Peckham and Hall—Thomsonite from Minnesota. 125 
agate mortar easily, but were scratched by quartz crystal; yet 
the percentage of silica was found to be no higher in the harder 
than the other specimens. The grain of such specimens, however, 
is exceedingly fine. Most frequently the hardness is between 
5 and 6. The specific gravity varics from 2°33 to 2°35; the 
water-worn and somewhat weathered pebbles have it a little 
lower, one or two as low as 2:2. The fracture of Numbers I 
and IT is fibrous; of Number III very uneven, and takes place in 
all directions with almost equal facility. They all gelatinize in 
hydrochloric acid to a thick jelly. Before the blowpipe they 
fuse easily and intumesce to a porous white enamel. In the 
closed tube, water to the amount of 11 to 12 per cent of the 
whole weight was given off at the heat of an ordinary spirit 
amp. Grains of native copper are frequently found in them, 
particularly in those of Number II, which, if the pebbles are 
transparent, exhibit under a low magnifying power arborescent 
groups of crystals, thrusting out their branches in every direc- 
tion through the enclosing mineral. In one instance an amyg- 
dule, about as large as a cranberry, contained at its center a mass 
of copper of this kind, one-third of its diameter. In this char- 
acteristic Number III resembles the prehnite of French River. 
umber —The amygdules of this type are perhaps of less 
common occurrence than other forms. Externally they look 
like porcelain with a slight creamy tint. Under the micro- 
Scope they appear for the most part translucent. Countless 
fine dark lines extend longitudinally through the thin section, 
rapidly disappearing to be replaced by others, like the cells in 
a longitudinal section of wood, which are probably caused in 
part by refraction of the light from the edges of minute densely 
packed crystals, from cavities, and from microlites. One notice- 
able result of these lines is to weaken the effect of the mineral 
©n polarized light. Not infrequently this opaque modification 
Of the mineral is banded with alternating zones, either trans- 
parent or yellow, or even with both; the transparency here 
Seems to be owing to an absence of the lines and microlites just 
hoticed ; while the yellow zones owe their color to globules of 
ferric oxide distributed through the mass. In the worn amyg- 
les the mineral often has a beautiful pearly luster. In 
minute quantities the ferric oxide gives the mineral a flesh- 
colored tint. 
