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Gueydan Formation
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Table I Classification of Fragmental Volcanic Rocks
The division of the above table headed "Epiclastic Vol canic Rocks" is equivalent to the "non-contemporaneous tuffs" of Green. 52 The rocks of this division commonly contain water-worn volcanic fragments as well as some non-volcanic rock fragments.
Pirsson has aptly divided tuffs into (1) Vitric tuffs, com-
52 Green, J. F. N., "The Vulcanicity of the Lake District," Proc. Geol. Assoc, Vol. 30, pp. 153-182, 1919.
Diameter and percent- age of component fragments.
Pyroclastic Rocks (Show no effects of ero- sion)
Epiclastic Volcanic Rocks (Composed wholly or in part of volcanic frag- mental material which
has been transported
and redeposited by wa-
ter.)
Vbove 256 mm. 50%
Very coarse volcanic
Volcanic boulder rock
breccia
256-32 mm. 50%
Coarse volcanic breccia
Coarse volcanic conglom-
erate
32- 4 mm. 50%
Medium volcanic breccia
Medium volcanic conglom-
erate
4- 1 mm. 50%
Fine volcanic breccia
Fine volcanic conglomerate
1-% mm. 50%
Coarse sand-tuff
Coarse tuffaceous sand-
(Volcanic ash when un-
stone
consolidated)
(Tuffaceous sand when
unconsolidated)
Vz-'bi mm. 50%
Medium sand-tuff
Medium tuffaceous sand-
stone
X k-V& mm. 50%
Fine sand-tuff
Fine tuffaceous sandstone
%-l/16 mm. 50%
Very fine sand-tuff
Very fine tuffaceous sand- stone
3elow 1/16 mm. 50%
Dust-tuff or mud-tuff
Tuffaceous shale
(Volcanic dust when un- consolidated)
(Tuffaceous clay when unconsolidated)
