LESSONS IN GEOLOGY. 357 



During the Algonkian era, the Lauren tian tableland was raised 

 above the sea level, as a permanent feature of the continent. 

 During the Silurian age, the Green Mountains were formed; and 

 the paloeozoic era closed with the elevation of the Appalachians, 

 extending the land surface of the continent westward to the region 

 of the Missouri, and southward to the Tennessee. The elevation 

 was doubtless slow, and possibly there were oscillations, but this 

 area seems never to have been below the sea level since the car- 

 boniferous age. 



The heat and pressure attending the elevation of the Appal- 

 achians, changed sediments to metamorphic rocks, as clays to 

 slates, sands to quartzite, limestones to marbles, peat beds to 

 anthracite coal, etc., the forces, the elevation and the metamor- 

 phism diminishing from the east toward the west. It is interest- 

 ing to note the similarity of events in North America and in 

 Europe ; the practical identity of the Algonkian rocks, the occur- 

 rence of evidences of a lost period, the similarity of the Silurian 

 and Devonian rocks, the identity of the millstone grit, the marshy 

 areas and other conditions for the formation of coal, with sim- 

 ilar forms of life, from beginning to end, including multitudes of 

 identical species ; and the eras of crumpling strata and mountain 

 formation seem to have been coincident. 



