The successional mode of speciation of Julian Huxley (1942) may be defined as gradual transformation of one species into a successor species, or several of them, during the course of geologic time without primarily involving geographic, ecologic, or adaptive segregation. The purpose of this paper is to present a particular case of such a mode of speciation and to discuss problems involved therein.

The Barrilla Mountains, in the northeastern part of the Davis Mountains of Trans-Pecos Texas, are composed of Tertiary volcanic materials. Five tuffs and five lava flows, 1500 feet thick occurring throughout the mountains, persist in thickness and lithologic characteristics. Their upper surfaces show little erosion. The lavas are chiefly silicic and soda rich. The volcanic succession is underlain by a Tertiary sandstone above Upper Cretaceous marine formations.

Gravitational observations were made of the pre-Cambrian Coal Creek serpentine mass in Blanco and Gillespie Counties, Texas, the geology of which had been mapped previously. The observed gravitational anomalies indicate roughly the depth of the serpentine mass below which it may possibly grade into its parent peridotitic rock. Probable correlation between the gravitational map and the other geologic features of the area is indicated.