254 ANNUAL OF SCIENTIFIC DISCOVERT. 



and depressing the north-eastern portion of the continent. The 

 waters of the ancient kike were not sufficient to cut out the chan- 

 nel of the Mississippi. The whole middle region, from the Gulf 

 to the Arctic region, was once covered by a sea, the subsidence or 

 withdrawal of which is indicated by the series of small lakes in Ne- 

 brar?ka. During the cretaceous period this area was under water, 

 and the streams from the land east ran west, and the Mississippi 

 was not then formed. These ancient rivers ran in beds older than 

 themselves, but the glacial period obliterated all these features. 

 The glacial drift extends probably as far south as Keokuk, where 

 is found a glacial moraine, 100 miles south of the limit of boulders, 

 as given by Prof. Whitney. The modified drift covers the whole 

 region as far as the Missouri River, beyond which no traces of the 

 glacial action are found. The source of supply of the glacial 

 moi-ture must have been on the south, and probably the glaciers 

 met the waters of the gulf. The glacial action of the Rocky 

 Mountains never extends more than 50 miles from their base. 

 The action of the great northern glacier made the water-courses 

 almost parallel ; the Lower Mississippi resulted from its with- 

 drawal, and marks the former limit of the continent. The diflfer- 

 ence of level of the Mississippi and Missouri Rivers, in the same 

 latitude, was 400 or 500 feet. The bluffs at Dubuque, Grand 

 Rapids, and some other places, show the effects of the erosion of 

 the river in its formation at the glacial period. 



GEOLOGY OF RUSSIAN AMERICA. 



W. H. Dall, in a letter from St. Michaels, Russian America, -dated 

 August 13, 1867, and published in the *' American Journal of 

 Science" for January, 1868, says: " Starting from Fort Youkon 

 and going down stream, we have on either hand low land sparsely 

 wooded with spruce, poplar, birch, and willow, with low hills in 

 the distance, gradually increasing in height and coming closer to 

 the river, where they finally come together 225 miles below Fort 

 Youkon ; and the river which previously has been very winding, 

 full of sloughs and large islands, and from 3 to 9 miles wide, here 

 becomes narrow, deep, and rapid, w^th one channel. These 

 mountains are known as the ' Ramparts,' and come close to the 

 water's edge, having an imposing appearance, though probably 

 not more than 1,500 to 2,500 feet high. They were entirely com- 

 posed of azoic rocks, of which a silvery greenish rock of talcose 

 appearance, but very hard, predominates. Quartz in seams, 

 slates, and quartzite rock are abundant ; and a rock resembling 

 granite, but with a superfluity of felspar and no mica, is rare. The 

 slates generally have a north-westerly dip. 



"True granite appears only once, near the termination of the 

 Ramparts, and forms a ledge extending across the river, and mak- 

 ing a rapid, — not, however, a dangerous one. Fifty miles or less 

 below the rapid the Ramparts terminate, and the Tanana River 

 comes in. From the end of the Ramparts to Koyoukuk River, 

 250 miles, the right bank presents in their order, conglomerate. 



