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NEW YORK STATE MUSEUM 



small area of metamorpkic Trenton and Caleiferous limestones, 

 but it is exceedingly improbable that any rocks which have been 

 subjected to metamorpkic changes are in any degree permeable. 

 This catckment must therefore be considered as underlain by an 

 impermeable formation. All of tke water falling upon it except 

 tkat absorbed by evaporation, chemical changes, etc. reappears as 

 runoff in the streams. It may be safely assumed that there are 

 no other losses. Nevertheless, the evaporation of this stream is 

 that tentatively placed upon other deforested areas. Moreover, 

 there is another interesting consideration of which brief note may 

 be taken at this place. In deference to the Water Supply Depart- 

 ment of the City of New York, the writer has used in computing 

 the monthly runoff the catchment area of 339 square miles. Mr 

 Yermeule, however, asserts that this area is not the true one. He 

 says the true area above old Croton dam is 353 square miles. 

 If we assume this to be true, it follows that the average runoff, 

 instead of being 22.8 inches, is over 4 per cent less, or is, roundly, 

 21.8 inches. This raises the evaporation from 26.6 inches to 27.6 

 inches. In his report on forests, Mr Yermeule has placed the 

 evaporation of his second Croton series, which the writer under- 

 stands him to consider more reliable, at 22.6 inches, a difference 

 of 5 inches from the foregoing figures, which it may be remarked 

 is based upon the latest revision and is presumably more likely 

 to be correct. 



On the upper Hudson river, with a catchment above Mechanic- 

 vine of 4500 square miles, the average rainfall for the fourteen 

 years from 1888 to 1901, inclusive, Avas about 44.2 inches, the 

 average runoff 23.3 inches, and the evaporation 20.9 inches. 

 Above Glens Falls this stream lies almost entirely in the Pre- 

 cambrian gneiss, from which it is improbable that there is any 

 loss of water. Its main tributary to the west, Sacandaga, is, by 

 observation, an exceedingly prolific water yielder. To the east, 

 the Battenkill and Hoosic rivers have a different geologic history. 

 The Battenkil] flows across the Hudson shales, the Georgia lime- 

 stones and shales, finally rising in the metamorphic Hudson and 

 Trenton formations. Tke H<><>sic river lias a similar geologic his- 



