C. A. Young—Spectrum of Brorsen’s Comet. 373 
At.a little distance from this shaft another boring was made, 
which shows eleven beds of coal in the same interval, which 
vary in thickness from two to thirty-one inches. 
One cannot join the exposures on the east, with those on the 
west side of the river, as the terraced plains of the Platte are 
very broad from Platteville to far below Evans. At the same 
time, the fucoids and the mollusks are fully characteristic of 
‘the Fox Hills group, and show that the rocks on both sides of 
the river are of the same age. ose on the east side have 
been regarded as without doubt belonging to the Laramie, not 
to the Fox Hills Group. 
The bluffs on the west side between the Thompson and the 
St. Vrain have been colored as Laramie on Dr. Hayden's map. 
These bluffs show Fox Hills fossils to the top; they contain no 
coal; no Jeaf-impressions were founc Dr. Hayden’s corps. 
It is difficult to understand, therefore, why the richly fossil- 
iferous sandstones of the bluffs should be colored as Laramie, 
while the underlying sandstones, without characteristic features, 
should be colored as Fox Hills. The bluffs along the Platte, 
both north and south from the Thompson and those on both 
sides of the Thompson, are Fox Hills and Fox Hills only. No 
higher rocks are exposed between Thompson and St. Vrain 
within five miles west from the Platte. 
Art. XLV.—WNote on the Spectrum of Brorsen’s Comet ; by 
Professor C. A. YounG, of Princeton, N. J. 
seen on the back-ground of even a very feeble spectrum. The 
. observation was made by placing the bar so that the bright 
ge of the band should be just visible as a thin line, the rest 
of the band being occulted. The instrument has also a scale 
like that of the ordinary chemical spectroscope, and the posi- 
1on of the micrometer-bar is determined both by the reading 
