F. W. Cope and C. S. Exley write: Sowerbutts is to be congratulated on the summary of his magnetic survey of the Butterton Dyke in North Staffordshire. It is a neat demonstration of the use of a magnetic gradiometer coupled with a microcomputer-based data-gathering system in a seemingly simple geological setting.
In his introductory remarks on exposures and the likely course of the dyke, Sowerbutts states “Magnetic surveys suggest the dyke is present at Keele University, 5 km NNW of the [Church Wood] quarry”. In point of fact, the dyke was well exposed within the University grounds in 1966 when an excavation was being made for a bacterial sewage tank. A photographic record was made, the petrography of the dolerite was described in detail, and the course of the dyke was plotted not only northwards from the spot where it was observed by Charles Darwin to the University outcrop, but southwards from Church Wood where twin intrusions are well exposed, over the ground covered by Sowerbutts’ magnetic survey; this 1 km length of outcrop was determined by boring with a post-auger (Cope 1966).
Sowerbutts uses a radiometric age for the dolerite of 52. ± 2 Ma (Evans 1969) which agrees with a previously determined age of 52.4 ± 1.4 Ma supplied by J. A. Miller (Cope 1966). It seems likely that both figures were based on the same assay, and they give an early Tertiary age for the intrusion.
Sowerbutts does not seem to have been aware of an exposure of the dyke which was visible in 1961 during the construction of the M6 motorway, or same or a closely related intrusion in a borehole to the north