The perthite problem has been dealt with. The treatment is based on the diffusibility in feldspar minerals. The values have previously been published by the author. It is concluded that the most stable orientation of perthite lamellae results from layer formation normal to the longest axis of the diffusion ellipsoid of the mineral, provided the diffusion body is a positive revolution ellipsoid. In the feldspars at temperatures from about 450° K to about 700° K this seems to be approximately the case. The accurate position of the longest axis is not stated with certainty, but it seems to be situated in the vicinity of the normal to (801). This position is in accordance with the perthite plane frequently found, viz. (801I) to (100).