May 25, 1900.] 



SCIENC:S. 



825 



The following papers were presented on the 

 regular program : 



Physiographic Development of the Black Hills: 



By Mr. N. H. Daeton. 



The principal period of uplift of the Black 

 Hills dome was in the earlier tertiary time. 

 During the progress of this uplift the larger 

 features of the present topography were devel- 

 oped. The main north and south divide lies 

 west of the apparent center of the uplift as the 

 dome now stands, the cause for which is not 

 yet ascertained. 



The area of deposition of the White River 

 deposits of the late Oligocene extended far up 

 the flanks and into the valleys of the Black 

 Hills apparently completely filling many of the 

 old depressions. Following White River depo- 

 sition the Black Hills were lifted both as a 

 whole and also with increased doming and the 

 new drainage was in part revived and in part 

 superimposed. In'the superimpositiou of drain- 

 age on the east side of the Hills there is evi- 

 dence of a general tilting to the northeast so 

 that portions of the revived pre-Oligocene val- 

 leys now appear to be robbed by their neighbors 

 to the north, the present channels ofl'setting at 

 intervals In that direction across former divides. 

 A period of early Pleistocene base-leveling is 

 recognized which had much to do with read- 

 justing the drainage on the east side of the 

 Hills. It cut deeply into the White River 

 deposits planing them off over wide areas and 

 depositing a mantle of gravels on plains now 

 adjoining the Hills at high levels, extending 

 up the valley as benches and passing over 

 many saddle-shaped divides. Leveling and 

 mapping of their features are now in progress 

 by Mr. Darton with a view to determining 

 quantitatively the amounts of uplift at the sev- 

 eral periods and their variations from place to 

 place through the Hills. 



River Terraces in Southwestern Colorado : By 



Me. a. C. Spencee. 



The rivers draining the San Juan mountains 

 emerge from deep canyons in palaeozoic and 

 older rocks upon a comparatively low-lying 

 region of younger rocks, comprising sandstones 

 and shales. These softer rocks have been 

 easily reduced by erosion and in the vicinity of 



the rivers terraces have been produced at alti- 

 tudes up to 500 feet above the present channels' 



The highest terraces may be correlated from 

 the Animas River at Durango, westward to the 

 Mancos River and McElmo Creek, and may be 

 recognized in the lower valley of the San 

 Miguel River. Similar terraces upon the Un- 

 compahgre River near Montrose, and along the 

 Gunnison and Grand rivers are also believed 

 to correspond. These facts are taken to indi- 

 cate the amount of recent erosion which the 

 rivers have accomplished, and as evidence of 

 regional uplift. There were several distinct 

 upward movements, all prior to the glaciation 

 of the San Juan mountains. 

 Some Coast Migrations in Southern California : 



By Me. Bailey Willis. 



The 102d meeting was held at the Cosmos 

 Club, May 2, 1900. 



The following papers were presented on the 

 regular program : 



A Reconnaissance from Pyramid Harbor to Forty- 

 mile River, Alaska : By Me. Alfred H. 

 Brooks. 



The route followed extends westward from 

 Lynn Canal along the northern front of the St. 

 Elias Range to the head of the White, Tanana 

 and Nabesna rivers. At the Nabesna river it 

 turned northward and, crossing the Tanana, 

 extended on to Eagle City on the Yukon. The 

 chief orographic features are the Coast Range 

 of Lynn Canal which extends westward beyond 

 Lake De Zar Diash, the St. Elias Range which 

 forms the Coast Range westward from Cross 

 Sound, the Nutzotin Mountains, which are a 

 minor range running parallel to the St. Elias 

 near the headwaters of the Tanana and White 

 rivers, the Mentasta Mountains, which are a 

 westward extension of the same range and con- 

 nect them with the Alaskan Range. From the 

 base of these mountains the Yukon Plateau ex- 

 tends northward, and is a dissected upland 

 sloping gently to the west. The drainage of 

 the region is taken by the Chilkat river to 

 Lynn Canal, by the Alsek southward to the 

 Pacific, and by the Tanana and White rivers 

 which are tributaries of the Yukon. The oldest 

 rocks of the region are the gneisses and crystal- 

 line schists forming a broad belt between the 



