Agate— Physical Properties and Origin 5 
the present time is in the mountain chain extending 
from Porto Alegre, State of Rio Grande do Sul, in 
Brazil, to the district of Salto in northern Uruguay. 
The agates of this region surpass in size and beauty 
any others known and they form at the present time 
the principal source of supply of agates for commer- 
cial purposes. 
A portion of the agate-bearing area in Uruguay 
was visited in 1926 by Associate Curator H. W. Nich- 
ols of the Museum on the Captain Marshall Field 
South American Expedition of that year. Mr. Nichols 
reports that the agates are dug from shallow pits in 
grazing lands, where they occur in considerable quan- 
tity. Associated with the agates are many hollow 
stones which are often lined with amethyst crystals. 
Some of these are of gem quality. Pieces of agate are 
also found in the stream beds of the region, fresh sup- 
plies being brought down by floods. The largest dig- 
gings at present are in the Catalan district of Artigas, 
Uruguay, which is about one hundred miles north of 
Salto. 
Agates of considerable beauty, though not of great 
size, are found in many places in the United States. 
Those of Agate Bay, Lake Superior, have rich colors 
and make attractive charms and other ornaments. 
Agates are found in the beds of many streams in 
Colorado, Montana and other regions of the Rocky 
Mountains. They occur all along the Mississippi River, 
especially in Minnesota, also along the Fox River in 
Blinois, in the trap rocks bordering the Connecticut 
River and on the coasts of California and Oregon. 
Method of Formation of Agate 
No one who gives much consideration to the sub- 
ject of agates, can fail to be impressed with the difficul- 
ty of explaining how they are formed. They appear to 
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