part of the period, because of the 

 more widespread clear water areas. During all this time the great 

 series of oolites were forming in England and the famous Solenhofen 

 lithographic limestone was being deposited in southern Germany. 



Just before the close of the period a considerable retrogression 

 of the sea set in, draining certain areas and leaving lakes or estu- 

 aries in certain other places. 



Other Continents. — Jurassic marine strata are known in 

 many places in Arctic lands, thus showing extensive sea waters 

 of that time there. 



A great marine transgression also affected Asia, so that exten- 

 sive areas of the continent became submerged, except mostly in 

 the central portion. Widespread Jurassic deposits are known in 

 Asia Minor, Siberia, India (especially the Himalayas), Persia, 

 Turkestan, and Japan. 



Jurassic rocks are also known in northern and eastern Africa, 

 western South America, Australia, and New Zealand. 



Climate 



In general the evidence from the character and distribution of 

 the organisms shows that the climate was comparatively mild. 

 Corals, for example, had a range several thousand miles farther 

 northward than they do today. A careful study of the migrations 

 of certain animals has, however, pretty well established the fact 

 that the Arctic Sea was notably colder than the Atlantic and 

 Pacific, but it is perhaps too much to say that the northern sea 

 was as cold as it is now. There was quite certainly some definition 

 of climatic zones, especially in later Jurassic time. 



Economic Products 



Considerable crude oil is obtained from Jurassic strata in south- 

 ern California. 



The great gold-bearing veins or lodes of the famous " Mother 



