162 



NEW YORK STATE MUSEUM 



Although evaporation played an important part in the fluctua- 

 tions, so far as appears from the report, this element was not 

 taken into account in drawing the foregoing conclusions. 



The error in precipitation, due to not fully taking into account 

 the amount of snow, should also be considered. 



Relation of Geologic Structure to Runoff 



Among the principal factors affecting stream flow should 

 be noted the structure and texture of the rocks, specially 

 those of the surface. For example, in regions with stiff, heavy, 

 clay soils, a larger proportion of the winter rainfall runs off 

 on the surface, passing immediately into the streams, than 

 is the case in regions with open, porous soils or extensive 

 sandy areas, while in summer a much smaller proportion runs 

 off. But such streams have a very much smaller ground- 

 water flow, from whence it results that the total runoff per 

 year is smaller than for streams with open, sandy soils. 

 The Genesee and Hudson rivers represent the extremes of 

 the State in this particular. A general knowledge of the 

 surface geology is therefore desirable in a study of the water 

 resources of the State. The relative position and area of the dif- 

 ferent geologic formations are best shown on the large Geologic 

 Map of New York prepared by Dr F. J. H. Merrill, State Geolo- 

 gist, in 1901 (scale 5 miles to the inch) . A similar but smaller map 

 by the same author, showing essentially the same features was also 

 printed in 1894 under authority of tlhe Regents of the University to 

 accompany the Report on the Mineral Exhibit of New York at the 

 World's Columbian Exposition, this being on the scale of approxi- 

 mately 14 miles to an inch. This map was also published with 

 Bulletin 15 of the State Museum and was reproduced in 1901 by 

 Edward A. Bond, State Engineer, in his report on the proposed 

 barge canal. On examining either of these maps one will note 

 the preponderance, so far as area is concerned, of two classes of 

 rocks — the ancient crystallines, which cover a large area in the 

 northern part of the State, and the conglomerates, sandstones and 

 shales of the Devonian, which form the greater part of the Appa- 

 lachian plateau, stretching from Lake Erie across the State to 



