J, L. Greenleaf — The Hydrology of the Mississippi. 29 



compounds pearceite, jordanite, proustite, dnfrenoysite and 

 sartorite are greater than those of the corresponding antimony 

 compounds, while xanthoconite and pyrostilpnite form an ex- 

 ception to this rule, which may, however, be due to inaccuracy 

 in the measurement of the small crystals of these exceedingly 

 rare minerals. The relations shown in the table may be acci- 

 dental, but the close approximation of the axial ratios to that 

 of chalcocite would seem to indicate that the sulphide of the 

 metal has had a controlling influence on the crystallization. 

 Sternbergite, Ao^^S-Fe^S^ with m^m = 60° 30' and enargite, 

 3Cu,S . As,S, wilh x,^x"\ 320^320 (twinning plane) = 60° 11' 

 mio^ht be added to the table. 



Laboratory of Mineralogy and Petrography. 

 Sheffield Scientific School, Xew Haven, March, 1896. 



Art. Y. — The Hydrology of the 21ississip>2?i ; by James L. 

 Greexleaf, C.E. 



A RIVER of the size of the Mississippi necessarily has many 

 and important tributaries, with their individual peculiarities of 

 watershed, channel, and variation in flow. Each of these trib- 

 utaries exerts its proportionate influence upon the stream which 

 is the resultant, and a knowledge of them is therefore essential 

 to a study of the hydrology of the main river. For this reason, 

 sixteen of the branches especially influential in forming the 

 character of the Mississippi, have been selected for a brief dis- 

 cussion. 



As a preliminary, it will be well to consider for a moment 

 the point of view from which the engineering profession ex- 

 amines the hydrology of a watershed. Whatever the purpose 

 that the engineer has in working upon a river, be it water 

 power, water supply of towns, irrigation, the training of the 

 current to prevent devastating overflows or to maintain a 

 depth for navigation ; in each and all of these problems two 

 fundamental considerations force themselves upon his attention. 

 They are the degree of regularity, and the amount of the 

 volume of flow from the tributary country. Thus, in the south, 

 the Mississippi River Commission and the Levee Boards find 

 the flow volume a vital feature in their efforts to confine the 

 stream in a definite channel. At the north, the Engineer Corps 

 of the United States Army are constructing a system of reser- 

 voirs for holding back the freshets and aiding navigation upon 

 the upper river during the season of low flow. 



