278 GEOLOGY. 



With such a basin arrangement of strata as we have 

 here and at Paris, you can see how artesian wells can be 

 successful in both of these localities, as noticed in Part 

 I., 120, to which I refer you for the explanation of their 

 operation. 



388. Tertiary Continent-making. In the Tertiary age 

 a great work was done in the building up of the conti- 

 nents. But it was not so much by the laying down of 

 deposits, though these were vast in thickness and over 

 extensive areas, as it was by elevating strata already 

 made, and crumpling them up in various quarters into 

 mountains. The work of this period is thus summed up 

 by Dana. " There was, 1, the finishing of the rocky sub- 

 stratum of the continents ; 2, the expansion of the conti- 

 nental areas to their full limits, or their permanent recov- 

 ery from the waters of the ocean ; 3, the elevation of 

 many of the great mountains of the globe, or consid- 

 erable portions of them, through a large part of their 

 height." He says, also, that " the mass of the earth 

 above the ocean's level was increased two or three fold 

 between the beginning and end of the Tertiary period." 

 In this country there was a mere fringe added along its 

 eastern and southern edge, but in the west, beyond the 

 Mississippi, there was an addition of vast extent. The 

 North American continent, with its triangular shape, had 

 now its great triangular skeleton of mountain ranges, 

 as Agassiz calls it, completed, shutting in that immense 

 area, the Mississippi Valley. The Laurentian Hills on the 

 north, making the base of the triangle, came into being 

 in that first long, dark age of the world, the Azoic age ; 

 the range of Alleghanies, making the eastern limb, were 

 raised probably at the close of the Carboniferous period ; 

 and the Rocky Mountains, the western limb, were thrust 

 up in the Tertiary age. The continent was now essen- 

 tially completed, the only great works yet to be done 

 being the filling up of a narrow gulf which extended to 

 where the city of St. Louis now is, from the material 



