n8 PHYSICAL GEOLOGY 



plain dotted with lakes, in which, especially along the margins, 

 the streams flow through deep valleys over falls and rapids. The 

 ruggedness of the southeastern margin of the peneplain, where 

 it borders the St. Lawrence, is due to the many valleys which 

 have been cut in it and which have given rise to the rough region 

 known as the Laurentian Mountains. The complicated and dis- 

 torted rocks of the region vary greatly in composition, but have, 

 nevertheless, been reduced to a common level, with the exception of 

 the residual domes and ridges (monadnocks) of the peneplain. The 

 interior peninsula of Labrador is so level that in an area of 200,000 

 square miles there is not a difference of general level of more than 

 300 or 400 feet. " The Canadian shield can be described as an ancient 

 peneplain which has undergone differential elevation; has been 

 denuded, and subsequently slightly incised around the uplifted 

 margin." (Wilson.) 



The Adirondacks were in part reduced to base level at the same 

 time, but in the eastern portion either the surface was not base- 

 leveled, or subsequent movements have raised it and given it vary* 

 ing altitudes. 



Rate of the Denudation of Continents. — The land surface of the 

 United States is being lowered at an average rate of about one inch 

 in 760 years, or of one foot in a little more than 9000 years. The 

 total amount carried to the sea each year from the United States is 

 approximately 270,000,000 tons of dissolved matter and 513,000,000 

 tons of suspended matter. " If this erosive action had been con- 

 centrated upon the Isthmus of Panama at the time of the American 

 occupation, it would have excavated the prism for an eighty-five 

 foot level canal in about seventy-three days." * 



How the Load of Streams is Measured. — Estimates such as the 

 above are obtained by measuring the amount of water discharged 

 by rivers, together with the minerals in solution and the insoluble 

 silt, and pebbles which are either carried in suspension or rolled along 

 the bottom. The volume of water discharged by a river is found by 

 multiplying the number of square feet in its cross section by the ve- 

 locity a second to obtain the discharge a second. The quantity of 

 silt is found by filtering samples of water from various portions of 

 the section at different times of the year, and the quantity of soluble 

 material is determined by evaporating samples of the water after 

 filtering. The difficulty in obtaining accurate results is due to the 

 1 U. S. Water-Supply Paper No. 234, p. 83. 



