4 
126 Peckham and Hall—Thomsonite from Minnesota. 
A mean of three analyses showed the composition of this 
mineral to be: 
AUIS Fintona ney onl: 40°45 
REO Hos Paes ert wt 29°50 
Cab neues. wwe 2. ltses 10 
KO ..8ide. nani aus 0-887 
PAO oe oe ee e706 
BOrid.aa ad eplaumatcaew cal OR 
99-985 
Even opaque white amygdules afforded a trace of ferric oxide, 
which increased to a few hundredths of one per cent when the 
tint was perceptibly flesh-red. 
Number I —Under this type nearly every specimen is fibrous 
and radiated. The masses are spherical or elliptical, with the 
point from which the crystalline fibers radiate on one side of 
the mass, or, as is perhaps more common, having several cen- 
mineral fills seams, or occupies cavities that run together; 
here, there are centers of radiation at frequent intervals and by 
a system of suture-like joints, the whole is made into a com- 
pact mass. Yet, solid as the mass may appear to be, a thin plate 
cut from it invariably separates into pieces along the line of 
these joints, giving the mineral an appearance of fragility while 
it is really as hard as agate. The fibers often interlock along 
the line of these joints. 
needles are broken up into short pieces by transverse fractures. 
They all taper out and disappear, the longest of them reaching 
no further than the middle of the mass. They act strongly on 
polarized light and contain some inclusions. These lines do 
not occur as developed crystals. 
Around the borders of many amygdules there are numerous 
small spherolites. They have probably formed around gran- 
ules of various foreign substances as nuclei. Their size is small ; 
to the naked eye they look like mere spots, but they are so 
numerous as to form an envelop almost entirely around the 
radiated concretions. 
A mean of three analyses gave— 
