This thesis describes a seismic refraction project undertaken in south west Britain in November 1966, designed to investigate crustal structure associated with a granite batholith. The results and interpretation of the data collected are presented in the form of crustal structure sections along the three lines of shots. These results are then compared with those obtained in other areas of the British Isles and the geological implications are discussed, suggestions for the design of future crustal structure experiments based upon the experience gained in south west England are also included. Several methods of analysis have been employed. Least squares straight lines have been fitted to first arrival travel time data and time term analyses have been applied to the major phases thus identified. Amplitude and velocity filtering measurements have been made of both first and secondary arrivals. The importance of a large amplitude secondary arrival, identified as a supercritical reflection from the Moho, is emphasised and an explanation of its amplitude characteristics is provided in terms of a lower crust exhibiting a gradual increase of velocity with depth. The average crustal velocity along the line of the south west England peninsula and between Land's End and Brittany is about 6.1 km/sec with an upper crustal velocity of about 5.8 km/sec. The crust is about 27 to 28 km thick with an almost horizontal Moho and a sub-Moho P(_n) velocity of about 8.07 km/sec. The granite batholith of south west England extends to a depth of about 11 km beneath which the lower crust exhibits a gradual increase of velocity with depth. For the region between Land's End and Ireland there is some evidence for a higher P(_n) velocity and average crustal velocity with a dipping Moho such that the thickness of the crust beneath southern Ireland may be about 30 km.