124 Peckham and Hall—Thomsonite from Minnesota. 
sii numerous, and in other cases few in number, even in 
the same bed of rock. The size varies from a microscopic 
point 1s a diameter of two or three inches. In one piece of the 
University of Minnesota, the number of amygdules distinctly 
visible to the unaided eye on a surface two inches square is sufli- 
cient to give more than 10,000,000 to the cubic foot. The 
largest in this area was about half an inch in diameter. The 
amygdules are generally much larger and more scattered than 
this specimen would indicate. Since they abound in the rock 
throughout many feet of its thickness and many miles of its ex- 
tent along the seated the supply appears to be inexhaustible; 
but practically the number of beach-pebbles, valuable as speci- 
mens, is quite Himnited: All the different varieties of thomsonite 
are so hard that they take a fine polish; and on account of this 
property and their often unique banded structure, they are 
much sought after by tourists and teeta as objects of rare 
beauty, and also for buttons, studs, et 
n our first visit to the beach oes the greater number of 
these pebbles occur, we at once recognized fragments of the 
large amygdules as thomsonite. Intermingled with these were 
— and oval pebbles, often more or less flattened, and 0. 
1 sizes from that of a pin’s head to that of a hickory nut, but 
for the most part of the size and form of beans and peas. 
Some of these were also recognized as thomsonite. The larger 
portion presented a great diversity of color and physical struct- 
ure; some being white and opaque, almost conchoidal in fract- 
ure, with but slight indications of a fibrous structure; others 
auies green mineral. In given portions of the rock formation, 
the amygdules were, for the most part, of the same genera 
character; in one place, being green and opaque; in another, 
without green bands; while in another, for the most part, 
beautifully variegated. Similar local peculiarities were 0 
served in reference to texture, some portions of the rock con- 
taining only those that were eid and fine a while others 
those that were uniformly coarser in texture. 
