The object of this investigation has been to determine the vertical displacement in the main fault of the Balcones Fault system West of the city of Austin, Texas. For this purpose a point on the fault line near the southern base of Mount Bonnel in Travis county was selected. The topography and geology of the surrounding region, including ten square miles, was mapped, the position of the fault determined and structural relations on either side studied. After establishing definite geological horizons and measuring the thicknesses of intervening formations, an estimate of the vertical displacement was made. This thesis gives an account of this work, comparing the results with those obtained by other investigators.