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University of Texas Bulletin
Evidence for a distant source. — Under the first hypothesis the source is distant and it is, therefore, useless to expect to find the remains of the volcano in the vicinity of the Guey dan outcrop. If this ancient volcanic vent is located farther inland than the Gueydan belt, it should be exposed as a volcanic plug, because the formations outcropping farther inland are older than the Gueydan and a volcanic pipe would have had to pass through these formations in order to deposit the eruptive debris on top of them. On the other hand, if this volcano is situated at a considerable distance nearer to the Gulf than the Gueydan belt it would be covered by several thousand feet of post-Lower Moicene sedimentary deposits and could be discovered only by drilling.
The evidence for a distant source is as follows:
(1) The bulk of the Gueydan pyroclastics consists of fine particles, below one millimeter in length. This does not necessarily indicate a distant source because volcanoes erupting pumiceous material similar to the Gueydan tuffs often throw out the debris in a series of violent explosions which comminute this debris into tiny particles. Krakatoa is an extreme case of this type of volcano.
(2) Negative evidence is furnished by the failure to discover volcanic necks or parts of cones in the immediate vicinity of the tuff outcrops.
(3) A number of volcanic plugs are exposed in Uvalde, Kinney, and Travis counties, 100 miles or more from the nearest Gueydan exposures and these would furnish a con venient source for the Tertiary tuffs provided there were no conflicting evidence. Jones 76 evidently believes that the volcanic boulders he has noted in northwestern Duval County came from the Uvalde County volcanoes, although he gives no facts in support of his statement. The volcanic necks have been described by Vaughan 77 in the Uvalde Folio
76 Jones, R. A., "The Relation of the Reynosa Escarpment to the Oil and Gas Fields of Webb and Zapata Counties, Texas," Bull. Am. Assoc. Petro. Geol., Vol. 7, No. 5, pp. 544-545, 1923.
"Vaughan, T. W., "The Uvalde Folio," U. S. Geol. Sur. Folio No. 64, pp. 1-7, 1900.
