Notes Upon Two Boulders of a Very Basic Eruptive 



Rock from the West Shore of Canandaigua 



Lake ; and their Contact Phenomena 



Upon the Trenton Limestone. 



By Prof. B. K. Emerson, Amherst College. 



[Communicated for the report of the State Geologist.] 



The rocks described below were sent to me by Prof. John M. Clarke 

 with the following note : 



' ' Specimens and sections marked : % 



*' 1 and 2. Boulder of Trenton hmestone from southern part of town of Canan- 

 daigua, N. Y. , on west shore of lake. 

 "1. Limestone at contact with dyke. 

 "2. Dyke. 



" X. Fragments of limestone. 

 '* 3 and 4. Boulder of eruptive, including small masses of sandstone from same 

 locality, about forty rods from other boulder. 

 "These are the only rocks of this character I have seen in the drift of west- 

 ern New York." 



The specimens had been sawed and broad surfaces polished, and this 

 greatly facilitated their study. Slides of an unusual size were also 

 sent, showing abundantly all the different tj^'pes present. 



The specimen numbered 1 — " limestone at contact mtli dyke " — is a 

 dull black, aphanitic, trap-like rock, effervescing slowly with acid and 

 showing minute pyrite grains and pale green, distant porphyritic spots 

 on the polished face. 



The largest spots reach the size of a pin-head. With the pocket lens 

 the thin sections show thickly and evenly disseminated elongate blades 

 of red brown color, with irregular ends and black, shapeless ore grains 

 and plates, and the porphyritic spots seem to be mainly marked by the 

 absence of the brown blades and the greater amount and larger size of 

 the black grains, and to be composed of interlaced, colorless and 

 elongate plates. 



