NO. 23.] CENTRAL CONNECTICUT IN THE GEOLOGIC PAST. II 



ing it to the sea. Layer after layer is buried beneath ever younger 

 layers, and, as sedimentary strata, with their record told by 

 fossils, the whole in after ages is reconverted into rock. 



But it follows that the uplands are being degraded always 

 toward the level of the sea; the marginal seas and basins are 

 likewise being filled. But in the lofty mountains the forces 

 which destroy the rocks work with greatest power ; the cliffs are 

 broken down to talus and the slopes subside more slowly into 

 soil-mantled hills. The hills in the course of ages flatten down, 

 so that the ultimate landscape has valleys which are broad and 

 ill-defined, separated by low and gently sloping hills, the whole a 

 surface of erosion which is known as a peneplain. Thus after 

 crustal uplift the landscape passes through an erosion cycle 

 from young and rugged mountains, to maturity marked by gentler 

 mountain slopes, thence to prolonged old age. The work of 

 erosion is then insignificant save for the ceaseless fretting of the 

 ocean on its shores. 



The length of the erosion cycle is, however, dependent upon 

 the durability of the rocks. Shales are soft and limestones 

 soluble. These will melt away in a fraction of the time needed ) 

 to destroy equal volumes of quartzite or granite-gneiss. An 

 erosion cycle in such soft rocks as the shales of central Connecti- 

 cut will in consequence pass into old age, while the erosion cycle 

 begun at the same time on the harder masses of crystalline rocks 

 which exist to the east and west is still in the stage of youth. 

 Erosion cycles of different beginnings, different lengths, and 

 different degrees of progress toward completion will therefore be 

 coexistent. 



But what are the causes which rejuvenate erosion and con- 

 tinue geologic change? The crust, through the action of vertical 

 or horizontal forces, periodically becomes broadly warped and 

 bowed; or it breaks into blocks, or is folded into mountain 

 ranges. Molten masses may invade the crust from the depths 

 below or pour out upon its surface. Thus the renewal of erosion 

 is dependent upon diastrophism and vulcanism, under which 

 terms are included all processes emanating from the inner earth. 



But, if the uplifts have been so great that erosion finally cuts 

 down to levels which were once miles below the surface, the 

 rocks then exposed are found to be contorted and mashed, crystal- 



