CONTEMPORANEITY OF PHYSICAL EVENTS 361) 



siderable stretches of the ohl slioreline tlie terrnces cire now completely 

 effaced. 



Although the epochs of the Hoiiiieville aiuL pre-Boiuieville terraces 

 have been correlated with tlie two assumed parts of the Glacial period, 

 there appears to ha\e been never any critical evidence achluced in sup[)ort 

 of the notion. Instead of the Glacial epoch being possibly bipartite, as 

 it was everywhere attem])ted to be ]n-oved when the Bonneville mono- 

 graph was written, it has since l)een found to bt^ even more complex and 

 to have had half a dozen distinct stages or advancements of the conti- 

 nental ice-sheet. Under these circumstances the interpretation of the 

 Bonneville facts obser\ed is not so satisfactory as it might otlierwise be. 



There chance to be^ not so very far away from the Great Salt Lake, 

 localities where lake terrace and glacial phenomena are directly associ- 

 ated. The Okanogan Yalley, in central Washington, is a glacier-M'orn 

 trough. Three hundred feet above the river level is a conspicuous con- 

 tinuous terrace which marks a slioreline of a lake Avhen the Columbia 

 Iliver was blocked to the south by a great ice-tongue extending out of the 

 valley of tlie present (dielaii Lake. Xow the Okanogan terraces are about 

 as well preserved as those of the Provo stage in the Bpnneville basin. 

 Since their appearance the plwsical surroundings, the character of the 

 formation, and the climatic conditions of the two localities have been to 

 all intents and purposes the same, it may be assumed that they were 

 formed about at the same time. The Pro^o stage probably comes within 

 the last quarter of Lake Bonneville's existence. Hence, according to this 

 test, only the Provo stage is to be brought within the limits of the Glacial 

 epoch. 



DEPAUPERATION OF MOLLUSC AN SHELLS 



In his reference of the ge]iesis of Lake Bonneville directly to glacial 

 causes, Gilbert lays great, if not chief, stress on supposed dwarfing of 

 certain molluscan shells found in the lake deposits. This line of argu- 

 ment is based mainly on E. E. Call's conclusions concerning a compari- 

 son of the lake shells with the same forms now living. These conclu- 

 sions are liighly misleading and seem to be really a >\'ar]iing of the facts 

 to fit a theory. Call's similar conclusions regarding the loess mollusks 

 of the Mississippi Yalley^'' are of like character and were, moreover, 

 drawn Avhen it was uui\ersally thought that the loess Avas a strictly 

 glacial deposit. As conclusively demonstrated recently by Prof. B. Shi- 

 mek,^^ CalPs results, in so far as Iowa is concerned at least, are entirely 

 erroneous and his conclusions wholJv unvarranted. So their bearinu" on 



12 Am. Jour. Sci. {'.'>). vol. xxiv, 1882, p. 202. 



" BuU. I.ab. Nat. Hist., Iowa State Univ., vol. v, lOol, p. 195. 



