C. 8. Hastings—Optical Constants of Glass. 269 
since, in 1874, Dr. Sterry Hunt, after a cursory examination 
of Ziegler’s Mine in Berks County, situated at the junction of 
the No. II limestone and the No. III slates, made the mistake, 
in a paper on “The Decay of Crystalline s” before the 
National Academy of Science, of supposing that the hydromica 
slates belonged to the Huronian Period :—a mistake into which 
so eminent an observer as himself would never have fallen had 
he been better acquainted with the region. 
At intervals along the junction of the limestones and slates 
there occurs a black carbonaceous shale, often decompose 
black or dark blue clay, which I have supposed to be the rep- 
resentative of the Utica shales. It consists of a very carbona- 
ceous hydromica slate (containing damourite), without any fos- 
sils and may not belong to the Utica Period at all. In no 
instance has it been found more than one to twelve feet thick, 
but it sometimes carries pyrite from which a portion of the iron 
ores, just mentioned, may have been derived. These shales 
are of no economic im 
opened at various points for the purpose of extracting them, as, 
however, they have been but very slightly examined, during the 
progress of the present Geological Survey of the State, I shall 
defer a more detailed description of them to some future time. 
Art. XXXIX.—On the Influence of Temperature on the Optical 
Constants of Glass ; by CHARLES S. Hastines, of the Jobns 
Hopkins University. 
A FORMULA connecting the refractive power of a body with 
its density, established by Newton, 1s well known. This, 
er. : 
More recently rear: Gladstone have made an extensive 
study of the changes produced in the refractive and as cor 
powers of various liquids by increase of temperature; @ study 
