452 MESOZOIC TIME. 



The dirt-bed of Portland, abounding in Mammalian remains, and 

 yet only five inches thick, shows strikingly what we ought to find in 

 the Coal formation, with its many scores of dirt-beds of far greater 

 thickness, if Mammals were then living. 



Climate. — The existence of Belemnites paxillosus and Ammonites 

 biplex (or closely-allied species) in the Arctic, in the Andes of South 

 America, and in Europe, indicates a remarkable uniformity of climate 

 over the globe in the Jurassic period. This has been made still more 

 striking, by the discoveiy, by Sir Edward Belcher, of the remains of 

 an Ichthyosaur on Exmouth Island, in 77° 16' N. and 96° W., 570 feet 

 above the sea ; and also by that of Captain Sherard Osborn, of two 

 bones of a species related to the Teleosaurs, on Bathurst Island, in 76° 

 22' N. and 104° W. No facts are yet ascertained, connected with the 

 geographical distribution of species, that sustain the idea of a diversity 

 of zones approaching in amount the present. The climate of the 

 Arctic regions in the Jurassic was probably at least warm-temperate. 



The existence of coral reefs in England, in the Oolitic era, consist- 

 ing of corals of the same grand groups with those of the existing 

 tropics, shows that the Coral-sea limit — marked off by the water- 

 isothermal of 68° F. as the average of the coldest winter month (see 

 page 41 and chart of the world) — extended north of part of the 

 British seas, or 30° (over 3,000 miles in distance) farther north than 

 its present most extra-tropical position just outside of the Bermudas. 

 The Gulf Stream was probably the cause of this long northward 

 stretch of tropical waters. The Oolytic isocryme of 68° F., accord- 

 ingly, would have had nearly the position of the present line of 44° 

 F., but with a little less northing and more leaning to the eastward. 

 The whole ocean was enough warmer to allow this ocean current to 

 bear the heat required for corals, as far north as northern England. 



4. Disturbances Closing the Jurassic Period. 



The igneous eruptions which made the trap ridges and trap dikes 

 that intersect the Connecticut River valley and other Triassic regions, 

 from Nova Scotia to South Carolina (described on page 417), may 

 have taken place at the close of the Jurassic period. All that the 

 facts definitely teach is that the outbreaks were subsequent, in part if 

 not wholly, to the deposition of the accompanying sandstone beds, and 

 anterior to the Cretaceous period. 



On the Pacific border, the evidences of disturbance, at this epoch, 

 are more positive ; and the results were of a grander character. The 

 Sierra Nevada, according to the facts brought out by Professor Whit- 

 ney, dates its existence from this time. As has been stated, Triassic and 



