W. H. Hobbs—Mineralogical Notes. 121 
mass volume than the oxyphyres. This relation of volumes 
would also be naturally dependant very greatly on the original 
composition of the magma; the theory is suggested for one of 
medium kind. 
_ All sorts and modifications of this process may of course be 
expected to occur, but the above expression of it may be 
assumed as the theoretically normal type, whose perfection or 
completeness will depend on local conditions, some of which 
have been indicated above. The writer has offered this hy- 
pothesis as to the formation of complementary rocks and radial 
dikes because it explains best their occurrence at those centers 
which have come under his observation, but he believes it to 
be of general application. 
Mineralogical and Petrographical Laboratory, Sheffield Scientific School, New 
Haven, May, 1895. 
Art. XIV.—WMineralogical Notes; by Wm. H. Hosss. With 
Analyses by Herman Schlundt and Louis Kahlenberg. 
1. Cerussite with superficial film of galena Jrom near Missoula, 
Mont. 
‘In February, 1894, I received from F. G. Bond, prospector 
at Missoula, Montana, a small specimen described as crystalline 
lead formed on the outside of a crystal. In the accompanying 
letter the following statements are made concerning it: 
“The sample was found in a very soft talco-slate with narrow 
bands of dark limestone running parallel with the slate. The 
slate is cut by a dike of bird’s eye porphyry that I traced over 
twenty miles. It is near this dike that our Iron Mountain mine 
is located.” 
The specimen sent is the only one that Mr. Bond had saved. 
The erystals referred to have a rather long prismatic habit 
with a length of 2-4™", and they show reéntrant angles in the 
prism zone, thus indicating that they are twinned individuals. 
They only rarely have terminal planes developed. They are 
white and somewhat translucent, but are partially covered by 
a glistening film of a metallic luster and very great tenuity. 
This thin superficial coating conforms perfectly to the form of 
the original erystal. A preliminary chemical examination 
showed that the material of the crystals is nearly pure lead car- 
bonate. On charcoal in the oxidizing flame of the blowpipe 
the mineral boils and turns red but cools to yellow. In the 
reducing flame it yields a button of metallic lead. The crys- 
tals are completely soluble with effervescence in dilute nitric 
acid. Further examination failed to reveal any base besides 
