proceedings: geological society 51 



Hudson River across the continental shelf; the origin of the "deeps" 

 of the Susquehanna River; the cause or causes of many Pleistocene 

 rearrangements of drainage in Pennsylvania, Ohio, West Virginia, 

 Kentucky, and Nebraska. The relation of these problems to the gen- 

 eral hypothesis may deserve discussion by the geologists who are most 

 familiar with the different parts of this extra-glacial belt. Munthe's 

 conclusion that Jamieson's hypothesis applies to Recent warpings in 

 northwestern Europe is another strong reason for its serious considera- 

 tion in the case of our own ice-cap area in eastern North America. 

 Similarly, the reported warping of the highest shore-line of Glacial Lake 

 Missoula (Montana-Idaho) and the problem of the Grand Coulee in 

 Washington State may possibly indicate still a third ice-cap which 

 deformed the earth's surface in the way Jamieson imagined. 

 The paper was discussed by several members of the Society. 



340TH MEETING 



The 340th meeting of the Society was held in the Auditorium of the 

 Cosmos Club on Wednesday evening, November 26, 1919. The pro- 

 gram consisted of the following three papers: 



D. F. Hewett: The Heart Mountain overthrust near Cody, Wyoming. 



This overthrust, recognized west of Cody by Dake in 191 6, proves 

 to be much more extensive than was at first suspected. Work in the 

 region in 191 9 shows that on two of the four principal summits of 

 McCulloch Peak (6,200 feet) 12 miles east of Cody, caps of Madison 

 and Bighorn (?) limestone 20 to 80 feet thick rest on beds that yield 

 Bridger fossils (Upper Eocene). As these residual masses of limestone 

 lie 28 miles east of the westernmost exposures of the fault, this distance 

 measures the minimum extent of the overthrust. Data collected to 

 date do not yield conclusive evidence whether the overthrust took 

 place before or after the deposition of the andesitic tuff and breccia 

 that make up the Absaroka Mountains. The extent of the over- 

 thrust is therefore comparable with the Bannock in southeastern Idaho, 

 the most extensive known in North America. The Bannock over- 

 thrust took place before the Wasatch (Lower Eocene) beds were laid 

 down, whereas the Heart Mountain overthrust can not have occurred 

 before the deposition of the Bridger (Upper Eocene) beds. 



Clyde P. Ross: Geology of the Lower Gila region, Arizona. 



The region described lies in southwestern Arizona between Gila and 

 Colorado rivers, south of Buckskin Mountains and west of Phoenix. 

 The rock formations are divided into four groups: (i) basal complex; 

 (2) intrusives of probable Mesozoic age; (3) Tertiary lavas and sedi- 

 mentary rocks; (4) Quaternary alluvium and basalt. The basal com- 

 plex is divided into highly metamorphosed schists included in batholithic 

 masses of granitic gneisses, a thick series of metamorphosed marine sed- 

 iments unconformably overlying the gneisses, and a series of compar- 

 atively slightly metamorphosed marine sediments of possible Paleozoic 

 age. The Tertiary lavas far exceed the sediments in amount, totalling 

 over 2,000 feet in thickness. They are latites, soda rhyolites, and 



