236 abstracts: geology 



Alaska Range, and diabase dikes, probably of Eocene age, are not uncom- 

 mon. Along the northern margin of the range the oldest recognized 

 Paleozoics rest unconformably on complexly folded metamorphoric sedi- 

 ments (Birch Creek schists). Still farther north these metamorphic 

 schists are succeeded by a great thickness of Paleozoics only in part 

 subdivided and correlated with the formations described above. The 

 Mesozoic horizons of the Alaska Range have in part been recognized 

 in the Matanuska Valley of the eastern part of the McKinley region. 

 An extensive river system was developed in this region during Eocene 

 times, and in this the sediments classed as Kenai were deposited. The 

 northern part of the province was peneplaned in later Tertiary times 

 and it is possible also that the same period of erosion may have base- 

 leveled the southern part of the province. Uplift followed and the 

 modern stream valleys were in part developed along the old Tertiary 

 drainage system. A. H. B. 



GEOLOGY. — Denudation and erosion of the southern Appalachian 

 region and the Monongahela Basin. Leonidas Chalmers Glenn. 

 Professional Paper U. S. Geological Survey No. 72. Pp. 137, with 

 maps and views. 1911. 

 Two kinds of aqueous erosion are recognized, namely, (1) stream ero- 

 sion, confined to the water courses, and (2) general surface erosion, 

 over the whole drainage basin. Stream erosion is continuous but varies 

 greatly in intensity with the amount of water. General surface erosion 

 is due almost entirely to the rain. During heavy rainfall a grass sod 

 absorbs less water than a good humous cover in forested areas and the 

 run-off from grass-land is greater than from forest. The force of the 

 rain drop is lessened in forested areas, and as the fallen drops gather 

 to flow down the slopes they are checked by the leaves and litter or are 

 absorbed by the spongy mass of soft moss or turf. The protective 

 effect of forest cover was noted in many parts of the region. In the 

 course of ages there has come about, in all but a very small part of the 

 area, such an adjustment between the steepness of the forested slope 

 and the average rainfall and other erosive agencies that the wasting of 

 the slope is reduced to a minimum. Streams that flow from such for- 

 est-clad graded slopes rise more slowly during rains than similar streams 

 in cleared areas; they also continue longer in flood and subside more 

 slowly. Removal of the forest disturbs the natural equilibrium, exces- 

 sive erosion takes place, the soil is swept away, and the mountain slopes 

 are scarred. 



