36 ROCHESTER ACADEMY OF SCIENCE. [Oct. 8, 



gives uniformity to the plant-bearing conditions. Exceptions may be 

 found in the areas of the Niagara and Corniferous limestones where 

 the drift is very thin or entirely wanting. It is true that the drift 

 partakes largely of the character of the rock formation underlying 

 immediately northward. But in this region the bands of different 

 rocks are not many miles in width, the changes in kind are frequent, 

 and the constituents of the drift are fairly distributed. Moreover, 

 the drift contains the product of the grinding and disintegration of 

 the crystalline Archean rocks, as well as the limestones and shales of 

 the region between here and the St. Lawrence. 



Following the southward drifting of the material by the ice came 

 the reversal and northward drifting of the finer material borne by 

 streams pouring into lake Warren, then later into lake Iroquois, and 

 now into the still lower lake Ontario. The Warren silts occur upon 

 the till over the area south of the " Ridge Road" and below 900 feet 

 altitude. The Iroquois silts occupy the surface between the present 

 lake Ontario and the beach altitude of 435-440 feet. This work of 

 stream and lake has thus helped to unify the flora of the region. 



The chief differentiation of the flora produced by differences 

 in the geological characters of the surface will be found upon tracts 

 that are washed sands or silts, such as the Chili sand knolls and the 

 Pittsford or the Mendon sand and gravel hills, and upon the tracts of 

 limestone above mentioned. 



The topographic relief of the surface is insufficient to greatly 

 affect the flora. 



CLIMATIC CONDITIONS. 



Statistics of Climate at Rochester. 



CONTRIBUTED BY MR. ORIN PARKER, WEATHER OBSERVER. 



The following statistics cover a period of twenty-five years, from 

 January 1, 1871, to January 1, 1896, and are obtained from the accu- 

 rate records of the Rochester Station of the United States Weather 

 Bureau. 



TEMPERATURE. (DEGREES FAHRENHEIT,) 



Highest recorded temperature go (July 20, 1894). 



Lowest " " —12 (Jan. 30, 1873, and Feb. 9, 1875.) 



Average variation between day and night 16 



Highest annual temperature 50 (1881) 



Lowest " " 44 ( 1 872) 



Average " " 47 



