Published length (L) and thickness (T) data on 135 laccolith and 21 granite intrusions define power-law relationships of the form L=kTa typical of systems exhibiting scale invariant (fractal) behaviour. Both data sets are characterised by an exponent a< 1 (0.88 ±0.1 for laccoliths and 0.80 ± 0.20 for plutons) that reflects an inherent preference for scale invariant tabular-sheet geometries. These power-law size relationships can be explained in mechanical terms by the need for an incoming magma sheet to travel laterally some distance before vertical thickening can occur. Sheet thickness is a function of available magma pressure which for an intrusion fed by a feeder dyke is proportional to the vertical magma transport distance.

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