Volcanic ash deposits, which also are known as _pumicite_, are loose and
powdery. They are made up mostly of material that is thrown into the air
when volcanoes erupt. If a volcano erupts with a violent explosion, the
nearby rocks are blown into powder. Molten lava also is hurled into the
air, where some of it immediately cools to become tiny bubbles and
particles of glass. The winds may carry some of this fine material far
away before depositing it.
Deposits of volcanic ash are white, bluish, greenish, yellowish, or
grayish, and some of them glisten like snow in the sunlight. They feel
rough and gritty. When examined under a microscope, this material shows
the tiny curved and sharp-cornered particles of the broken volcanic
glass. Deposits of volcanic ash may also contain clay, silt, sand, or
other impurities.
Volcanoes, which may have been located in the Davis Mountains and in
other areas of west Texas and in northern Mexico, erupted during
Tertiary time. The volcanic ash that we find at the surface today in
some of the Tertiary formations in Texas could have come from these
volcanoes. Tertiary volcanic ash deposits occur in the Texas Gulf
Coastal Plain (such as in Brazos, Fayette, Karnes, Polk, Starr, Trinity,
and other counties) and in the Trans-Pecos country of west Texas.
Volcanic ash deposits of Quaternary (Pleistocene) age, which are less
than a million years old, are found in a number of counties on the Texas
High Plains. Farther to the east, ash deposits occur in Baylor, Dickens,
Kent, and Wilbarger counties. This volcanic ash may have come from a
volcano that erupted in northern New Mexico during Quaternary time.
Volcanic ash or pumicite has several commercial uses. Some is used to
make pozzolan cement, and some is used in sweeping compounds, cleansing
and scouring powders, and abrasive soaps. Pumicite has been mined in
Dickens, Scurry, Starr, and several other counties of Texas.
Wad. _See_ Manganese Minerals.
Wood Opal. _See_ Opal.