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. These were slightly deformed by the Laramide revolution, subsequently beveled, and everywhere covered by the sands of coalescing streams. The sandstone contains well-rounded chert and quartzite pebbles.
Broad folds and normal faults succeeded the extrusions of the youngest lava.