332 W. LeConte Stevens — New Apparatus for 



be said, as the latter resembles the Campton rock very strongly 

 in structure. Specimens of each were kindly given me for 

 comparison by Professor Hitchcock and Dr. Harrington. 



The following analysis by the writer shows higher silica, 

 than the two dikes above referred to, also more alumina and 

 iron, but less lime. Allowing some of the lime to the horn 

 blende, the feldspar appears to be within the oligoclase limits 

 of the plagioclase series. A number of extinctions on oo P^ of 

 from 7°-9° indicated the same. 



SiO„ 48-19 



Al 2 3 --_'. 16-79 



Fe 2 3 18-37 



CaO 6-85 



MgO 1-32 



K„0 1-11 



Na 3 5-59 



Loss on ignition 2*31 



100-53 



Soluble in HC1 before fusion 23-5 



OfthisFe O 18-0 



"2 3 



All the iron determined as sesquioxide. 



Noticing in the Report on Mining Industry, 10th Census, p. 

 118, PI. XX YIII, a trap dike figured as cutting the Palmer 

 Ore Bed near Lake Champlain, I procured specimens of the 

 dike, but on microscopic examination they proved to be dia- 

 base. 



Stray bowlders of rock similar to the Forest of Dean dike 

 are to be seen not unfrequently in field work in the region, but 

 no other definite locality is as yet known to the writer. 

 Geological Laboratory, Cornell 'University. 



Aet. XXIX. — New Lecture Apparatus for demonstration 

 of Reflection and Refraction; by W. LeConte Stevens. 



The apparatus of which a brief description is here offered 

 is so simple, and in every particular so much more convenient 

 than any other designed for the same purpose, that the writer 

 deems it worth bringing to the attention of his fellow-teachers 

 in physics. 



The refracting medium is a hemi-cylinder of crown glass, 

 carefully polished, and mounted so as to turn on its axis hori- 

 zontally. The radius is 2 -5 cm , the length* 5 cm . The axis passes 

 through the center of a circle of white card-board, whose radius 

 may be 20 cm or 30 cm . Each of its quadrants is graduated from 



