The use of ultrasonic velocity measurements (velocity of compression and shear sound waves of ultrasonic frequencies) is able to differentiate between fissuration-resistant (basalts) and fissurable ones (granite, marbles). The experimental fissuration of rocks was carried out by heating them. A fissuration index is defined and calculated from the compression wave data. A plot of this index against the porosity values for unweathered granites shows that below 1% porosity small changes in porosity are very important in the degradation of mechanical characteristics of the granites. See also 21-914 for entire proceedings. -- AATA