Amygdales in Columbia River lavas near Freedom, Idaho

Incomplete study of seven amygdales from the Columbia River lava‐flows along Slate Creek, a tributary of Salmon River, near Freedom in north‐central Idaho, reveals that these small objects are of unusual geological and mineralogical interest. This paper includes an outline of the geology of the area from which the amygdales came, a description of the amygdales, and a brief account of the periodic tilting of a large fault‐block as revealed by them.

Slate Creek enters Salmon River near the eastern border of the Columbia Plateau. In this locality the thick Lower Middle Miocene lava‐flows of the plateau country are interbedded with the sediments of many local lakes formed periodically as successive flows dammed the streams flowing westward from the higher country to the east. A section measured near the mouth of Slate Creek shows the volcanic rocks there to be 2600 feet thick. At some places on the Plateau farther from its borders the flows aggregate more than twice that thickness.