The Little Avon river rises on the scarp slope of the Cotswolds near Hawkesbury, 14 miles north-east of Bristol, and flows in a westerly direction over Mesozoic strata, until half a mile south-east of Wickwar it cuts through the Triassic mantle and exposes to view three inliers of Wenlock Limestone (Whittard & Smith, 1944, p. 65 and map). The West Gloucestershire Water Company was interested in the valley as a possible site for an impounded reservoir, and as part of a preliminary survey a trial-shaft was sunk into the Wenlock Limestone of the southernmost inlier situated near Sturt Bridge. The shaft penetrated 28 feet of dolomitized siltstones and crinoidal limestones, and at a depth of 4 feet it entered a band of calcareous chamositic siltstone averaging 9 inches in thickness. The shaft had been completed and the sides boarded before we heard it had been sunk; the hand-specimens were therefore obtained from the tip-heaps, but there is no doubt that all the material originated from the same horizon.

In the early days of X-ray crystallography the study of the atomic and molecular structure of crystals was of primary importance. It was more necessary to know the arrangement of the atoms in, say, rocksalt, quartz, diamond, or graphite, than to know the exact distances apart of the atoms, or the small deviations from the mean atomic structure which may result from heat movements, eccentricity of growth, mechanical distortion, and other factors.

Now, however, the range of problems is widening, and X-ray methods are being used as a research tool by many workers whose main interest is in other branches of science. It is necessary, therefore, to point out that the familiar Bragg law, nλ = 2d sin θ, which relates order of spectrum n, wave-length λ, crystal spacing d, and diffraction angle θ in an ideal crystal, though still of course applicable, must be applied with some caution to crystals which are not ideal in either structure or texture, and which may be of irregular or eccentric shape.