University of Texas Bulletin
irregularities on their surfaces and frequently give only a suggestion of a rounded or elliptical form. They are sepa rated from each other by ramifying dendritic masses and flings of a yellowish or pale greenish substance which con sists of a mixture of bentonitic and opaline material. Some of the tuff beds are composed almost entirely of such round ish lumps, which give to the rock a conglomeratic appear ance. In a few localities veins of light bluish barite occur in the joints or cracks between the lumps instead of the opal and bentonite. The large lumps are composed of the same kind of tuff as the matrix in which they are included and they possibly represent fragments of a dehydrated and somewhat solidified crust which was formed on the upper surface of the mud flow and later engulfed ; these fragments were subsequently comminuted and partly rounded by rolling over and over in the more liquid volcanic mud below. Also, the spheroidal masses may have been formed near the ends of mud flows by a process sintilar to that which produces a mass of irregular blocks of the solidified crust in the ends of lava flows. In other words, the more liquid mud may have pushed forward, incorporated fragments of the partially solidified miud at the front of the flow, and rounded them by rubbing them against one another as the mud flow proceeded. It is impossible to ascertain which of these plausible explanations is the true one, and it is probable that several processes have contributed to the for mation of the larger lumps. These larger lumps are com monly composed of a number of smaller lumps. Lacroix 40 has described and photographed tuffs composed of such pisolite-like bodies of various sizes up to nearly 1 cm. in diameter. He states that they have been formed by the cohesive action of rain drops. Many of the smaller pisolite like bodies in the Fant member of the Gueydan tuff have probably originated in this manner. Some of the smaller lumps are pumice pebbles, rounded, no doubt, by the attri tional action of finer particles and other pumice fragments
40 Lacroix, A., "La Montagne Pelee et ses Eruptions," Paris, Masson et Cie, p. 420 and fig. 181, 1904.