proceedings: geological society 109 



havior have led to the conclusion that only one compound of ferric 

 oxide and water is known, Fe203.H20, which exists in two pure crystal- 

 line forms, goethite and lepidocrocite. The fibrous material commonly 

 known as limonite is really fibrous goethite with additional water, 

 silica, etc., held in capillaries. It has been possible to find a series of 

 specimens representing the expectable properties of such impure fibrous 

 goethite with variable water content. Turgite appears to be mix- 

 crystals of Fe203.H20 and hematite, with properties varying according 

 to the composition. 



The amorphous brown iron ores when air dried contain sub-micro- 

 scopic pores from which water has escaped, but they still hold in these 

 pores variable amounts of water in excess of the formula Fe203.H20. 

 They often show marked double refraction due to strain, but are readily 

 distinguished from fibrous goethite by lack of splintery fracture and 

 by lower refractive index. 



All these minerals except well-crystallized goethite often occur in 

 very close association. 



J. B. Mertie, Jr.: Repeated stream piracy in the Tolovana and 

 Hess River Basins, Alaska. The theme of particular interest to which 

 this paper was devoted is an example of stream piracy eflfected by one 

 stream at the expense of another; and a subsequent repetition of the 

 same process under diflferent physiographic conditions whereby the 

 second stream recovered a considerable proportion of its former drain- 

 age. The present watershed therefore represents the third recognizable 

 period of stability in the physiographic history of the two drainage 

 basins. 



The two streams in question are Livengood Creek, a tributary of 

 Tolovana River, and the south fork of Hess River, in the Tolovana 

 district, northwest of Fairbanks, Alaska. Gold-placer mining on 

 Livengood Creek has furnished the necessary underground data on the 

 configuration of bedrock under the gravels. An abnormally steep 

 bedrock gradient at the lower end of the old bench channel on Liven- 

 good Creek justifies the inference that this stream in its earliest recog- 

 nizable stage was much shorter than at present. The pronounced 

 back-hand drainage of its present upper tributaries is regarded as cor- 

 roboratory evidence of this hypothesis. Also the present divide 

 between the two streams has been found by drilling to be silt-filled. 

 An original stream piracy is thus deduced, whereby Livengood Creek 

 stole the headwater tributaries of the south fork of Hess River. The 

 depth to bedrock in the present silt-filled divide, and the depth to 

 bedrock in a narrow gorge in the lower part of the south fork of 

 Hess River, together with the elevations at these two localities, show 

 exactly how much of the upper drainage of the south fork of Hess River 

 was pirated. 



The new physiographic condition that brought about the original 

 piracy was a progressive drowning or inundation of the stream valleys 



