Disintegration of isolated clasts of tonalite and granodiorite in some glacial and glaciofluvial deposits in southwestern British Columbia has been examined. Degree of disintegration is highly variable and neighboring clasts of similar lithology may be completely unaltered. Friability of some clasts is so great that the concept of transport from an already weathered parent bedrock is dismissed. Disintegration is associated with the biotite content of the rock and with numerous microfractures radiating from expanded biotite crystals. It is concluded that alteration and expansion of biotite has been responsible for the microfractures and has facilitated further chemical weathering of the rock. Optical, X-ray, and scanning electron-microscope studies indicate that the biotite has been converted first to trioctohedral vermiculite, and minor biotite-vermiculite random mixed-layer mineral by leaching of potassium and expansion of the phyllosilicate lattice, followed by precipitation of hydroxyl-Al and -Fe(?) in the interlayer position. Mildly acid solutions percolating downward from the present, or former, soil surface, is considered to be the prime agent for the alteration. Differences in biotite content, in grain size, in degree of rounding and of polishing of the clast surfaces, and perhaps in hydrothermal alterations of the rock prior to glacial transport account for the variable degree of disintegration at their present sites.