The basic properties (light refractive index, density, mechanical strength, etc.) of fused silica are changed by the influence of high-intensity light from the glass transparency region capable of generating excitons by multiphoton absorption. The self-trapped exciton decay near the microcavity in the fused silica structure leads to the stable elementary intrinsic defect pair (nonbridging oxygen atom and three-fold-coordinated silicon atom) generation. At the large- enough light intensities near such a microcavity with a defect, the next exciton can be self- trapped. Then the next elementary defect can appear in the microcavity, and a chemical bond between it and the previously generated defect is formed. In such a way 'the darning' of the microcavity takes place and leads to the increase of the density and the light refractive index in this material. If the opposite elementary defects meet in the microcavity, then the Si-O bond is renewed, whereas if equal defects meet in the microcavity, then weaker 0-0 (peroxy bridges) an Si-Si bonds are formed. Because of this, during the illumination more and more of the above-mentioned weaker chemical bonds appear in the fused silica network leading to the decrease of the mechanical strength of the material.