A large number of density measurements of the more important rock-types exposed at the surface in Ayrshire and certain neighbouring areas provide information for the interpretation of gravity measurements. The three methods employed of determining density are (1) laboratory measurements of rock samples, (2) gravimeter measurements in mineshafts, and (3) gravimeter measurements over topographic features.
The first method, the most widely used, is the only one applicable to many of the geological formations. A total of 600 specimens measured cover all the common rock types of the Lower and Upper Palaeozoic systems and their associated igneous rocks. Use of the second method is restricted to Carboniferous rocks, in which records from four mine-shafts—Littlemill 5, Mauchline I, Auchincruive I, and Houldsworth—cover much of the Carboniferous succession in south Ayrshire. The third method was seldom applied as there are few suitable topographic features, independent of geological structure, in Ayrshire.
The results of the measurements are summarized, and their reliability discussed. Two contrasts of rock density of regional importance occur in the sedimentary succession of south Ayrshire—the first at the plane of unconformity between Upper and Lower Old Red Sandstone, and the second between Lower Old Red Sandstone and the Lower Palaeozoic greywackes. A marked contrast of local importance occurs between the Permian sandstones of the Mauchline Basin and the underlying lavas and Carboniferous rocks. In north Ayrshire the most important density contrast lies between the Upper Palaeozoic sediments and the associated dense igneous rocks—the Clyde Plateau Lavas, the Millstone Grit lavas, and the thick dolerite intrusions.
