Viewing magma as a natural critical fluid decomposing spinodally, it is possible to consider a magmatic rock as a system formed by one phase embedded in a background of the other phase. Furthermore, because critical phenomenology predicts fractal structures in magmatic rocks, the possibility of inquiring into magma behaviour on cooling it is offered. Small angle neutron scattering experiments have been carried out on several magmatic rocks from different localities. The results obtained, if on the one hand indicate the utility of the fractal concept in studying very complex structural details in magmatic rocks, on the other hand, confirm the validity of the small angle neutron scattering technique, for this kind of study. In particular, the analysis of the scattering data excludes the possibility that, at the molecular level, our volcanic and plutonic rocks are fractal volumes; no inference is possible about the particle-matrix interface. In addition, comparing between themselves the experimental results of the plutonic rocks with those of the volcanic ones, it is possible to notice a quite similar behaviour for all samples. This fact, clearly suggests that magma behaviour on cooling occurs by means of a unique and iterative mechanism. This is consistent with the hypothesis that the scattering originates from droplets with similar surface fractal dimension and cooled at or near critical condition by spinodal mechanism.