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Gueydan Formation
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While the mud flow was in motion. A few of these rounded pumice lapilli may have had an original rounded form. Lacroix 41 states that the pumice lapilli from Mont Pelee, both from vertical projections and inclined fiery clouds,- are ordinarily rounded on account of their slight hardness and their fragility due to their porous structure. It is, there fore, more probable that the Gueydan pumice lapilli were rounded after their expulsion from the volcano. Many of the very small rounded bodies are hollow and are evidently glass bubbles. These smallest spherules are abundant in tuff beds where only a few of the larger tuff lumps occur.
The vesicular cavities, which are common in this tuff, are very irregular to rounded or flat ellipsoidal in shape and may be lined or completely filled with soft waxy, cream colored to light gray, or pink bentonite. Crusts of trans parent, botryoidal hyalite opal or mixtures of bentonite and opal are present in other cavities. In a few of the tuff beds in Live Oak, McMullen, Webb, and Starr counties some of the cavities are lined with tiny glittering plates of tridy mite or with a mixture of botryoidal hyalite and tridymite. These minerals are discernible with a hand lens, but the crystal form of the tridymite cannot be distinguished with out a microscope. The tubular cavities probably represent channels along wihich steam and other vapors — originally included in the mud flow — escaped. In certain beds these tubular cavities, now filled with opal and bentonite, are quite prominent and resemble small smooth-surfaced rootlets crossing each other at various angles. Some dendrites of manganese and limonitic concretions are seen in this tuff. No fossils have been found in the mud-flow tuff beds.
(2) A number of beds of light gray to white or grayish pink, very dense textured, chert-like silicified tuff or "por cellanite" are present at many localities in the Fant mem ber, especially near its base. Although this rock has a hardness of 5 or 6, and breaks with a conchoidal fracture like that of chert, it is cut by closely spaced, irregular, con choidal joint cracks which cause it to break down readily
41 Op dt., p.. 371.
