CHAPTER V. 



THE FLORA OF THE EAKLY MESOZOIC. 



GREAT physical changes occurred at the close of the 

 Carboniferous age. The thick beds of sediment that had 

 been accumulating in long lines along the primitive con- 

 tinents had weighed down the earth's crust. Slow sub- 

 sidence had been proceeding from this cause in the coal- 

 formation period, and at its close vast wrinklings occurred, 

 only surpassed by those of the old Laurentian time. 

 Hence in the Appalachian region of America we haye the 

 Carboniferous beds thrown into abrupt folds, their shales 

 converted into hard slates, their sandstones into quartzite 

 and their coals into anthracite, and all this before the 

 deposition of the Triassic Eed Sandstones which consti- 

 tute the earliest deposit of the great succeeding Mesozoic 

 period. In like manner the coal-fields of Wales and 

 elsewhere in western Europe have suffered similar treat- 

 ment, and apparently at the same time. 



This folding is, however, on both sides of the Atlantic 

 limited to a band on the margin of the continents, and to 

 certain interior lines of pressure, while in the middle, as 

 in Ohio and Illinois in America, and in the great interior 

 plains of Europe, the coal-beds are undisturbed and un- 

 altered. In connection with this we have an entire 

 change in the physical character of the deposits, a great 

 elevation of the borders of the continents, and probably 

 a considerable deepening of the seas, leading to the estab- 

 lishment of general geographical conditions which still 

 remain, though they have been temporarily modified by 

 subsequent subsidences and re-elevations. 



